Delamination of Geryonia ovumFig. 70. Diagrammatic figure shewing the delamination of the ovum of Geryonia.(Copied from Fol.)cs.segmentation cavity;a.endoplasm;b.ectoplasm. The dotted lines shew the course of the next planes of division.
Fig. 70. Diagrammatic figure shewing the delamination of the ovum of Geryonia.(Copied from Fol.)
cs.segmentation cavity;a.endoplasm;b.ectoplasm. The dotted lines shew the course of the next planes of division.
The development of Geryonia (Carmarina) hastata has been studied by Fol (No.155) and Metschnikoff (No.163)[77]. The ovum, when laid, is invested by a delicate vitelline membrane and mucous covering. Its protoplasm is formed of an outer granular and dense layer, and a central mass of a more spongy character. The segmentation is complete and regular, and up to the time when thirty-two segments have appeared each segment is composed of both constituents of the protoplasm of the ovum. A segmentation cavity appears when sixteen segments are formed, and becomes somewhat larger at the stage with thirty-two. At this stage the process of delamination commences. Each of the thirty-two segments, as shewn in the accompanying diagram (fig. 70), becomes divided into two unequal parts. The smaller of these is formed almost entirely of granular material; the larger contains portions of both kinds of protoplasm. In the next segmentation the thirty-two large cells only are concerned, and in each of these the line of division passes between the granular and the transparent protoplasm. The sixty-four lenticular masses of granular protoplasm thus formed constitute an outer closed epiblastic vesicle, within which thethirty-two masses of transparent protoplasm form an hypoblastic vesicle. The embryo at this stage is shewn in optical section infig. 71.
Embryo of GeryoniaFig. 71. Embryo of Geryonia after delamination. (After Fol.)ep.epiblast;hy.hypoblast.
Fig. 71. Embryo of Geryonia after delamination. (After Fol.)
ep.epiblast;hy.hypoblast.
The epiblastic vesicle now grows rapidly, while the hypoblastic vesicle remains nearly passive and becomes somewhat lens-shaped. At one point its wall comes in close contact with the epiblast. Elsewhere a wide cavity is developed between the two vesicles which becomes filled with gelatinous tissue. At this period cilia appear on the surface, and the larva becomes a planula.
The succeeding changes lead rapidly to the formation of a typical Medusa. Where the epiblast and hypoblast are in contact the former layer becomes thickened and forms a disc-shaped structure. The centre of this becomes somewhat protuberant, fuses with the hypoblast and then becomes perforated to form the mouth (fig. 72o). The edge of the disc forms a thickened ridge, the rudiment of the velum (v), which is entirely formed of epiblast. At its edge six tentacles (t) arise, into which are continued solid prolongations of the wall of the now somewhat hexagonal gastric chamber. The hypoblastic axes of the tentacles soon lose their connection with the gastric wall.
Optical section of GeryoniaFig. 72. Optical section through the oral pole of Geryonia after the appearance of the gelatinous tissue of the disc.(After Fol.)o.mouth;v.velum;t.tentacle.The shaded part represents the gelatinous tissue.
Fig. 72. Optical section through the oral pole of Geryonia after the appearance of the gelatinous tissue of the disc.(After Fol.)
o.mouth;v.velum;t.tentacle.
The shaded part represents the gelatinous tissue.
Up to this time the larva has retained a more or less spherical form, and the cavity on the under side of the umbrella has not yet become developed. The latter now becomes established by the whole disc assuming a vaulted form with the concavity directed downwards. The lining of the cavity so formed is derived from the epiblast of the disc already spoken of.
The exact mode of formation of the gastrovascular canals has not been worked out. It has however been established by the researches of theHertwigs (No.146) and Claus (No.153) that the radial and circular vessels of this system are connected together in adult Medusæ by an hypoblastic lamella; so that these canals would seem to be the remnants of an once-continuous gastric cavity. This mode of formation is established in the case of the medusiform buds; and it would therefore seem, as pointed out by the Hertwigs, a fair deduction that it occurs in the larva—a conclusion which is confirmed by the primitive extension of the gastric cavity to the edge of the disc at the time when its walls give rise to the solid axes of the tentacles. In the course of the subsequent retirement of the gastric cavity from the edge of the disc the gastrovascular canals probably take their origin, though Fol was unable to follow the changes which result in their formation.
On the completion of the above changes the larva has become a fully formed Medusa, but it undergoes a not inconsiderable metamorphosis before the attainment of the adult state.
Three-days’ larva of ÆginopsisFig. 73. A three-days’ larva of Æginopsis with two tentacles.(After Metschnikoff.)m.mouth;t.tentacle.
Fig. 73. A three-days’ larva of Æginopsis with two tentacles.(After Metschnikoff.)
m.mouth;t.tentacle.
Two species of Æginidæ have been studied by Metschnikoff (163),viz.Polyxenia leucostyla(Ægineta flavescens), andÆginopsis mediterranea. In both of these forms the segmentation results in the formation of an elongated two-layered ciliated planula, without a central cavity. The two ends of this grow out into two long processes—the rudiments of a pair of at first aborally directed arms—which contain a solid hypoblastic axis (fig. 73). At this stage the larva closely resembles the larva of Tubularia. An alimentary cavity is hollowed out in the centre of the hypoblast which soon opens by a wide oral aperture (m). A second pair of arms becomes formed, which are at first much shorter than the original pair; with their formation a radial symmetry is acquired. Sense organs become at the same timedeveloped, and the whole embryo assumes a medusiform character. Fresh tentacles arise, the velum and cavity of the umbrella become established, but these changes do not involve any points of very special interest.
Siphonophora.The development of the Siphonophora has been the subject of careful investigation by Haeckel (158) and Metschnikoff (163). The ova are large and usually (except Hippopodius) without a membrane.
They are formed of a peripheral denser layer of protoplasm and a central spongy mass. They usually undergo their entire development in the water. In some instances they have been successfully reared by artificial impregnation.
As an example of the Calycophoridæ I shall take Epibulia aurantiaca, a form allied to Diphyes, the development of which has been studied by Metschnikoff[78].
Illustration: Three larval stages of Epibulia aurantiacaFig. 74. Three larval stages of Epibulia aurantiaca.(After Metschnikoff.)A. Planula stage.B. Six-days’ larva with nectocalyx (nc) and tentacle (t).C. Somewhat older larva with gastric cavity.ep.epiblast;hy.hypoblast;so.somatocyst;nc.nectocalyx;t.tentacle;c.large yolk cells;po.polypite.
Fig. 74. Three larval stages of Epibulia aurantiaca.(After Metschnikoff.)
A. Planula stage.B. Six-days’ larva with nectocalyx (nc) and tentacle (t).C. Somewhat older larva with gastric cavity.
ep.epiblast;hy.hypoblast;so.somatocyst;nc.nectocalyx;t.tentacle;c.large yolk cells;po.polypite.
There is a regular segmentation, unaccompanied by the formation of a segmentation cavity. At its close the ovum becomes a spherical ciliated embryo. This embryo soon becomes elongated, and its cells differentiate themselves into a central and a peripheral layer—the epiblast and the hypoblast (fig. 74A). At this stage the larva has the typical planula form. The epiblast is especially thickened at a pole, which may be called the oral pole, and towards the side of this, which will be spoken of as the ventral side. Adjoining this thickened layer of epiblast a special thin layer of hypoblast becomes differentiated, which in opposition to the main mass of large nutritive cells forms the true hypoblastic epithelium (fig. 74B,hy). On this thickening two prominences make their appearance (fig. 74B). The oral of these is the rudiment of a tentacle (t), and the aboral of a nectocalyx (nc).
Epibulia aurantiaca with large nectocalyxFig. 75. An advanced larva of Epibulia aurantiaca with one large nectocalyx.(After Metschnikoff.)so.somatocyst;nc.second imperfectly developed nectocalyx;hph.hydrophyllium;po.polypite;t.tentacle.
Fig. 75. An advanced larva of Epibulia aurantiaca with one large nectocalyx.(After Metschnikoff.)
so.somatocyst;nc.second imperfectly developed nectocalyx;hph.hydrophyllium;po.polypite;t.tentacle.
The former of these elongates itself in succeeding stages into a process of both epiblast and hypoblast. The central part of the nectocalyx on the other hand appears to originate from a thickening of the epiblast in which the cavity of the bell becomes subsequently hollowed out. Between this part and the external epiblast which gives origin to the outermost layer of the nectocalyx a layer of hypoblast is interposed. When the nectocalyx has become to a certain extent established a cavity—the commencement of theprimitive gastrovascular cavity of the adult—appears in the general hypoblast between the epithelial and nutritive layers in the immediate neighbourhood of its attachment. This cavity becomes prolonged into the nectocalyx to form the four gastrovascular canals; while the hypoblast at the upper end of the nectocalyx forms the somatocyst (fig. 74C,so). The primitive enteric cavity once formed rapidly extends, especially in an oral direction (fig. 74C), and forms a widish cavity in the oral part of the embryo. At the pole of this part (fig. 74,po) is eventually formed the opening of the mouth, and the contained cavity becomes in a special sense the gastric cavity. This region of the embryo may be spoken of as the polypite. The nectocalyx grows with great rapidity and soon forms by far the most prominent part of the larva (fig. 75). The true gastric region or polypite (fig. 75,po) continues also to grow, and a mouth becomes formed at its extremity. The aboral end of the original body of the embryo gradually atrophies.
Stephanomia pictumFig. 76. Two stages in the development of Stephanomia pictum.(After Metschnikoff.)A. Stage after the delamination.ep.epiblastic invagination to form pneumatocyst.B. Later stage after the formation of the gastric cavity in the solid hypoblast,po.polypite;t.tentacle;pp.pneumatophore;ep.epiblastic invagination to form pneumatocyst;hy.hypoblast surrounding pneumatocyst.
Fig. 76. Two stages in the development of Stephanomia pictum.(After Metschnikoff.)
A. Stage after the delamination.ep.epiblastic invagination to form pneumatocyst.B. Later stage after the formation of the gastric cavity in the solid hypoblast,po.polypite;t.tentacle;pp.pneumatophore;ep.epiblastic invagination to form pneumatocyst;hy.hypoblast surrounding pneumatocyst.
At the junction of the nectocalyx and polypite the cœnosarc becomes formed, and rudiments of a second nectocalyx (nc) andsecond polypite early become visible; while a hydrophyllium is formed as a bud which covers over the first polypite and tentacle (hph). With the development of the hydrophyllium the first segment, if the term may so be used, is complete. The second segment of which a rudiment is already present as a second polypite is intercalated between the first segment and the nectocalyces.
Amongst the Physophoridæ there is a considerable range of variation in development; though the variations concern for the most part not very important points. The simplest type hitherto observed is that ofStephanomia(Halistemma)pictum. The segmentation and formation of a two-layered planula (fig. 76) take place in the usual way. Between the solid central mass of nutritive hypoblast cells and the epiblast an epithelial hypoblastic layer becomes interposed which undergoes a special thickening at the aboral pole. At this pole a solid involution of epiblast next becomes formed, to which a layer of hypoblast becomes applied. The structure so formed is the rudiment of the pneumatocyst (ep). In the next stage the air-cavity of the pneumatocyst becomes established within the epiblast.
The gastrovascular cavity is formed in the midst of the nutritive hypoblast cells, which then become rapidly absorbed leaving the gastrovascular cavity entirely enclosed by the epithelial layer of hypoblast (fig. 76B).
By the above changes the more important organs of the larva have become established. The one end forms the pneumatophore, and the other, the oral part, the polypite. Between the two there is already present the rudiment of a tentacle, and a second tentacle soon becomes formed. The mouth arises as a perforation at the oral end of the larva.
The pneumatophore contains a prolongation of the gastrovascular cavity, the fluid in which bathes the outer hypoblastic wall of the pneumatocyst. It has however no communication with the enclosed cavity of the pneumatocyst. In the later developmental stages the size of the pneumatophore becomes immensely reduced in comparison with the remainder of the larva.
The development of Physophora agrees closely with that of Stephanomia except in one somewhat important point,viz.in the development of aprovisional hydrophyllium. This arises as a prominence at the aboral pole, containing a prolongation of the gastrovascular cavity. Between the epiblast and hypoblast of the prominence gelatinous tissue becomes deposited, and the hydrophyllium is thus converted into a large umbrella-like organ enclosing the polypite. The two together have a close resemblance to an ordinary Medusa, the polypite forming the manubrium, and the hydrophyllium the umbrella. The hydrophyllium is eventually thrown off.
An important type of Physophorid development is exemplified in Crystalloides, a genus closely allied to Agalma. In this type the greater part of the original ovum, instead of directly giving rise to the polypite, becomes a kind of yolk-sack, from which the polypite is secondarily budded (fig. 77,yk).Agalma sarsiiis in this respect intermediate between Crystalloides and Physophora. Both these types are remarkable for developing a series of provisional hydrophyllia (fig. 77,h.ph.). In both genera the first of these develops as in Physophora, and for a long time is the only one functional.
The conclusions to be drawn from the above description may be summed up as follows. In all the Siphonophora, so far observed, the starting point for further development is a typical ciliated two-layered planula. The inner layer or hypoblast is mainly formed of large nutritive cells. From these cells an epithelial hypoblastic layer becomes secondarily differentiated, the exact relations of which differ somewhat in the various types. The nutritive cells themselves do not appear to become directly converted into the permanent hypoblastic tissues. The development of the adult from the planula commences by the thickening of the epiblastic layer, usually at one pole (the future proximal or aboral pole), and the formation at this pole of a series of bud-like structures (in the growth of which both embryonic layers have a share), which become converted into the hydrophyllia, nectocalyces etc. The main oral part of the planula becomes generally converted into the polypite, though in some instances (Crystalloides) it remains as a yolk-sack, and only secondarily gives rise to a polypite.
Two very different views have been taken as to the nature of the various component parts of the Siphonophora, and the embryological evidence has been appealed to by both sides in confirmation of their views. By Huxley and Metschnikoff the various parts—nectocalyces, hydrophyllia, hydrocysts, polypites, generative gonophores etc. are regarded as simple organs, while by Leuckart, Haeckel, Claus etc. they are regarded as so many different individuals forming a compound stock. The differencebetween these two views is not merely as to the definition of an individual[79]. The question really is, are these parts originally derived by the modification of complete zooids like the gonophores and trophosomes of the fixed Hydrozoa stocks, or are they structures derived from the modification of the tentacles or some other parts of a single zooid?
Larva of CrystalloidesFig. 77. Larva of Crystalloides,(After Haeckel.)h.ph.hydrophyllium;h.hydrocyst;t.tentacle;pp.pneumatophore;po.polypite;yk.yolk-sack.
Fig. 77. Larva of Crystalloides,(After Haeckel.)
h.ph.hydrophyllium;h.hydrocyst;t.tentacle;pp.pneumatophore;po.polypite;yk.yolk-sack.
The difficulty of deciding this point on embryological evidence depends on the fact that ontologically a tentacle and a true bud arise in the same way,viz.as papilliform outgrowths containing prolongations of both the primitive germinal layers. The balance of evidence is nevertheless in my opinion in favour of regarding the Siphonophora as compound stocks, and the views of Claus on this subject (Zoologie,p.271) appear to me the most satisfactory.
The most primitive condition is probably that like Physophora in an early stage with an hydrophyllium enclosing a polypite (cf.Haeckel and Metschnikoff). In this condition the whole larva may be compared to a single Medusa in which the primitive hydrophyllium represents the umbrella of the Medusa, and the polypite the manubrium. The tentacle which appears so early is probably not to be regarded as a modified zooid, but as a true tentacle. The absence of a ring of tentacles is correlated with the bilateral symmetry of the Siphonophora.
The primitive zooid of a Siphonophora stock is thus a Medusa. Like Sarsia and Wilsia this Medusa must be supposed to have been capable of budding. The ordinary nectocalyces by their resemblance to the umbrellas of typical Medusæ are clearly such buds of the medusiform type. The same may be said of the pneumatophore, which, as pointed out by Metschnikoff, is identical in its development with a nectocalyx. Both are formed by asolid process of epiblast in which a cavity—the cavity of the nectocalyx or pneumatocyst—is eventually hollowed out. Around this there appears a double layer of hypoblast containing a prolongation of the gastrovascular cavity; and this is in its turn enclosed by a layer of epiblast which forms the covering of the convex surface of the nectocalyx and the external epiblast of the pneumatophore.
The generative gonophores are clearly also zooids, and the hydrophyllia are probably a rudimentary form of umbrella. In many cases (Epibulia, Stephanomia, Halistemma etc.) the hydrophyllium of the primitive polypite (manubrium) is absent. In such instances it is necessary to suppose that the umbrella of the primitive zooid of the whole colony has become aborted. Leuckart originally took a somewhat different view from the above in that he regarded the starting point of the Siphonophora to be a compound fixed Hydrozoon stock, which became detached and free-swimming.
Acraspeda[80]. The embryonic development of several of the forms of the Acraspeda has been investigated by Kowalevsky (No.147) and Claus (No.153). Their observations seem to point to an invaginate gastrula being characteristic of this group.
Amongst the forms with alternations of generations and a fixed larval form Chrysaora and Cassiopea have been most fully investigated. The ovum of the former undergoes the first embryonic phases while still in the ovary. In the latter it is enclosed amongst the oral processes. A complete and more or less regular segmentation leads to the formation of a single-walled blastosphere with a small segmentation cavity. The wall of the blastosphere next becomes invaginated, giving rise to an archenteron (fig. 78A). The blastopore soon closes up, and the archenteron is converted into a closed sack completely isolated from the epiblast (fig. 78B). The surface of the larva becomes in the meantime covered with cilia. The free larval stage thus reached is similar to the ordinary Hydrozoon planula. After the closure of the blastopore the larva becomes elongated, and one end becomes narrowed. By this narrowed extremity the larva soon attaches itself, and at the opposite and broader end a fresh involution of the epiblast appears (fig. 78C); this gives rise to the stomodæum, which is placed in communication with the archenteron on the absorption of the septum dividing them. The relation of the stomodæum to the original blastopore has not been determined.
At the point of attachment there is developed a peculiar pedal disc, and around the mouth there appears a fold of epiblast which gives rise to an oral disc (fig. 78D). Two tentacles first make their appearance, but one of these is primarily much the largest, though eventually the second overtakes it in its growth. A second pair of tentacles next becomes formed, giving to the larva a 4-radial symmetry. Between these four new tentacles subsequently sprout out, and in the intermediate planes four ridge-like thickenings of the hypoblast, projecting into the cavity of the stomach, make their appearance. They imperfectly divide the stomach into four chambers, to each of which one of the primary tentacles corresponds; they may be regarded as homologous with the mesenteries of the Actinozoa. The number of tentacles goes on increasing somewhat irregularly up to sixteen. All the tentacles contain a solid hypoblastic axis. Muscular elements are developed from the epiblast.
Four stages of ChrysaoraFig. 78. Four stages in the development of Chrysaora.(After Claus.)A. Gastrula stage.B. Stage after closure of blastopore.C. Fixed larva with commencing stomodæum.D. Fixed larva with mouth, short tentacles, etc.ep.epiblast;hy.hypoblast;st.stomodæum;m.mouth;bl.blastopore.
Fig. 78. Four stages in the development of Chrysaora.(After Claus.)
A. Gastrula stage.B. Stage after closure of blastopore.C. Fixed larva with commencing stomodæum.D. Fixed larva with mouth, short tentacles, etc.
ep.epiblast;hy.hypoblast;st.stomodæum;m.mouth;bl.blastopore.
With the above changes the so-called Hydra tuba or Scyphistoma form is reached (videfig. 85). The peculiar strobilization of this form is dealt with in the section devoted to the metamorphosis.
Aurelia is stated by Kowalevsky to develop in the same way as Cassiopea; and the one stage of Rhizostoma observed is that in which it has a (probably invaginate) gastrula form.
In Pelagia the ovum directly gives rise to a form like the parent. The segmentation and the invagination take place nearly as in Cassiopea, but the archenteric cavity is relatively much smaller, and the large space between it and the epiblast becomes filled with the gelatinous tissue which forms the umbrella. The blastopore does not appear to close but to become directly converted into the mouth. As in Cassiopea the larva takes a somewhat four-sided pyramidal form. The mouth is placed at the base. The pyramid becomes subsequently flatter, and at the four corners four tentacles grow out which increase to eight by division. The flattening continues till the larva reaches a form hardly to be distinguished from the Ephyra resulting from the strobilization of the fixed Scyphistoma form of other Acraspeda.
Alcyonidæ.In the Alcyonidæ the segmentation appears always to lead to the formation of a solid morula, which becomes a planula by delamination. The true enteric cavity is formed by an absorption of the central cells, but the axial portion of the gastric cavity and mouth are formed by an epiblastic invagination.
The development of these types has been mainly studied by Kowalevsky (147), and my knowledge of his results is derived from German abstracts of the original Russian memoirs.
InAlcyonium palmatumthe impregnation is external. The segmentation is very exceptional in character. It commences with the formation of a series of irregular prominences on the surface of the ovum, which become segmented off to form a superficial layer of epiblast cells. The inner mass of protoplasm then divides up into polygonal cells to form the hypoblast, which would thus seem to be formed by a kind of delamination. InClavularia crassa(No.168) there is a complete segmentation followed by a delamination. The larva ofAl. palmatumelongates and becomes ciliated, and so assumes the characters of a typical planula. The central hypoblast is formed of an outer granular stratum with imperfectly differentiated cells—the true hypoblast—and an inner homogeneous mass with vacuoles.
Some of the larvæ become fixed, while others coalesce together and form a large mass, the fate of which has not been further studied. An invagination of epiblast takes place at the free end of the fixed larva, which gives rise to the so-called gastric cavity,i.e.the axial portion of the general enteric cavity, which would appear to be in reality a kind of stomodæum. Around the gastric cavity the hypoblast forms eight mesenteries, the chambers between which are filled with the homogeneous material which occupied the centre of the ovum in the previous stage. It is to be presumed, though not stated, that by an absorption of the blind end of the stomodæal invagination the gastric chamber is placed in freecommunication with the spaces between the mesenteries[81]. During the next stage the young Alcyonium also acquires eight tentacles, which arise as hollow papillæ opening into the eight mesenteric chambers. By this stage also the matter filling up the mesenteric chambers is nearly absorbed.
Between the epiblast and hypoblast there is formed an homogeneous membrane, which penetrates in between the two layers of hypoblast which form the mesenteries. On the outer side of this membrane, and therefore presumably derived from the epiblast, is a layer of connective-tissue cells, which eventually gives rise to the abundant gelatinous tissue (cœnenchyma) in which the skeletal elements are deposited. InSympodium coralloidesKowalevsky (No.168) has shewn still more completely the derivation of the stellate mesoblast cells from the epiblast. He finds that the calcareous spicula develop in these cells as in the mesoblast cells of sponges. The branched gastrovascular canals in this tissue are outgrowths of the primitive enteric cavity. A layer of circular muscles is formed at a late period from the epiblast, but the longitudinal muscles of the mesenteries on the inner side of the homogeneous membrane are regarded by Kowalevsky as hypoblastic.
A ciliated planula with delaminated hypoblast is also found in Gorgonia andCorallium rubrum. In the former genus at the time when the larva becomes fixed, the hypoblast is formed of two strata, an outer one of columnar cells, and an inner one of round ciliated cells lining a central enteric cavity. The inner layer is believed by Kowalevsky to become eventually absorbed and to be homologous with the inner granular mass of Alcyonium.
Zoantharia.Amongst the Zoantharia several forms have been investigated by Kowalevsky (147) and Lacaze Duthiers (170), of which some are stated by the former author to pass through an invaginate gastrula stage, while in other instances the hypoblast is probably formed by delamination.
To the first group belongs an edible form of Sea Anemone found near Messina, Cerianthus, and perhaps also Caryophyllium. In the first of these segmentation results in the formation of a blastosphere. A normal invagination obliterating the segmentation cavity then ensues, and the blastopore narrows to form the mouth. The borders of the mouth bend inwards and so give rise to the gastric cavity (stomodæum) which as in the Alcyonidæ is lined by epiblast. Simultaneously with the formation of the mouth there appear the two first mesenteries.
In Cerianthus the segmentation is unequal, the early stages are the same as in the Actinia just described, but the hypoblast cells give riseto a mass of fatty material filling up the enteric cavity, which becomes eventually absorbed.
In the majority of the Zoantharia so far investigated, including species of Actinia, Sagartia, Bunodes, Astroides, Astræa, the segmentation, which is often unequal[82]and not accompanied by the formation of a segmentation cavity, results in a solid two-layered ciliated planula. In these forms the impregnation takes place in the ovary, and the early stages of development are passed through in the maternal tissues.
One end of the planula becomes somewhat oval and develops a special bunch of cilia. At the other end a shallow depression appears, which becomes deeper and forms an involution lined by epiblast. This involution is the stomodæum, and becomes the so-called gastric cavity. The true enteric cavity lined by hypoblast is for some time filled with yolk material. The larva always swims with the aboral end directed forwards.
Between the two embryonic layers a homogeneous membrane is formed, similar to that already described in the Alcyonidæ.
The further development of the larvæ especially concerns the formation of mesenteries, tentacles and calcareous skeleton. With reference to this subject the observations of Lacaze Duthiers are especially valuable and striking.
In the adult it is usually possible to recognise in the tentacles a symmetry of six. There are six primary tentacles, six secondary, twelve tertiary, twenty-four quaternary, etc. In the hard septa of the skeleton the same law is followed up to the third cycle, but beyond that, in the cases where the point can be verified, there appear to be only twelve septa in each additional cycle. The observations of Lacaze Duthiers have shewn that this symmetry is only secondarily acquired and does not in the least correspond with the succession of the parts in development.
His observations were conducted on three species of Zoantharia without a skeleton,viz.Actinia mesembryanthemum, Sagartia, and Bunodes gemmacea; while Astroides calycularis served as the type for his investigations on the corallum. It will be convenient to commence with his results on Actinia mesembryanthemum which served as his type.
The free cylindrical embryo, with the aboral end directed forwards in swimming, first becomes somewhat flattened and the mouth elongated. A bilateral symmetry is thus brought about. Two mesenteries now make their appearance transversely to the long axis of the mouth, which divide the enteric cavity into twounequal chambers. The mesenteries consist of a fold of hypoblast with a prolongation of the epiblast between the twolimbs of the fold. The larger chamber next becomes divided by two fresh mesenteries into three, and a similar division then takes place in the smaller chamber. The stage with six chambers is almost immediately succeeded by one with eight, owing to the appearance of two fresh mesenteries in the second-formed set of chambers. At the stage with eight chambers there is a marked period of repose. The number of chambers is increased to ten by the division of the third-formed set of chambers, and to twelve by the division of the fourth-formed set. It will be observed that the number of the chambers increases in arithmetical progression by the continual addition of two, alternately cut off from the primitive large and small chambers. The freshly formed chambers are always formed immediately on one side of the primitive mesenteries. The stages with six and ten are of very short duration. The two primitive chambers are necessarily at the ends of the long axis of the mouth. After the division of the enteric cavity into twelve chambers, these chambers become about equal in size, and the formation of the tentacles commences. The law regulating the appearance of the tentacles is nearly the same as that for the mesenteries, but is not quite so precise. One tentacle makes its appearance for each chamber. The most remarkable feature in the appearance of the tentacles is due to the fact that the tentacle surmounting the primitive largest chamber arises before any of the others, and long retains its supremacy (fig. 80A). This fact, coupled with the inequality of the two primitive chambers, supplies some grounds for speculating on a possible descent of the Cœlenterata from bilaterally symmetrical forms with distinctly differentiated dorsal and ventral surfaces. The supremacy of the first-formed tentacle is not confined to the Actinozoa, but as has already been indicated, is also found in the Scyphistoma (p.166) of the Acraspeda.
Two stages of Actinia mesembryanthemumFig. 80. Two stages in the development Of Actinia mesembryanthemum.(After Lacaze Duthiers.)In the younger ciliated embryo A, viewed from the side, only one tentacle is developed.m.mouth.The older larva B is viewed from the face when 24 tentacles have just become established. The letters shew the true order of succession of the tentacles; buteandfare transposed.
Fig. 80. Two stages in the development Of Actinia mesembryanthemum.(After Lacaze Duthiers.)
In the younger ciliated embryo A, viewed from the side, only one tentacle is developed.m.mouth.The older larva B is viewed from the face when 24 tentacles have just become established. The letters shew the true order of succession of the tentacles; buteandfare transposed.
After the twelve tentacles have become established they become secondarily divided into two cycles of six respectively larger and smaller tentacles, which alternate with each other. The two tentacles pertaining to the two original chambers belong to the cycle of larger tentacles. The mesenteric filaments appear first of all on the primary pair of septa. The increase in the number of tentacles and chambers from 12 to 24 has been found to take place in a veryremarkable and unexpected way. The law is expressed by Lacaze Duthiers as follows. “The appearance of the new chambers is not, as has been believed, a consequence of the production of a single chamber between each of the twelve already existing chambers, but of the birth of two new chambers in each of the six elements (chambers) of the smaller cycle.” The result of this law is that a pair of tentacles of the third cycle is placed in every alternate space, between a large and a small tentacle, of the two already existing cycles, which may conveniently be called the first and second cycles (fig. 80B).
The twenty-four tentacles formed in the above manner are obviously at first very irregularly arranged (fig. 80B), but they soon acquire a regular arrangement in three graduated cycles of 6, 6 and 12. The first cycle of the six largest tentacles is the large cycle of the previous stage, but the two other cycles are heterogeneous in their origin, each of them being composed partly of the twelve tentacles last formed, and partly of the six tentacles of the second cycle of the previous stage.
The further law of multiplication has been thus expressed by Lacaze Duthiers: “The number of chambers and still later that of the corresponding tentacles is carried from 24‑48 and from 48‑96 by the birth of a pair of elements in each of the 12 or 24 chambers, above which are placed the smallest tentacles which together constitute the fourth or fifth cycle. Since, after the formation of each fresh cycle, the arrangement of the tentacles again becomes symmetrical, it is obvious that all the equal sized cycles except the first are formed of tentacles entirely heterogeneous as to age.”
The fixation of the free-swimming larva takes place during the period when the tentacles are increasing from 12 to 24.
The general formation of the chambers in Bunodes and Sagartia is nearly the same as in Actinia.
In the two types of Actinozoa with an embolic gastrula stage the laws as to the formation of the tentacles do not appear to be the same as those regulating the forms observed by Lacaze Duthiers.
In Cerianthus four tentacles are formed simultaneously at the period when only four chambers are present. In Arachnitis (Edwarsia) the succession of the tentacles is stated (A. Agassiz,166) to resemble that in Cerianthus. There are originally four tentacles, and at one extremity of the long axis of the mouth are the oldest tentacles, while at the other tentacles are constantly added in pairs. An odd tentacle is always found at the extremity of the mouth opposite the oldest tentacles.
In the other species with an embolic gastrula eight tentacles would seem to appear simultaneously at the period when eight chambers are present; though on this point Kowalevsky’s description is not very clear. The presence of such a stage would seem to indicate a close affinity to the Alcyonidæ.
Amongst the sclerodermatous Actinozoa, except Caryophyllium, the embryo closely resembles that of the delaminate Malacodermata. The firststages occur in the ovary, and the larva is dehisced into the body cavity as a two-layered ciliated planula.
The laws affecting the formation of the first twelve tentacles and septa appear to be nearly the same as for the Malacodermata. The hard parts begin as a rule to be formed when twelve tentacles have appeared, at which period also the fixation of the larva takes place. On fixation the larva becomes very much flattened.
The first parts of the corallum to appear are twelve of the septa, which arise simultaneously in folds of the enteric wall in the chambersbetween the mesenteries, and correspond therefore with the tentacles and not, as might be supposed, with the mesenteries. Each septum is formed by the coalescence of three calcareous plates which originate in separate centres of calcification. The concrescence of the three produces a Y-shaped plate with the single limb directed inwards and the two limbs outwards (fig. 81). The theca does not arise till after the septa have become formed, and is at first a somewhat membranous cup quite distinct from the septa. The columella is formed still later by the coalescence of a series of nodules which are formed in a central axis enclosed by the inner ends of the septa.
Larva of Astroides calycularisFig. 81. Larva of Astroides calycularis shortly after it has become attached.(After Lacaze Duthiers.)The figure shews the development of the Y-shaped septa in the intervals between the mesenteries. The position of the latter is indicated by the faint shading. The theca has become developed externally.
Fig. 81. Larva of Astroides calycularis shortly after it has become attached.(After Lacaze Duthiers.)
The figure shews the development of the Y-shaped septa in the intervals between the mesenteries. The position of the latter is indicated by the faint shading. The theca has become developed externally.
After the formation of the theca the septa become divided into two cycles by the predominant growth of six of them. On the coalescence of the septa with the theca the space between the two limbs of the Y becomes filled up with calcareous tissue. The law of the formation of the third cycle of septa (12‑24) has not been worked out, so that it is not possible to state whether it follows the peculiar principles regulating the growth of the tentacles.
The whole of the skeletal parts occupy a position between the epiblast and hypoblast, and are exactly homologous in this respect with the skeleton of the Alcyonidæ. By Lacaze Duthiers they are however believed to originate in the hypoblast, but from the observations of Kowalevsky there be little doubt that they arise in the connective tissue between the two embryonic layers which is probably epiblastic in origin.
A peculiar larva, probably belonging to the Actinozoa, has been described by Semper[83]. It has an elongated form and is provided with a longitudinal ridge of cilia. There is a mouth at one end of the body and an anus at the opposite extremity. The mouth leads into an œsophagus, which opensfreely into a stomach with six mesenteries. In the skin are numerous thread-cells. A mesotrochal worm-like larva, also provided with thread-cells, and found at the same time, was conjectured by Semper to be a younger form of this larva.
Ctenophora.The ovum of the Ctenophora is formed of an outer granular protoplasmic layer and an inner spongy mass with fatty spherules. It is enveloped in a delicate vesicle, the diameter of which is very much greater than that of the contained ovum. This vesicle appears to be filled with sea-water, in which the ovum floats.
Fertilized ova may usually be easily obtained by keeping the captured adults in water from 12‑24 hours. The two main authorities on the development of these forms (Kowalevsky,No.147and178and Agassiz,No.172) are unfortunately at variance on one or two of the most fundamental points. It seems however that the embryonic layers are formed by a kind of epibolic gastrula; while the true gastric cavity, as distinct from the gastrovascular, is formed by an invagination, and deserves therefore to be regarded as a form of stomodæum.