Chapter 10

(1) Truefissionor longitudinal division of an individual into two equal and similar daughter-individuals is not common but occurs inGastroblasta, where it has been described in detail by Arnold Lang [30].

(2)Autotomy, sometimes termed transverse fission, is the name given to a process of unequal fission in which a portion of the body separates off with subsequent regeneration. InTubulariaby a process of decapitation the hydranths may separate off and give rise to a separate individual, while the remainder of the body grows a new hydranth. Similarly inSchizocladiumportions of the hydrocaulus are cut off to form so-called “spores,” which grow into new individuals (see Allman [1]).

A, B, C, E, F, In vertical section.

D, Sketch of external view.

st, Stomach.

m, Manubrium.

t. Tentacle.

s.o, Sense organ.

v, Velum.

s.c, Sub-umbral cavity.

n.s, Nervous system.

A, B, C, D, F, Successive stages in vertical section.

E, Transverse section of a stage similar to D.

Gc, Entocodon.

s.c, Cavity of entocodon, forming the future sub-umbral cavity.

st, Stomach.

r.c, Radial canal.

c.c, Circular canal.

e.l, Endoderm lamella.

m, Manubrium.

v, Velum.

t, Tentacle.

(3)Vegetative buddingis almost universal in the Hydromedusae. By budding is understood the formation of a new individual from a fresh growth of undifferentiated material. It is convenient to distinguish buds that give rise to polyps from those that form medusae.

(a)The Polyp.—The buds that form polyps are very simple in mode of formation. Four stages may be distinguished; the first is a simple outgrowth of both layers, ectoderm and endoderm, containing a prolongation of the coelenteric cavity; in the second stage the tentacles grow out as secondary diverticula from the side of the first outgrowth; in the third stage the mouth is formed as a perforation of the two layers; and, lastly, if the bud is to be separated, it becomes nipped off from the parent polyp and begins a free existence.(b)The Medusae.—Two types of budding must be distinguished—the direct, so-called, palingenetic type, and theindirect, so-called coenogenetic type.The direct type of budding is rare, but is seen inCuninaandMillepora. InCuninathere arises, first, a simple outgrowth of both layers, as in a polyp-bud (fig. 43, A); in this the mouth is formed distally as a perforation (B); next the sides of the tube so formed bulge out laterally near the attachment to form the umbrella, while the distal undilated portion of the tube represents the manubrium (C); the umbrella now grows out into a number of lobes or lappets, and the tentacles and tentaculocysts grow out, the former in a notch between two lappets, the latter on the apex of each lappet (D, E); finally, the velum arises as a growth of the ectoderm alone, the whole bud shapes itself, so to speak, and the little medusa is separated off by rupture of the thin stalk connecting it with the parent (F). The direct method of medusa-budding only differs from the polyp-bud by its greater complexity of parts and organs.The indirect mode of budding (figs. 44, 45) is the commonest method by which medusa-buds are formed. It is marked by the formation in the bud of a characteristic structure termed theentocodon(Knospenkern,Glockenkern).Fig. 45.—Modifications of the method of budding shown in fig. 44, with solid Entocodon (Gc.) and formation of an ectotheca (ect.).The first stage is a simple hollow outgrowth of both body-layers (fig. 44, A); at the tip of this is formed a thickening of the ectoderm, arising primitively as a hollow ingrowth (fig. 44, B), but more usually as a solid mass of ectoderm-cells (fig. 45, A). The ectodermal ingrowth is the entocodon (Gc.); it bulges into, and pushes down, the endoderm at the apex of the bud, and if solid it soon acquires a cavity (fig. 44, C,s.c.). The cavity of the entocodon increases continually in size, while the endoderm pushes up at the sides of it to form a cup with hollow walls, enclosing but not quite surrounding the entocodon, which remains in contact at its outer side with the ectoderm covering the bud (fig. 44, D,v). The next changes that take place are chiefly in the endoderm-cup (fig. 44, D, E); the cavity between the two walls of the cup becomes reduced by concrescence to form the radial canals (r.c.), ring-canal (c.c.), and endoderm-lamella (e.l., fig. 44, E), and at the same time the base of the cup is thrust upwards to form the manubrium (m), converting the cavity of the entocodon into aspace which is crescentic or horse-shoe-like in section. Next tentacles (t, fig. 44, F) grow out from the ring-canal, and the double plate of ectoderm on the distal side of the entocodon becomes perforated, leaving a circular rim composed of two layers of ectoderm, the velum (v) of the medusa. Finally, a mouth is formed by breaking through at the apex of the manubrium, and the now fully-formed medusa becomes separated by rupture of the stalk of the bud and swims away.Fig. 46.—Diagrams to show the significance of the Entocodon in Medusa-buds. (Modified from a diagram given by A. Weismann.)I, Ideally primitive method of budding, in which the mouth is formed first (Ia), next the tentacles (Ib), and lastly the umbrella.II, Method. ofCunina; (a) the mouth arises, next the umbrella (b), and lastly the tentacles (c).III, Hypothetical transition from II to the indirect method with an entocodon; the formation of the manubrium is retarded, that of the umbrella hastened (IIIa,b).IV,a,b,c, budding with an entocodon (cf. fig. 44).V, Budding with a solid entocodon (cf. fig. 45).If the bud, however, is destined to give rise not to a free medusa, but to a gonophore, the development is similar but becomes arrested at various points, according to the degree to which the gonophore is degenerate. The entocodon is usually formed, proving the medusoid nature of the bud, but in sporosacs the entocodon may be rudimentary or absent altogether. The process of budding as above described may be varied or complicated in various ways; thus a secondary, amnion-like, ectodermal covering or ectotheca (fig. 45, C,ect.) may be formed over all, as inGarveia, &c.; or the entocodon may remain solid and without cavity until after the formation of the manubrium, or may never acquire a cavity at all, as described above for the gonophores.Phylogenetic Significance of the Entocodon.—It is seen from the foregoing account of medusa-budding that the entocodon is a very important constituent of the bud, furnishing some of the most essential portions of the medusa; its cavity becomes the sub-umbral cavity, and its lining furnishes the ectodermal epithelium of the manubrium and of the sub-umbral cavity as far as the edge of the velum. Hence the entocodon represents a precocious formation of the sub-umbral surface, equivalent to the peristome of the polyp, differentiated in the bud prior to other portions of the organism which must be regarded as antecedent to it in phylogeny.If the three principal organ-systems of the medusa, namely mouth, tentacles and umbrella, be considered in the light of phylogeny, it is evident that the manubrium bearing the mouth must be the oldest, as representing a common property of all the Coelentera, even of the gastrula embryo of all Enterozoa. Next in order come the tentacles, common to all Cnidaria. The special property of the medusa is the umbrella, distinguishing the medusa at once from other morphological types among the Coelentera. If, therefore, the formation of these three systems of organs took place according to a strictly phylogenetic sequence, we should expect them to appear in the order set forth above (fig. 46, Ia,b,c). The nearest approach to the phylogenetic sequence is seen in the budding ofCunina, where the manubrium and mouth appear first, but the umbrella is formed before the tentacles (fig. 46, IIa,b,c). In the indirect or coenogenetic method of budding, the first two members of the sequence exhibited by Cunina change places, and the umbrella is formed first, the manubrium next, and then the tentacles; the actual mouth-perforation being delayed to the very last (fig. 46, IVa,b,c). Hence the budding of medusae exemplifies very clearly a common phenomenon in development, a phylogenetic series of events completely dislocated in the ontogenetic time-sequence.The entocodon is to be regarded, therefore, not as primarily an ingrowth of ectoderm, but rather as an upgrowth of both body-layers, in the form of a circular rim (IVa), representing the umbrellar margin; it is comparable to the bulging that forms the umbrella in the direct method of budding, but takes place before a manubrium is formed, and is greatly reduced in size, so as to become a little pit. By a simple modification, the open pit becomes a solid ectodermal ingrowth, just as in Teleostean fishes the hollow medullary tube, or the auditory pit of other vertebrate embryos, is formed at first as a solid cord of cells, which acquires a cavity secondarily. Moreover, the entocodon, however developed, gives rise at first to a closed cavity, representing a closing over of the umbrella, temporary in the bud destined to be a free medusa, but usually permanent in the sessile gonophore. As has been shown above, the closing up of the sub-umbral cavity is one of the earliest degenerative changes in the evolution of the gonophore, and we may regard it as the umbrellar fold taking on a protective function, either temporarily for the bud or permanently for the gonophore.To sum up, the entocodon is a precocious formation of the umbrella, closing over to protect the organs in the umbrellar cavity. The possession of an entocodon proves the medusa-nature of the bud, and can only be explained on the theory that gonophores are degenerate medusae, and is inexplicable on the opposed view that medusae are derived from gonophores secondarily set free. In the sporosac, however, the medusa-individual has become so degenerate that even the documentary proof, so to speak, of its medusoid nature may have been destroyed, and only circumstantial evidence of its nature can be produced.

(b)The Medusae.—Two types of budding must be distinguished—the direct, so-called, palingenetic type, and theindirect, so-called coenogenetic type.

The direct type of budding is rare, but is seen inCuninaandMillepora. InCuninathere arises, first, a simple outgrowth of both layers, as in a polyp-bud (fig. 43, A); in this the mouth is formed distally as a perforation (B); next the sides of the tube so formed bulge out laterally near the attachment to form the umbrella, while the distal undilated portion of the tube represents the manubrium (C); the umbrella now grows out into a number of lobes or lappets, and the tentacles and tentaculocysts grow out, the former in a notch between two lappets, the latter on the apex of each lappet (D, E); finally, the velum arises as a growth of the ectoderm alone, the whole bud shapes itself, so to speak, and the little medusa is separated off by rupture of the thin stalk connecting it with the parent (F). The direct method of medusa-budding only differs from the polyp-bud by its greater complexity of parts and organs.

The indirect mode of budding (figs. 44, 45) is the commonest method by which medusa-buds are formed. It is marked by the formation in the bud of a characteristic structure termed theentocodon(Knospenkern,Glockenkern).

The first stage is a simple hollow outgrowth of both body-layers (fig. 44, A); at the tip of this is formed a thickening of the ectoderm, arising primitively as a hollow ingrowth (fig. 44, B), but more usually as a solid mass of ectoderm-cells (fig. 45, A). The ectodermal ingrowth is the entocodon (Gc.); it bulges into, and pushes down, the endoderm at the apex of the bud, and if solid it soon acquires a cavity (fig. 44, C,s.c.). The cavity of the entocodon increases continually in size, while the endoderm pushes up at the sides of it to form a cup with hollow walls, enclosing but not quite surrounding the entocodon, which remains in contact at its outer side with the ectoderm covering the bud (fig. 44, D,v). The next changes that take place are chiefly in the endoderm-cup (fig. 44, D, E); the cavity between the two walls of the cup becomes reduced by concrescence to form the radial canals (r.c.), ring-canal (c.c.), and endoderm-lamella (e.l., fig. 44, E), and at the same time the base of the cup is thrust upwards to form the manubrium (m), converting the cavity of the entocodon into aspace which is crescentic or horse-shoe-like in section. Next tentacles (t, fig. 44, F) grow out from the ring-canal, and the double plate of ectoderm on the distal side of the entocodon becomes perforated, leaving a circular rim composed of two layers of ectoderm, the velum (v) of the medusa. Finally, a mouth is formed by breaking through at the apex of the manubrium, and the now fully-formed medusa becomes separated by rupture of the stalk of the bud and swims away.

I, Ideally primitive method of budding, in which the mouth is formed first (Ia), next the tentacles (Ib), and lastly the umbrella.

II, Method. ofCunina; (a) the mouth arises, next the umbrella (b), and lastly the tentacles (c).

III, Hypothetical transition from II to the indirect method with an entocodon; the formation of the manubrium is retarded, that of the umbrella hastened (IIIa,b).

IV,a,b,c, budding with an entocodon (cf. fig. 44).

V, Budding with a solid entocodon (cf. fig. 45).

If the bud, however, is destined to give rise not to a free medusa, but to a gonophore, the development is similar but becomes arrested at various points, according to the degree to which the gonophore is degenerate. The entocodon is usually formed, proving the medusoid nature of the bud, but in sporosacs the entocodon may be rudimentary or absent altogether. The process of budding as above described may be varied or complicated in various ways; thus a secondary, amnion-like, ectodermal covering or ectotheca (fig. 45, C,ect.) may be formed over all, as inGarveia, &c.; or the entocodon may remain solid and without cavity until after the formation of the manubrium, or may never acquire a cavity at all, as described above for the gonophores.

Phylogenetic Significance of the Entocodon.—It is seen from the foregoing account of medusa-budding that the entocodon is a very important constituent of the bud, furnishing some of the most essential portions of the medusa; its cavity becomes the sub-umbral cavity, and its lining furnishes the ectodermal epithelium of the manubrium and of the sub-umbral cavity as far as the edge of the velum. Hence the entocodon represents a precocious formation of the sub-umbral surface, equivalent to the peristome of the polyp, differentiated in the bud prior to other portions of the organism which must be regarded as antecedent to it in phylogeny.

If the three principal organ-systems of the medusa, namely mouth, tentacles and umbrella, be considered in the light of phylogeny, it is evident that the manubrium bearing the mouth must be the oldest, as representing a common property of all the Coelentera, even of the gastrula embryo of all Enterozoa. Next in order come the tentacles, common to all Cnidaria. The special property of the medusa is the umbrella, distinguishing the medusa at once from other morphological types among the Coelentera. If, therefore, the formation of these three systems of organs took place according to a strictly phylogenetic sequence, we should expect them to appear in the order set forth above (fig. 46, Ia,b,c). The nearest approach to the phylogenetic sequence is seen in the budding ofCunina, where the manubrium and mouth appear first, but the umbrella is formed before the tentacles (fig. 46, IIa,b,c). In the indirect or coenogenetic method of budding, the first two members of the sequence exhibited by Cunina change places, and the umbrella is formed first, the manubrium next, and then the tentacles; the actual mouth-perforation being delayed to the very last (fig. 46, IVa,b,c). Hence the budding of medusae exemplifies very clearly a common phenomenon in development, a phylogenetic series of events completely dislocated in the ontogenetic time-sequence.

The entocodon is to be regarded, therefore, not as primarily an ingrowth of ectoderm, but rather as an upgrowth of both body-layers, in the form of a circular rim (IVa), representing the umbrellar margin; it is comparable to the bulging that forms the umbrella in the direct method of budding, but takes place before a manubrium is formed, and is greatly reduced in size, so as to become a little pit. By a simple modification, the open pit becomes a solid ectodermal ingrowth, just as in Teleostean fishes the hollow medullary tube, or the auditory pit of other vertebrate embryos, is formed at first as a solid cord of cells, which acquires a cavity secondarily. Moreover, the entocodon, however developed, gives rise at first to a closed cavity, representing a closing over of the umbrella, temporary in the bud destined to be a free medusa, but usually permanent in the sessile gonophore. As has been shown above, the closing up of the sub-umbral cavity is one of the earliest degenerative changes in the evolution of the gonophore, and we may regard it as the umbrellar fold taking on a protective function, either temporarily for the bud or permanently for the gonophore.

To sum up, the entocodon is a precocious formation of the umbrella, closing over to protect the organs in the umbrellar cavity. The possession of an entocodon proves the medusa-nature of the bud, and can only be explained on the theory that gonophores are degenerate medusae, and is inexplicable on the opposed view that medusae are derived from gonophores secondarily set free. In the sporosac, however, the medusa-individual has become so degenerate that even the documentary proof, so to speak, of its medusoid nature may have been destroyed, and only circumstantial evidence of its nature can be produced.

4.Germinal Budding.—This method of budding is commonly described as budding from a single body-layer, instead of from both layers. The layer that produces the bud is invariably the ectoderm,i.e.the layer in which, in Hydromedusae, the generative cells are lodged; and in some cases the buds are produced in the exact spot in which later the gonads appear. From these facts, and from those of the sporogony, to be described below, we may regard budding to this type as taking place from the germinal epithelium rather than from ordinary ectoderm.

(a)The Polyp.—Budding from the ectoderm alone has been described by A. Lang [29] inHydraand other polyps. The tissues of the bud become differentiated into ectoderm and endoderm, and the endoderm of the bud becomes secondarily continuous with that of the parent, but no part of the parental endoderm contributes to the building up of the daughter-polyp. Lang regarded this method of budding as universal in polyps, a notion disproved by O. Seeliger [52] who went to the opposite extreme and regarded the type of budding described by Lang as non-existent. In view, however, both of the statements and figures of Lang and of the facts to be described presently for medusae (Margellium), it is at least theoretically possible that both germinal and vegetative budding may occur in polyps as well as in medusae.(b)The Medusa.—The clearest instance of germinal budding is furnished byMargellium (Rathkea) octopunctatum, one of theMargelidae. The budding of this medusa has been worked out in detail by Chun (Hydrozoa, [1]), to whom the reader must be referred for the interesting laws of budding regulating the sequence and order of formation of the buds.The buds ofMargelliumare produced on the manubrium in each of the four interradii, and they arise from the ectoderm, that is to say, the germinal epithelium, which later gives rise to the gonads. The buds do not appear simultaneously but successively on each of the four sides of the manubrium, thus:and secondary buds may be produced on the medusa-buds before the latter are set free as medusae. Each bud arises as a thickening of the epithelium, which first forms two or three layers (fig. 47, A), and becomes separated into a superficial layer, future ectoderm, surrounding a central mass, future endoderm (fig. 47, B). The ectodermal epithelium on the distal side of the bud becomes thickened, grows inwards, and forms a typical entocodon (fig. 37, D, E, F). The remaining development of the bud is just as described above for the indirect method of medusa-budding (fig. 47, G, H). When the bud is nearly complete, the body-wall of the parent immediately below it becomes perforated, placing the coelenteric cavity of the parent in secondary communication with that of the bud (H), doubtless for the better nutrition of the latter.Especially noteworthy in the germinal budding ofMargelliumis the formation of the entocodon, as in the vegetative budding of the indirect type.

(b)The Medusa.—The clearest instance of germinal budding is furnished byMargellium (Rathkea) octopunctatum, one of theMargelidae. The budding of this medusa has been worked out in detail by Chun (Hydrozoa, [1]), to whom the reader must be referred for the interesting laws of budding regulating the sequence and order of formation of the buds.

The buds ofMargelliumare produced on the manubrium in each of the four interradii, and they arise from the ectoderm, that is to say, the germinal epithelium, which later gives rise to the gonads. The buds do not appear simultaneously but successively on each of the four sides of the manubrium, thus:and secondary buds may be produced on the medusa-buds before the latter are set free as medusae. Each bud arises as a thickening of the epithelium, which first forms two or three layers (fig. 47, A), and becomes separated into a superficial layer, future ectoderm, surrounding a central mass, future endoderm (fig. 47, B). The ectodermal epithelium on the distal side of the bud becomes thickened, grows inwards, and forms a typical entocodon (fig. 37, D, E, F). The remaining development of the bud is just as described above for the indirect method of medusa-budding (fig. 47, G, H). When the bud is nearly complete, the body-wall of the parent immediately below it becomes perforated, placing the coelenteric cavity of the parent in secondary communication with that of the bud (H), doubtless for the better nutrition of the latter.

Especially noteworthy in the germinal budding ofMargelliumis the formation of the entocodon, as in the vegetative budding of the indirect type.

5.Sporogony.—This method of reproduction has been described by E. Metchnikoff inCuninaand allied genera. In individuals either of the male or female sex, germ-cells which are quite undifferentiated and neutral in character, become amoeboid, and wander into the endoderm. They divide each into two sister-cells, one of which—the spore—becomes enveloped by the other. The spore-cell multiplies by division, while the enveloping cell is nutrient and protective. The spore cell gives rise to a “spore-larva,” which is set free in the coelenteron and grows into a medusa. Whether sporogony occurs also in the polyp or not remains to be proved.

6.Sexual Reproduction and Embryology.—The ovum of Hydromedusae is usually one of a large number of oögonia, and grows at the expense of its sister-cells. No regular follicle is formed, but the oöcyte absorbs nutriment from the remaining oögonia. InHydrathe oöcyte is a large amoeboid cell, which sends out pseudopodia amongst the oögonia and absorbs nutriment from them. When the oöcyte is full grown, the residual oögonia die off and disintegrate.

A, The epithelium becomes two-layered.

B, The lower layer forms a solid mass of cells, which (C) becomes a vesicle, the future endoderm, containing the coelenteric cavity (coel), while the outer layer furnishes the future ectoderm.

D, E, F, a thickening of the ectoderm on the distal side of the bud forms an entocodon (Gc).

G,H, Formation of the medusae.

s.c, Sub-umbral cavity.

r.c, Radial canal.

st, Stomach, which in H acquires a secondary communication with the digestive cavity of the mother.

cc, Circular canal.

v, Velum.

t, Tentacle.

The spermatogenesis and maturation and fertilization of the germ-cells present nothing out of the common and need not be described here. These processes have been studied in detail by A. Brauer [2] forHydra.

The general course of the development is described in the articleHydrozoa. We may distinguish the following series of stages: (1) ovum; (2) cleavage, leading to formation of a blastula; (3) formation of an inner mass or parenchyma, the future endoderm, by immigration or delamination, leading to the so-called parenchymula-stage; (4) formation of an archenteric cavity, the future coelenteron, by a splitting of the internal parenchyma, and of a blastopore, the future mouth, by perforation at one pole, leading to the gastrula-stage; (5) the outgrowth of tentacles round the mouth (blastopore), leading to the actinula-stage; and (6) the actinula becomes the polyp or medusa in the manner described elsewhere (see articlesHydrozoa,PolypandMedusa). This is the full, ideal development, which is always contracted or shortened to a greater or less extent. If the embryo is set free as a free-swimming, so-called planula-larva, in the blastula, parenchymula, or gastrula stage, then a free actinula stage is not found; if, on the other hand, a free actinula occurs, then there is no free planula stage.The cleavage of the ovum follows two types, both seen inTubularia(Brauer [3]). In the first, a cleavage follows each nuclear division; in the second, the nuclei multiply by division a number of times, and then the ovum divides into as many blastomeres as there are nuclei present. The result of cleavage in all cases is a typical blastula, which when set free becomes oval and develops a flagellum to each cell, but when not set free, it remains spherical in form and has no flagella.The germ-layer formation is always by immigration or delamination, never by invagination. When the blastula is oval and free-swimming the inner mass is formed by unipolar immigration from the hinder pole. When the blastula is spherical and not set free, the germ-layer formation is always multipolar, either by immigration or by delamination,i.e.by tangential division of the cells of the blastoderm, as inGeryonia, or by a mixture of immigration and delamination, as inHydra,Tubularia, &c. The blastopore is formed as a secondary perforation at one spot, in free-swimming forms at the hinder pole. Formation of archenteron and blastopore may, however, be deferred till a later stage (actinula or after).The actinula stage is usually suppressed or not set free, but it is seen inTubularia(fig. 48), where it is ambulatory, inGonionemus(Trachomedusae), and inCunina(Narcomedusae), where it is parasitic.Modified from a plate by L. Agassiz,Contributions to Nat. Hist. U.S., iv.Fig. 48.—Free Actinula ofTubularia.In Leptolinae the embryonic development culminates in a polyp, which is usually formed by fixation of a planula (parenchymula), rarely by fixation of an actinula. The planula may fix itself (1) by one end, and then becomes the hydrocaulus and hydranth, while the hydrorhiza grows out from the base; or (2) partly by one side and then gives rise to the hydrorhiza as well as to the other parts of the polyp; or (3) entirely by its side, and then forms a recumbent hydrorhiza from which a polyp appears to be budded as an upgrowth.In Trachylinae the development produces always a medusa, and there is no polyp-stage. The medusa arises direct from the actinula-stage and there is no entocodon formed, as in the budding described above.Life-cycles of the Hydromedusae.—The life-cycle of the Leptolinae consists of an alternation of generations in which non-sexual individuals, polyps, produce by budding sexual individuals, medusae, which give rise by the sexual process to the non-sexual polyps again, so completing the cycle. Hence the alternation is of the type termed metagenesis. The Leptolinae are chiefly forms belonging to the inshore fauna. The Trachylinae, on the other hand, are above all oceanic forms, and have no polyp-stage, and hence there is typically no alternation in their life-cycle. It is commonly assumed that the Trachylinae are forms which have lost the alternation of generations possessed by them ancestrally, through secondary simplification of the life-cycle. Hence the Trachylinae are termed “hypogenetic” medusae to contrast them with the metagenetic Leptolinae. The whole question has, however, been argued at length by W. K. Brooks [4], who adduces strong evidence for a contrary view, that is to say, for regarding the direct type of development seen in Trachylinae as more primitive, and the metagenesis seen in Leptolinae as a secondary complication introduced into the life-cycle by the acquisition oflarval budding. The polyp is regarded, on this view, as a form phylogenetically older than the medusa, in short, as nothing more than a sessile actinula. In Trachylinae the polyp-stage is passed over, and is represented only by the actinula as a transitory embryonic stage. In Leptolinae the actinula becomes the sessile polyp which has acquired the power of budding and producing individuals either of its own or of a higher rank; it represents a persistent larval stage and remains in a sexually immature condition as a neutral individual, sex being an attribute only of the final stage in the development, namely the medusa. The polyp of the Leptolinae has reached the limit of its individual development and is incapable of becoming itself a medusa, but only produces medusa-buds; hence a true alternation of generations is produced. In Trachylinae also the beginnings of a similar metagenesis can be found. Thus inCunina octonaria, the ovum develops into an actinula which buds daughter-actinulae; all of them, both parent and offspring, develop into medusae, so that there is no alternation of generations, but only larval multiplication. InCunina parasitica, however, the ovum develops into an actinula, which buds actinulae as before, but only the daughter-actinulae develop into medusae, while the original, parent-actinula dies off; here, therefore, larval budding has led to a true alternation of generations. InGonionemusthe actinula becomes fixed and polyp-like, and reproduces by budding, so that here also an alternation of generations may occur. In the Leptolinae we must first substitute polyp for actinula, and then a condition is found which can be compared to the case ofCunina parasiticaor Gonionemus, if we suppose that neither the parent-actinula (i.e.founder-polyp) nor its offspring by budding (polyps of the colony) have the power of becoming medusae, but only of producing medusae by budding. For further arguments and illustrations the reader must be referred to Brooks’s most interesting memoir. The whole theory is one mostintimately connected with the question of the relation between polyp and medusa, to be discussed presently. It will be seen elsewhere, however, that whatever view may be held as to the origin of metagenesis in Hydromedusae, in the case of Scyphomedusae (q.v.) no other view is possible than that the alternation of generations is the direct result of larval proliferation.To complete our survey of life-cycles in the Hydromedusae it is necessary to add a few words about the position ofHydraand its allies. If we accept the view thatHydrais a true sexual polyp, and that its gonads are not gonophores (i.e.medusa-buds) in the extreme of degeneration, then it follows from Brooks’s theory thatHydramust be descended from an archaic form in which the medusan type of organization had not yet been evolved.Hydramust, in short, be a living representative of the ancestor of which the actinula-stage is a transient reminiscence in the development of higher forms. It may be pointed out in this connexion that the fixation ofHydrais only temporary, and that the animal is able at all times to detach itself, to move to a new situation, and to fix itself again. There is no difficulty whatever in regardingHydraas bearing the same relation to the actinula-stage of other Hydromedusae that a Rotifer bears to a trochophore-larva or a fish to a tadpole.

The cleavage of the ovum follows two types, both seen inTubularia(Brauer [3]). In the first, a cleavage follows each nuclear division; in the second, the nuclei multiply by division a number of times, and then the ovum divides into as many blastomeres as there are nuclei present. The result of cleavage in all cases is a typical blastula, which when set free becomes oval and develops a flagellum to each cell, but when not set free, it remains spherical in form and has no flagella.

The germ-layer formation is always by immigration or delamination, never by invagination. When the blastula is oval and free-swimming the inner mass is formed by unipolar immigration from the hinder pole. When the blastula is spherical and not set free, the germ-layer formation is always multipolar, either by immigration or by delamination,i.e.by tangential division of the cells of the blastoderm, as inGeryonia, or by a mixture of immigration and delamination, as inHydra,Tubularia, &c. The blastopore is formed as a secondary perforation at one spot, in free-swimming forms at the hinder pole. Formation of archenteron and blastopore may, however, be deferred till a later stage (actinula or after).

The actinula stage is usually suppressed or not set free, but it is seen inTubularia(fig. 48), where it is ambulatory, inGonionemus(Trachomedusae), and inCunina(Narcomedusae), where it is parasitic.

In Leptolinae the embryonic development culminates in a polyp, which is usually formed by fixation of a planula (parenchymula), rarely by fixation of an actinula. The planula may fix itself (1) by one end, and then becomes the hydrocaulus and hydranth, while the hydrorhiza grows out from the base; or (2) partly by one side and then gives rise to the hydrorhiza as well as to the other parts of the polyp; or (3) entirely by its side, and then forms a recumbent hydrorhiza from which a polyp appears to be budded as an upgrowth.

In Trachylinae the development produces always a medusa, and there is no polyp-stage. The medusa arises direct from the actinula-stage and there is no entocodon formed, as in the budding described above.

Life-cycles of the Hydromedusae.—The life-cycle of the Leptolinae consists of an alternation of generations in which non-sexual individuals, polyps, produce by budding sexual individuals, medusae, which give rise by the sexual process to the non-sexual polyps again, so completing the cycle. Hence the alternation is of the type termed metagenesis. The Leptolinae are chiefly forms belonging to the inshore fauna. The Trachylinae, on the other hand, are above all oceanic forms, and have no polyp-stage, and hence there is typically no alternation in their life-cycle. It is commonly assumed that the Trachylinae are forms which have lost the alternation of generations possessed by them ancestrally, through secondary simplification of the life-cycle. Hence the Trachylinae are termed “hypogenetic” medusae to contrast them with the metagenetic Leptolinae. The whole question has, however, been argued at length by W. K. Brooks [4], who adduces strong evidence for a contrary view, that is to say, for regarding the direct type of development seen in Trachylinae as more primitive, and the metagenesis seen in Leptolinae as a secondary complication introduced into the life-cycle by the acquisition oflarval budding. The polyp is regarded, on this view, as a form phylogenetically older than the medusa, in short, as nothing more than a sessile actinula. In Trachylinae the polyp-stage is passed over, and is represented only by the actinula as a transitory embryonic stage. In Leptolinae the actinula becomes the sessile polyp which has acquired the power of budding and producing individuals either of its own or of a higher rank; it represents a persistent larval stage and remains in a sexually immature condition as a neutral individual, sex being an attribute only of the final stage in the development, namely the medusa. The polyp of the Leptolinae has reached the limit of its individual development and is incapable of becoming itself a medusa, but only produces medusa-buds; hence a true alternation of generations is produced. In Trachylinae also the beginnings of a similar metagenesis can be found. Thus inCunina octonaria, the ovum develops into an actinula which buds daughter-actinulae; all of them, both parent and offspring, develop into medusae, so that there is no alternation of generations, but only larval multiplication. InCunina parasitica, however, the ovum develops into an actinula, which buds actinulae as before, but only the daughter-actinulae develop into medusae, while the original, parent-actinula dies off; here, therefore, larval budding has led to a true alternation of generations. InGonionemusthe actinula becomes fixed and polyp-like, and reproduces by budding, so that here also an alternation of generations may occur. In the Leptolinae we must first substitute polyp for actinula, and then a condition is found which can be compared to the case ofCunina parasiticaor Gonionemus, if we suppose that neither the parent-actinula (i.e.founder-polyp) nor its offspring by budding (polyps of the colony) have the power of becoming medusae, but only of producing medusae by budding. For further arguments and illustrations the reader must be referred to Brooks’s most interesting memoir. The whole theory is one mostintimately connected with the question of the relation between polyp and medusa, to be discussed presently. It will be seen elsewhere, however, that whatever view may be held as to the origin of metagenesis in Hydromedusae, in the case of Scyphomedusae (q.v.) no other view is possible than that the alternation of generations is the direct result of larval proliferation.

To complete our survey of life-cycles in the Hydromedusae it is necessary to add a few words about the position ofHydraand its allies. If we accept the view thatHydrais a true sexual polyp, and that its gonads are not gonophores (i.e.medusa-buds) in the extreme of degeneration, then it follows from Brooks’s theory thatHydramust be descended from an archaic form in which the medusan type of organization had not yet been evolved.Hydramust, in short, be a living representative of the ancestor of which the actinula-stage is a transient reminiscence in the development of higher forms. It may be pointed out in this connexion that the fixation ofHydrais only temporary, and that the animal is able at all times to detach itself, to move to a new situation, and to fix itself again. There is no difficulty whatever in regardingHydraas bearing the same relation to the actinula-stage of other Hydromedusae that a Rotifer bears to a trochophore-larva or a fish to a tadpole.

The Relation of Polyp and Medusa.—Many views have been put forward as to the morphological relationship between the two types of person in the Hydromedusae. For the most part, polyp and medusa have been regarded as modifications of a common type, a view supported by the existence, among Scyphomedusae (q.v.), of sessile polyp-like medusae (Lucernaria, &c.). R. Leuckart in 1848 compared medusae in general terms to flattened polyps. G. J. Allman [1] put forward a more detailed view, which was as follows. In some polyps the tentacles are webbed at the base, and it was supposed that a medusa was a polyp of this kind set free, the umbrella being a greatly developed web or membrane extending between the tentacles. A very different theory was enunciated by E. Metchnikoff. In some hydroids the founder-polyp, developed from a planula after fixation, throws out numerous outgrowths from the base to form the hydrorhiza; these outgrowths may be radially arranged so as to form by contact or coalescence a flat plate. Mechnikov considered the plate thus formed at the base of the polyp as equivalent to the umbrella, and the body of the polyp as equivalent to the manubrium, of the medusa; on this view the marginal tentacles almost invariably present in medusae are new formations, and the tentacles of the polyp are represented in the medusa by the oral arms which may occur round the mouth, and which sometimes,e.g.inMargelidae, have the appearance and structure of tentacles. Apart from the weighty arguments which the development furnishes against the theories of Allman and Mechnikov, it may be pointed out that neither hypothesis gives a satisfactory explanation of a structure universally present in medusae of whatever class, namely the endoderm-lamella, discovered by the brothers O. and R. Hertwig. It would be necessary to regard this structure as a secondary extension of the endoderm in the tentacle-web, on Allman’s theory, or between the outgrowths of the hydrorhiza, on Mechnikov’s hypothesis. The development, on the contrary, shows unequivocally that the endoderm-lamella arises as a local coalescence of the endodermal linings of a primitively extensive gastral space.

The question is one intimately connected with the view taken as to the nature and individuality of polyp, medusa and gonophore respectively. On this point the following theories have been put forward.

1. The theory that the medusa is simply an organ, which has become detached and has acquired a certain degree of independence, like the well-known instance of the hectocotyle of the cuttle-fish. On this view, put forward by E. van Beneden and T. H. Huxley, the sporosac is the starting-point of an evolution leading up through the various types of gonophores to the free medusa as the culminating point of a phyletic series. The evidence against this view may be classed under two heads: first, comparative evidence; hydroids very different in their structural characters and widely separate in the systematic classification of these organisms may produce medusae very similar, at least so far as the essential features of medusan organization are concerned; on the other hydroids closely allied, perhaps almost indistinguishable, may produce gonophores in the one case, medusae in the other; for example,Hydractinia(gonophores) andPodocoryne(medusae),Tubularia(gonophores) andEctopleura(medusae),Coryne(gonophores) andSyncoryne(medusae), and so on. If it is assumed that all these genera bore gonophores ancestrally, then medusa of similar type must have been evolved quite independently in a great number of cases. Secondly, there is the evidence from the development, namely, the presence of the entocodon in the medusa-bud, a structure which, as explained above, can only be accounted for satisfactorily by derivation from a medusan type of organization. Hence it may be concluded that the gonophores are degenerate medusae, and not that the medusae are highly elaborated gonophores, as the organ-theory requires.2. The theory that the medusa is an independent individual, fully equivalent to the polyp in this respect, is now universally accepted as being supported by all the facts of comparative morphology and development. The question still remains open, however, which of the two types of person may be regarded as the most primitive, the most ancient in the race-history of the Hydromedusae. F. M. Balfour put forward the view that the polyp was the more primitive type, and that the medusa is a special modification of the polyp for reproductive purposes, the result of division of labour in a polyp-colony, whereby special reproductive persons become detached and acquire organs of locomotion for spreading the species. W. K. Brooks, on the other hand, as stated above, regards the medusa as the older type and looks upon both polyp and medusa, in the Hydromedusae, as derived from a free-swimming or floating actinula, the polyp being thus merely a fixed nutritive stage, possessing secondarily acquired powers of multiplication by budding.The Hertwigs when they discovered the endoderm-lamella showed on morphological grounds that polyp and medusa are independent types, each produced by modification in different directions of a more primitive type represented in development by the actinula-stage. If a polyp, such asHydra, be regarded simply as a sessile actinula, we must certainly consider the polyp to be the older type, and it may be pointed out that in the Anthozoa only polyp-individuals occur. This must not be taken to mean, however, that the medusa is derived from a sessile polyp; it must be regarded as a direct modification of the more ancient free actinula form, without primitively any intervening polyp-stage, such as has been introduced secondarily into the development of the Leptolinae and represents a revival, so to speak, of an ancestral form or larval stage, which has taken on a special role in the economy of the species.

2. The theory that the medusa is an independent individual, fully equivalent to the polyp in this respect, is now universally accepted as being supported by all the facts of comparative morphology and development. The question still remains open, however, which of the two types of person may be regarded as the most primitive, the most ancient in the race-history of the Hydromedusae. F. M. Balfour put forward the view that the polyp was the more primitive type, and that the medusa is a special modification of the polyp for reproductive purposes, the result of division of labour in a polyp-colony, whereby special reproductive persons become detached and acquire organs of locomotion for spreading the species. W. K. Brooks, on the other hand, as stated above, regards the medusa as the older type and looks upon both polyp and medusa, in the Hydromedusae, as derived from a free-swimming or floating actinula, the polyp being thus merely a fixed nutritive stage, possessing secondarily acquired powers of multiplication by budding.

The Hertwigs when they discovered the endoderm-lamella showed on morphological grounds that polyp and medusa are independent types, each produced by modification in different directions of a more primitive type represented in development by the actinula-stage. If a polyp, such asHydra, be regarded simply as a sessile actinula, we must certainly consider the polyp to be the older type, and it may be pointed out that in the Anthozoa only polyp-individuals occur. This must not be taken to mean, however, that the medusa is derived from a sessile polyp; it must be regarded as a direct modification of the more ancient free actinula form, without primitively any intervening polyp-stage, such as has been introduced secondarily into the development of the Leptolinae and represents a revival, so to speak, of an ancestral form or larval stage, which has taken on a special role in the economy of the species.

Systematic Review of the Hydromedusae

Order I.Eleutheroblastea.—Simple polyps which become sexually mature and which also reproduce non-sexually, but without any medusoid stage in the life-cycle.

The sub-order includes the familyHydridae, containing the common fresh-water polyps of the genusHydra. Certain other forms of doubtful affinities have also been referred provisionally to this section.

Hydra.—This genus comprises fresh-water polyps of simple structure. The body bears tentacles, but shows no division into hydrorhiza, hydrocaulus or hydranth; it is temporarily fixed and has no perisarc. The polyp is usually hermaphrodite, developing both ovaries and testes in the same individual. There is no free-swimming planula larva, but the stage corresponding to it is passed over in an enveloping cyst, which is secreted round the embryo by its own ectodermal layer, shortly after the germ-layer formation is complete,i.e.in the parenchymula-stage. The envelope is double, consisting of an external chitinous stratified shell, and an internal thin elastic membrane. Protected by the double envelope, the embryo is set free as a so-called “egg,” and in Europe it passes the winter in this condition. In the spring the embryo bursts its shell and is set free as a minute actinula which becomes aHydra.Many species are known, of which three are common in European waters. It has been shown by C. F. Jickeli (28) that the species are distinguishable by the characters of their nematocysts. They also show characteristic differences in the egg (Brauer [2]). InHydra viridisthe polyp is of a green colour and produces a spherical egg with a smooth shell which is dropped into the mud.H. griseais greyish in tint and produces a spherical egg with a spiky shell, which also is dropped into the mud.H. fusca(=H. vulgaris) is brown in colour, and produces a bun-shaped egg, spiky on the convex surface, and attached to a water-weed or some object by its flattened side. Brauer found a fourth species, similar in appearance toH. fusca, but differing from the three other species in being of separate sexes, and in producing a spherical egg with a knobby shell, which is attached like that ofH. fusca.The fact already noted that the species ofHydracan be distinguished by the characters of their nematocysts is a point of great interest. In each species, two or three kinds of nematocysts occur, some large, some small, and for specific identification the nematocysts must be studied collectively in each species. It is very remarkable that this method of characterizing and diagnozing species has never been extended to the marine hydroids. It is quite possible that the characters of the nematocysts might afford data as useful to the systematist in this group as do the spicules of sponges, for instance. It would be particularly interesting to ascertain how the nematocysts of a polyp are related to those possessed by the medusa budded from it, and it is possible that in this manner obscure questions of relationship might be cleared up.Fig. 49.—Diagram showing possible modifications of persons of a gymnoblasticHydromedusa. (After Allman.)a, Hydrocaulus (stem).b, Hydrorhiza (root).c, Enteric cavity.d, Endoderm.e, Ectoderm.f, Perisarc, (horny case).g, Hydranth (hydriform person) expanded.g′, Hydranth (hydriform person) contracted.h, Hypostome, bearing mouth at its extremity.k, Sporosac springing from the hydrocaulus.k′, Sporosac springing from m, a modified hydriform person (blastostyle): the genitalia are seen surrounding the spadix or manubrium.l, Medusiform person or medusa.m, Blastostyle.Protohydrais a marine genus characterized by the absence of tentacles, by a great similarity toHydrain histological structure, and by reproduction by transverse fission. It was found originally in an oyster-farm at Ostend. The sexual reproduction is unknown. For further information see C. Chun (Hydrozoa[1]. Pl. I.).Polypodium hydriformeUssow is a fresh-water form parasitic on the eggs of the sterlet. A “stolon” of unknown origin produces thirty-two buds, which become as manyPolypodia; each has twenty-four tentacles and divides by fission repeated twice into four individuals, each with six tentacles. The daughter-individuals grow, form the full number of twenty-four tentacles and divide again. The polyps are free and walk on their tentacles. See Ussow [54].Tetraplatia volitansViguier is a remarkable floating marine form. See C. Viguier [56] and Delage and Hérouard (Hydrozoa [2]).HaleremitaSchaudinn. See F. Schaudinn [50] and Delage and Hérouard (Hydrozoa[2]).In all the above-mentioned genera, with the exception ofHydra, the life-cycle is so imperfectly known that their true position cannot be determined in the present state of our knowledge. They may prove eventually to belong to other orders. Hence only the genusHydracan be considered as truly representing the order Eleutheroblastea. The phylogenetic position of this genus has been discussed above.

Many species are known, of which three are common in European waters. It has been shown by C. F. Jickeli (28) that the species are distinguishable by the characters of their nematocysts. They also show characteristic differences in the egg (Brauer [2]). InHydra viridisthe polyp is of a green colour and produces a spherical egg with a smooth shell which is dropped into the mud.H. griseais greyish in tint and produces a spherical egg with a spiky shell, which also is dropped into the mud.H. fusca(=H. vulgaris) is brown in colour, and produces a bun-shaped egg, spiky on the convex surface, and attached to a water-weed or some object by its flattened side. Brauer found a fourth species, similar in appearance toH. fusca, but differing from the three other species in being of separate sexes, and in producing a spherical egg with a knobby shell, which is attached like that ofH. fusca.

The fact already noted that the species ofHydracan be distinguished by the characters of their nematocysts is a point of great interest. In each species, two or three kinds of nematocysts occur, some large, some small, and for specific identification the nematocysts must be studied collectively in each species. It is very remarkable that this method of characterizing and diagnozing species has never been extended to the marine hydroids. It is quite possible that the characters of the nematocysts might afford data as useful to the systematist in this group as do the spicules of sponges, for instance. It would be particularly interesting to ascertain how the nematocysts of a polyp are related to those possessed by the medusa budded from it, and it is possible that in this manner obscure questions of relationship might be cleared up.

a, Hydrocaulus (stem).

b, Hydrorhiza (root).

c, Enteric cavity.

d, Endoderm.

e, Ectoderm.

f, Perisarc, (horny case).

g, Hydranth (hydriform person) expanded.

g′, Hydranth (hydriform person) contracted.

h, Hypostome, bearing mouth at its extremity.

k, Sporosac springing from the hydrocaulus.

k′, Sporosac springing from m, a modified hydriform person (blastostyle): the genitalia are seen surrounding the spadix or manubrium.

l, Medusiform person or medusa.

m, Blastostyle.

Protohydrais a marine genus characterized by the absence of tentacles, by a great similarity toHydrain histological structure, and by reproduction by transverse fission. It was found originally in an oyster-farm at Ostend. The sexual reproduction is unknown. For further information see C. Chun (Hydrozoa[1]. Pl. I.).

Polypodium hydriformeUssow is a fresh-water form parasitic on the eggs of the sterlet. A “stolon” of unknown origin produces thirty-two buds, which become as manyPolypodia; each has twenty-four tentacles and divides by fission repeated twice into four individuals, each with six tentacles. The daughter-individuals grow, form the full number of twenty-four tentacles and divide again. The polyps are free and walk on their tentacles. See Ussow [54].

Tetraplatia volitansViguier is a remarkable floating marine form. See C. Viguier [56] and Delage and Hérouard (Hydrozoa [2]).

HaleremitaSchaudinn. See F. Schaudinn [50] and Delage and Hérouard (Hydrozoa[2]).

In all the above-mentioned genera, with the exception ofHydra, the life-cycle is so imperfectly known that their true position cannot be determined in the present state of our knowledge. They may prove eventually to belong to other orders. Hence only the genusHydracan be considered as truly representing the order Eleutheroblastea. The phylogenetic position of this genus has been discussed above.

Order II.Hydroidea seu Leptolinae.—Hydromedusae with alternation of generations (metagenesis) in which a non-sexual polyp-generation (trophosome) produces by budding a sexual medusa-generation (gonosome). The polyp may be solitary, but more usually produces polyps by budding and forms a polyp-colony. The polyp usually has the body distinctly divisible into hydranth, hydrocaulus and hydrorhiza, and is usually clothed in a perisarc. The medusae may be set free or may remain attached to the polyp-colony and degenerate into a gonophore. When fully developed the medusa is characterized by the sense organs being composed entirely of ectoderm, developed independently of the tentacles, and innervated from the sub-umbral nerve-ring.

The two kinds of persons present in the typical Hydroidea make the classification of the group extremely difficult, for reasons explained above. Hence the systematic arrangement that follows must be considered purely provisional. A natural classification of the Hydroidea has yet to be put forward. Many genera and families are separated by purely artificial characters, mere shelf-and-bottle groupings devised, for the convenience of the museum curator and the collector. Thus many subdivisions are diagnosed by setting free medusae in one case, or producing gonophores in another, although it is very obvious, as pointed out above, that a genus producing medusae may be far more closely allied to one producing gonophores than to another producing medusae, or vice versa, and that in some cases the production of medusae or gonophores varies with the season or the sex. Moreover, P. Hallez [22] has recently shown that hydroids hitherto regarded as distinct species are only forms of the same species grown under different conditions.

Sub-Order 1. Hydroidea Gymnoblastea (Anthomedusae).—Trophosome without hydrothecae or gonothecae, with monopodial type of budding. Gonosome with free medusae or gonophores; medusae usually with ocelli, never with otocysts. The gymnoblastic polyp usually has a distinct perisarc investing the hydrorhiza and the hydrocaulus, sometimes also the hydranth as far as the bases of the tentacles (Bimeria); but in such cases the perisarc forms a closely-fitting investment or cuticule on the hydranth, never a hydrotheca standing off from it, as in the next sub-order. The polyps may be solitary, or form colonies, which may be of the spreading or encrusting type, or arborescent, and then always of monopodial growth and budding. In some cases, any polyp of the colony may bud medusae; in other cases, only certain polyps, the blastostyles, have this power. When blastostyles are present, however, they are never enclosed in special gonothecae as in the next sub-order. In this sub-order the characters of the hydranth are very variable, probably owing to the fact that it is exposed and not protected by a hydrotheca, as in Calyptoblastea.

Speaking generally, three principal types of hydranth can be distinguished, each with subordinate varieties of form.1. Club-shaped hydranths with numerous tentacles, generally scattered irregularly, sometimes with a spiral arrangement, or in whorls (“verticillate”).(a) Tentacles filiform; type ofClava(fig. 5),Cordylophora, &c.(b) Tentacles capitate, simple; type ofCoryneandSyncoryne;Myriothelais an aberrant form with some of the tentacles modified as “claspers” to hold the ova.(c) Tentacles capitate, branched, wholly or in part; type ofCladocoryne.(d) Tentacles filiform or capitate, tending to be arranged in definite whorls; type ofStauridium(fig. 2),CladonemaandPennaria.2. Hydranth more shortened, daisy-like in form, with two whorls of tentacles, oral and aboral.(a) Tentacles filiform, simple, radially arranged or scattered irregularly; type ofTubularia(fig. 4),Corymorpha(fig. 3),Nemopsis,Pelagohydra, &c.(b) Tentacles with a bilateral arrangement, branched tentacles in addition to simple filiform ones; type ofBranchiocerianthus.3. Hydranth with a single circlet of tentacles.(a) With filiform tentacles; the commonest type, seen inBougainvillea(fig. 13),Eudendrium, &c.(b) With capitate tentacles; type ofClavatella.4. Hydranth with tentacles reduced below four; type ofLar(fig. 11),Monobrachium, &c.

Speaking generally, three principal types of hydranth can be distinguished, each with subordinate varieties of form.

1. Club-shaped hydranths with numerous tentacles, generally scattered irregularly, sometimes with a spiral arrangement, or in whorls (“verticillate”).

(a) Tentacles filiform; type ofClava(fig. 5),Cordylophora, &c.(b) Tentacles capitate, simple; type ofCoryneandSyncoryne;Myriothelais an aberrant form with some of the tentacles modified as “claspers” to hold the ova.(c) Tentacles capitate, branched, wholly or in part; type ofCladocoryne.(d) Tentacles filiform or capitate, tending to be arranged in definite whorls; type ofStauridium(fig. 2),CladonemaandPennaria.

(a) Tentacles filiform; type ofClava(fig. 5),Cordylophora, &c.

(b) Tentacles capitate, simple; type ofCoryneandSyncoryne;Myriothelais an aberrant form with some of the tentacles modified as “claspers” to hold the ova.

(d) Tentacles filiform or capitate, tending to be arranged in definite whorls; type ofStauridium(fig. 2),CladonemaandPennaria.

2. Hydranth more shortened, daisy-like in form, with two whorls of tentacles, oral and aboral.

(a) Tentacles filiform, simple, radially arranged or scattered irregularly; type ofTubularia(fig. 4),Corymorpha(fig. 3),Nemopsis,Pelagohydra, &c.(b) Tentacles with a bilateral arrangement, branched tentacles in addition to simple filiform ones; type ofBranchiocerianthus.

(a) Tentacles filiform, simple, radially arranged or scattered irregularly; type ofTubularia(fig. 4),Corymorpha(fig. 3),Nemopsis,Pelagohydra, &c.

(b) Tentacles with a bilateral arrangement, branched tentacles in addition to simple filiform ones; type ofBranchiocerianthus.

3. Hydranth with a single circlet of tentacles.

(a) With filiform tentacles; the commonest type, seen inBougainvillea(fig. 13),Eudendrium, &c.(b) With capitate tentacles; type ofClavatella.

(a) With filiform tentacles; the commonest type, seen inBougainvillea(fig. 13),Eudendrium, &c.

(b) With capitate tentacles; type ofClavatella.

4. Hydranth with tentacles reduced below four; type ofLar(fig. 11),Monobrachium, &c.

TheAnthomedusain form is generally deep, bell-shaped. The sense organs are typically ocelli, never otocysts. The gonads are borne on the manubrium, either forming a continuous ring (Codonid type), or four masses or pairs of masses (Oceanid type). The tentacles may be scattered singly round the margin of the umbrella (“monerenematous”) or arranged in tufts (“lophonematous”); in form they may be simple or branched (Cladonemid type); in structure they may be hollow (“coelomerinthous”); or solid (“pycnomerinthous”). When sessile gonophores are produced, they may show all stages of degeneration.

Classification.—Until quite recently the hydroids (Gymnoblastea) and the medusae (Anthomedusae) have been classified separately, since the connexion between them was insufficiently known. Delage and Hérouard (Hydrozoa[2]) were the first to make an heroic attempt to unite the two classifications into one, to which Hickson (Hydrozoa[4]) has made some additions and slight modifications. The classification given here is for the most part that of Delage and Hérouard. It is certain, however, that no such classification can be considered final at present, but must undergo continual revision in the future. With this reservation we may recognize fifteen well-characterized families and others of more doubtful nature. Certain discrepancies must also be noted.1.Margelidae(= medusa-familyMargelidae+ hydroid familiesBougainvillidae,Dicorynidae,BimeridaeandEudendridae). Trophosome arborescent, with hydranths ofBougainvillea-type; gonosome free medusae or gonophores, the medusae with solid tentacles in tufts (lophonematous). Common genera are the hydroidBougainvillea(figs. 12, 13), and the medusaeHippocrene(budded fromBougainvillea),Margelis,Rathkea(fig. 24), andMargellium. Other hydroids areGarveia,Bimeria,EudendriumandHeterocordyle, with gonophores, andDicorynewith peculiar sporosacs.After Haeckel,System der Medusen, by permission of Gustav Fischer.Fig. 52.—Tiara pileata, L. Agassiz.2.Podocorynidae(= medusa-familiesThamnostomidaeandCytaeidae+ hydroid familiesPodocorynidaeandHydractiniidae). Trophosome encrusting with hydranths ofBougainvillea-type, polyps differentiated into blastostyles, gastrozoids and dactylozoids; gonosome free medusae or gonophores. The typical genus is the well-known hydroidPodocoryne, budding the medusa known asDysmorphosa;Thamnostylus,Cytaeis, &c., are other medusae with unknown hydroids.Hydractinia(figs. 9, 10) is a familiar hydroid genus, bearing gonophores.3.Cladonemidae.—Trophosome, polyps with two whorls of tentacles, the lower filiform, the upper capitate; gonosome, free medusae, with tentacles solid and branched. The type-genusCladonema(fig. 20) is a common British form.4.Clavatellidae.—Trophosome, polyps with a single whorl of capitate tentacles; gonosome, free medusae, with tentacles branched, solid.Clavatella(fig. 21), with a peculiar ambulatory medusa is a British form.5.Pennariidae.—Trophosome, polyps with an upper circlet of numerous capitate tentacles, and a lower circlet of filiform tentacles.Pennaria, with a free medusa known asGlobiceps, is a common Mediterranean form.Stauridium(fig. 2) is a British hydroid.6.Tubulariidae.—Trophosome, polyps with two whorls of tentacles, both filiform.Tubularia(fig. 4), a well-known British hydroid, bears gonophores.7.Corymorphidae(including the medusa-familyHybocodonidae).—Trophosome solitary polyps, with two whorls of tentacles; gonosome, free medusae or gonophores.Corymorpha(fig. 3), a well-known British genus, sets free a medusa known asSteenstrupia(fig. 22). Here belong the deep-sea generaMonocaulusandBranchiocerianthus, including the largest hydroid polyps known, both genera producing sessile gonophores.After Haeckel,System der Medusen, by permission of Gustav Fischer.Fig. 53.—Pteronema darwinii. The apex of the stomach is prolonged into a brood pouch containing embryos.8.Dendroclavidae.—Trophosome, polyp with filiform tentacles in three or four whorls.Dendroclava, a hydroid, produces the medusa known asTurritopsis.9.Clavidae(including the medusa-familyTiaridae(figs. 27 and 51). Trophosome, polyps with scattered filiform tentacles; gonosome, medusae or gonophores, the medusae with hollow tentacles.Clava(fig. 5), a common British hydroid, produces gonophores; so also doesCordylophora, a form inhabiting fresh or brackish water.Turrisproduces free medusae.Amphinemais a medusan genus of unknown hydroid.10.Bythotiaridae.—Trophosome unknown; gonosome, free medusae, with deep, bell-shaped umbrella, with interradial gonads on the base of the stomach, with branched radial canals, and correspondingly numerous hollow tentacles.Bythotiara,Sibogita.11.Corynidae(= hydroid familiesCorynidae,SyncorynidaeandCladocorynidae+ medusan familySarsiidae).—Trophosome polyps with capitate tentacles, simple or branched, scattered or verticillate; gonosome, free medusae or gonophores.Coryne, a common British hydroid, produces gonophores;Syncoryne, indistinguishable from it, produces medusae known asSarsia(fig. 51).Cladocoryneis another hydroid genus;CodoniumandDipurena(fig. 50) are medusan genera.12.Myriothelidae.—The genusMyriothelais a solitary polyp with scattered capitate tentacles, producing sporosacs.13.Hydrolaridae.—Trophosome (only known in one genus), polyps with two tentacles forming a creeping colony; gonosome, free medusae with four, six or more radial canals, giving off one or more lateral branches which run to the margin of the umbrella, with the stomach produced into four, six or more lobes, upon which the gonads are developed; the mouth with four lips or with a folded margin; the tentacles simple, arranged evenly round the margin of the umbrella. The remarkable hydroidLar(fig. 11) grows upon the tubes of the wormSabellaand produces a medusa known asWillia. Another medusan genus isProboscidactyla.14.Monobrachiidae.—The genusMonobrachiumis a colony-forming hydroid which grows upon the shells of bivalve molluscs, each polyp having but a single tentacle. It buds medusae, which, however, are as yet only known in an immature condition (C. Mereschkowsky [41]).15.Ceratellidae.—Trophosome polyps forming branching colonies of which the stem and main branches are thick and composed of a network of anastomosing coenosarcal tubes covered by a common ectoderm and supported by a thick chitinous perisarc; hydranths similar to those ofCoryne; gonosome, sessile gonophores.Ceratella, an exotic genus from the coast of East Africa, New South Wales and Japan. The generaDehitellaGray andDendrocoryneInaba should perhaps be referred to this family; the last-named is regarded by S. Goto [16] as the type of a distinct family,Dendrocorynidae.Doubtful families, or forms difficult to classify, are: Pteronemidae, Medusae of Cladonemid type, with hydroids for the most part unknown. The British genusGemmaria, however, is budded from a hydroid referable to the familyCorynidae.Pteronema(fig. 53).Nemopsidae, for the floating polypNemopsis, very similar toTubulariain character; the medusa, on the other hand, is very similar toHippocrene(Margelidae). See C. Chun (Hydrozoa[1]).Pelagohydridae, for the floating polypPelagohydra, Dendy, from New Zealand. The animal is a solitary polyp bearing a great number of medusa-buds. The body, representing the hydranth of an ordinary hydroid, has the aboral portion modified into a float, from which hangs down a proboscis bearing the mouth. The float is covered with long tentacles and bears the medusa-buds. The proboscis bears at its extremity a circlet of smaller oral tentacles. Thus the affinities of the hydranth are clearly, as Dendy points out,with a form such asCorymorpha, which also is not fixed but only rooted in the mud. The medusae, on the other hand, have the tentacles in four tufts of (in the buds) five each, and thus resemble the medusae of the familyMargelidae. See A. Dendy [12].Fig. 54.—Diagram showing possible modifications of the persons of a Calyptoblastic Hydromedusa. Lettersatohsame as in fig. 49.i, The horny cup or hydrotheca of the hydriform persons;l, medusiform person springing fromm, a modified, hydriform person (blastostyle);n, the horny case or gonangium enclosing the blastostyle and its buds. This and the hydrothecaigive origin to the nameCalyptoblastea. (After Allman.)Perigonimus.—This common British hydroid belongs by its characters to the familyBougainvillidae; it produces, however, a medusa of the genusTiara(fig. 52), referable to the familyClavidae; a fact sufficient to indicate the tentative character of even the most modern classifications of this order.

1.Margelidae(= medusa-familyMargelidae+ hydroid familiesBougainvillidae,Dicorynidae,BimeridaeandEudendridae). Trophosome arborescent, with hydranths ofBougainvillea-type; gonosome free medusae or gonophores, the medusae with solid tentacles in tufts (lophonematous). Common genera are the hydroidBougainvillea(figs. 12, 13), and the medusaeHippocrene(budded fromBougainvillea),Margelis,Rathkea(fig. 24), andMargellium. Other hydroids areGarveia,Bimeria,EudendriumandHeterocordyle, with gonophores, andDicorynewith peculiar sporosacs.

2.Podocorynidae(= medusa-familiesThamnostomidaeandCytaeidae+ hydroid familiesPodocorynidaeandHydractiniidae). Trophosome encrusting with hydranths ofBougainvillea-type, polyps differentiated into blastostyles, gastrozoids and dactylozoids; gonosome free medusae or gonophores. The typical genus is the well-known hydroidPodocoryne, budding the medusa known asDysmorphosa;Thamnostylus,Cytaeis, &c., are other medusae with unknown hydroids.Hydractinia(figs. 9, 10) is a familiar hydroid genus, bearing gonophores.

3.Cladonemidae.—Trophosome, polyps with two whorls of tentacles, the lower filiform, the upper capitate; gonosome, free medusae, with tentacles solid and branched. The type-genusCladonema(fig. 20) is a common British form.

4.Clavatellidae.—Trophosome, polyps with a single whorl of capitate tentacles; gonosome, free medusae, with tentacles branched, solid.Clavatella(fig. 21), with a peculiar ambulatory medusa is a British form.

5.Pennariidae.—Trophosome, polyps with an upper circlet of numerous capitate tentacles, and a lower circlet of filiform tentacles.Pennaria, with a free medusa known asGlobiceps, is a common Mediterranean form.Stauridium(fig. 2) is a British hydroid.

6.Tubulariidae.—Trophosome, polyps with two whorls of tentacles, both filiform.Tubularia(fig. 4), a well-known British hydroid, bears gonophores.

7.Corymorphidae(including the medusa-familyHybocodonidae).—Trophosome solitary polyps, with two whorls of tentacles; gonosome, free medusae or gonophores.Corymorpha(fig. 3), a well-known British genus, sets free a medusa known asSteenstrupia(fig. 22). Here belong the deep-sea generaMonocaulusandBranchiocerianthus, including the largest hydroid polyps known, both genera producing sessile gonophores.

8.Dendroclavidae.—Trophosome, polyp with filiform tentacles in three or four whorls.Dendroclava, a hydroid, produces the medusa known asTurritopsis.

9.Clavidae(including the medusa-familyTiaridae(figs. 27 and 51). Trophosome, polyps with scattered filiform tentacles; gonosome, medusae or gonophores, the medusae with hollow tentacles.Clava(fig. 5), a common British hydroid, produces gonophores; so also doesCordylophora, a form inhabiting fresh or brackish water.Turrisproduces free medusae.Amphinemais a medusan genus of unknown hydroid.

10.Bythotiaridae.—Trophosome unknown; gonosome, free medusae, with deep, bell-shaped umbrella, with interradial gonads on the base of the stomach, with branched radial canals, and correspondingly numerous hollow tentacles.Bythotiara,Sibogita.

11.Corynidae(= hydroid familiesCorynidae,SyncorynidaeandCladocorynidae+ medusan familySarsiidae).—Trophosome polyps with capitate tentacles, simple or branched, scattered or verticillate; gonosome, free medusae or gonophores.Coryne, a common British hydroid, produces gonophores;Syncoryne, indistinguishable from it, produces medusae known asSarsia(fig. 51).Cladocoryneis another hydroid genus;CodoniumandDipurena(fig. 50) are medusan genera.

12.Myriothelidae.—The genusMyriothelais a solitary polyp with scattered capitate tentacles, producing sporosacs.

13.Hydrolaridae.—Trophosome (only known in one genus), polyps with two tentacles forming a creeping colony; gonosome, free medusae with four, six or more radial canals, giving off one or more lateral branches which run to the margin of the umbrella, with the stomach produced into four, six or more lobes, upon which the gonads are developed; the mouth with four lips or with a folded margin; the tentacles simple, arranged evenly round the margin of the umbrella. The remarkable hydroidLar(fig. 11) grows upon the tubes of the wormSabellaand produces a medusa known asWillia. Another medusan genus isProboscidactyla.

14.Monobrachiidae.—The genusMonobrachiumis a colony-forming hydroid which grows upon the shells of bivalve molluscs, each polyp having but a single tentacle. It buds medusae, which, however, are as yet only known in an immature condition (C. Mereschkowsky [41]).

15.Ceratellidae.—Trophosome polyps forming branching colonies of which the stem and main branches are thick and composed of a network of anastomosing coenosarcal tubes covered by a common ectoderm and supported by a thick chitinous perisarc; hydranths similar to those ofCoryne; gonosome, sessile gonophores.Ceratella, an exotic genus from the coast of East Africa, New South Wales and Japan. The generaDehitellaGray andDendrocoryneInaba should perhaps be referred to this family; the last-named is regarded by S. Goto [16] as the type of a distinct family,Dendrocorynidae.

Doubtful families, or forms difficult to classify, are: Pteronemidae, Medusae of Cladonemid type, with hydroids for the most part unknown. The British genusGemmaria, however, is budded from a hydroid referable to the familyCorynidae.Pteronema(fig. 53).

Nemopsidae, for the floating polypNemopsis, very similar toTubulariain character; the medusa, on the other hand, is very similar toHippocrene(Margelidae). See C. Chun (Hydrozoa[1]).

Pelagohydridae, for the floating polypPelagohydra, Dendy, from New Zealand. The animal is a solitary polyp bearing a great number of medusa-buds. The body, representing the hydranth of an ordinary hydroid, has the aboral portion modified into a float, from which hangs down a proboscis bearing the mouth. The float is covered with long tentacles and bears the medusa-buds. The proboscis bears at its extremity a circlet of smaller oral tentacles. Thus the affinities of the hydranth are clearly, as Dendy points out,with a form such asCorymorpha, which also is not fixed but only rooted in the mud. The medusae, on the other hand, have the tentacles in four tufts of (in the buds) five each, and thus resemble the medusae of the familyMargelidae. See A. Dendy [12].

Perigonimus.—This common British hydroid belongs by its characters to the familyBougainvillidae; it produces, however, a medusa of the genusTiara(fig. 52), referable to the familyClavidae; a fact sufficient to indicate the tentative character of even the most modern classifications of this order.

Sub-order II. Hydroidea Calyptoblastea (Leptomedusae).—Trophosome with polyps always differentiated into nutritive and reproductive individuals (blastostyles) enclosed in hydrothecae and gonothecae respectively; with sympodial type of budding. Gonosome with free medusae or gonophores; the medusae typically with otocysts, sometimes with cordyli or ocelli (figs. 54, 55).

ge, Genital glands.

M, Manubrium.

ot, Otocysts.

rc, The four radiating canals.

Ve, The velum.

The calyptoblastic polyp of the nutritive type is very uniform in character, its tendency to variation being limited, as it were, by the enclosing hydrotheca. The hydranth almost always has a single circlet of tentacles, like theBougainvillea-type, in the preceding sub-order; an exception is the curious genusClathrozoon, in which the hydranth has a single tentacle. The characteristic hydrotheca is formed by the bud at an early stage (fig. 56); when complete it is an open cup, in which the hydranth develops and can be protruded from the opening for the capture of food, or is withdrawn into it for protection. Solitary polyps are unknown in this sub-order; the colony may be creeping or arborescent in form; if the latter, the budding of the polyps, as already stated, is of the sympodial type, and either biserial, forming stems capable of further branching, or uniserial, forming pinnules not capable of further branching. In the biserial type the polyps on the two sides of the stem have primitively an alternating, zigzag arrangement; but, by a process of differential growth, quickened in the 1st, 3rd, 5th, &c., members of the stem, and retarded in the 2nd, 4th, 6th, &c., members, the polyps may assume secondarily positions opposite to one another on the two sides of the stem. Other variations in the mode of growth or budding bring about further differences in the building up of the colony, which are not in all cases properly understood and cannot be described in detail here. The stem may contain a single coenosarcal tube (“monosiphonic”) or several united in a common perisarc (“polysiphonic”). An important variation is seen, in the form of the hydrotheca itself, which may come off from the main stem by a stalk, as inObelia, or may be sessile, without a stalk, as inSertularia.

In many Calyptoblastea there occur also reduced defensive polyps or dactylozoids, which in this sub-order have received the special name ofsarcostyles. Such are the “snake-like zoids” ofOphiodesand other genera, and as such are generally interpreted the “machopolyps” of thePlumularidea. These organs are supported by cuplike structures of the perisarc, termed nematophores, regarded as modified hydrothecae supporting the specialized polyp-individuals. They are specially characteristic of the familyPlumularidae.

The medusa-buds, as already stated, are always produced from blastostyles, reduced non-nutritive polyps without mouth or tentacles. An apparent, but not real, exception isHalecium halecinum, in which the blastostyle is produced from the side of a nutritive polyp, and both are enclosed in a common theca without a partition between them (Allman [1] p. 50, fig. 24). The gonotheca is formed in its early stage in the same way as the hydrotheca, but the remains of the hydranth persists as an operculum closing the capsule, to be withdrawn when the medusae or genital products are set free (fig. 56).

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