fig245Fig. 245.—Diagram of the structure ofLoxosoma, seen from the oesophageal side.×about 70.a, Anus;b, buds;e, excretory organ;f, foot-gland;g, ganglion;gn, generative organs;o, orifice of vestibule;oe, oesophagus;s, stomach;t, retracted tentacles.
Fig. 245.—Diagram of the structure ofLoxosoma, seen from the oesophageal side.×about 70.a, Anus;b, buds;e, excretory organ;f, foot-gland;g, ganglion;gn, generative organs;o, orifice of vestibule;oe, oesophagus;s, stomach;t, retracted tentacles.
Fig. 245.—Diagram of the structure ofLoxosoma, seen from the oesophageal side.×about 70.a, Anus;b, buds;e, excretory organ;f, foot-gland;g, ganglion;gn, generative organs;o, orifice of vestibule;oe, oesophagus;s, stomach;t, retracted tentacles.
InPedicellinathe plane of the lophophore is at right angles to the stalk, which is separated from its calyx by a marked constriction. InLoxosomathe lophophore is set obliquely,[542]and there is no constriction at the base of the calyx. InUrnatellawe find an intermediate condition, the lophophore resembling that ofLoxosoma, while the constriction at the base of the calyx is similar to that ofPedicellina. Since the latter is known to pass in its development[543]through a stage with an oblique lophophore, it may be presumed thatLoxosomais a more archaic form thanPedicellina. In other respects, the structure of the Entoprocta is very constant, whatever the genus.
A pair of ciliated excretory tubes open into the vestibule. These are similar in structure to the "head-kidneys" of the larvae of Polychaet worms, or to the excretory organs of adult Rotifers. Flame-cells have been described by Davenport in the stalk ofUrnatella, but it is not known whether they are connected with the excretory tubes of the calyx. The animals are either hermaphrodite or have separate sexes, and the generative organs open by ducts of their own into the vestibule. The nervous system consists of a ganglion placed between the mouth and the anus, giving off a set of nerves, many of which end in delicate tactile hairs placed on the tentacles or other parts of the body.[544]
POLYZOA (continued)
FRESH-WATER POLYZOA—PHYLACTOLAEMATA—OCCURRENCE—STRUCTURE OFCRISTATELLA—DIVISION OF COLONY—MOVEMENTS OF COLONY—RETRACTION AND PROTRUSION OF POLYPIDES IN POLYZOA—STATOBLASTS—TABLE FOR DETERMINATION OF GENERA OF FRESH-WATER POLYZOA—REPRODUCTIVE PROCESSES OF POLYZOA—DEVELOPMENT—AFFINITIES—METAMORPHOSIS—BUDDING.
Fresh-water Polyzoa.—Although the Gymnolaemata are ordinarily marine animals, fresh-water examples from this Order are not altogether wanting. The Ctenostomata among the typically marine groups show the most tendency to stray into fresh-water.
AlcyonidiumandBowerbankia(Fig. 238) flourish in estuaries, whileVictorellaandPaludicella(Fig. 250) are only known as fresh or brackish water forms.Victorellawas named after the Victoria Docks in London, where it was first found; more recently it has also been discovered in other parts of England and on the Continent.[545]
The systematic position of the generaHislopiaandNorodonia,[546]which have been described from fresh water of India and China respectively, is at present uncertain. The undoubted CheilostomeMembraniporahas, however, a British representative (M. monostachys), which occurs in brackish water, in ditches on the coast of East Anglia. It is there known to form "friable, irregularly-shaped, sponge-like masses," which grow on water-plants.[547]
The Entoprocta, as we have seen, are represented in fresh water by the genusUrnatella.
ThePhylactolaemataare an exclusively fresh-water group, and they are believed by Kraepelin[548]to have been derived from the Ctenostomata. Many of their special peculiarities can, with great probability, be regarded as adaptations to a fresh-water existence. This is particularly clear in the all but universal habit of dying down in the winter, and in the occurrence of the so-called statoblasts (Fig. 251), which are hard-shelled reproductive bodies, absolutely restricted to the Phylactolaemata, and capable of resisting the winter's cold and even a certain amount of drying up. Phylactolaemata have indeed been recorded from the tropics; but it is not yet sufficiently clear how they there behave in these respects. F. Müller[549]has found these animals in Brazil, where they are said to be more common at certain periods of the year than at others. Stuhlmann has found them in Tropical Africa (Victoria Nyanza, etc.);[550]and Meissner[551]has discovered the sessile statoblasts ofPlumatellaon the shells preserved in the Berlin Museum, of species of the MolluscAetheriafrom various localities in Africa. Fresh-water representatives of a considerable number of other groups of animals agree with the Phylactolaemata in the possession of reproductive bodies which are protected by hard coats. Such, for instance, are the ephippian ova ofDaphnia—bodies which have an extraordinary external similarity to statoblasts—the gemmules of Spongillidae, the winter-eggs of Rhabdocoels and Rotifers, and the cysts of Protozoa. The evolution of these bodies in so many widely different cases may have been due to the selection of variations calculated to minimise the dangers attendant on the drying up of the water in summer, or on its freezing in winter.
The Phylactolaemata are by no means uncommon, although they can seldom be found without a careful search. Their presence may often be detected by taking advantage of the property of the free statoblasts of rising to the top of the water, where they can be discovered by skimming the surface with a fine hand-net.
The colonies themselves are usually found attached to water-plants, roots of trees or stones. Most of them flourish best ina zone not more than two feet below the surface. Certain species show a preference for floating leaves, such as those of water-lilies, where they are not liable to be dried up by alterations in the level of the water. Some forms (e.g.Plumatella, Fig. 246) are, however, able to withstand being dried for some time. Most species prefer shady places, and accordingly settle on the lower sides of leaves and sticks. Others (e.g.Cristatella, Fig. 247) have no objection to the direct rays of the sun. Most forms prefer still water, but one or two are found in running water.
Fredericellais a common constituent of the deep-water fauna of Swiss Lakes (down to over forty fathoms); and reaches there a size considerably larger than the shallow-water form of the same species.Paludicellais common at thirteen fathoms. These two genera, withPlumatella, have been found in absolute darkness, under a pressure of 2½-5½ atmospheres, in the Hamburg aqueduct. The Polyzoa and other organisms growing in the water-supply of Hamburg were accused of being concerned in the spreading of cholera, during the recent epidemic, by choking up the water-pipes, and creating obstructions which formed a favourable nidus for the development of cholera-germs.
The colony may take the form of a series of delicate, branching tubes (Plumatella,Fredericella), of more massive aggregations of parallel tubes (as in the Alcyonelloid forms ofPlumatella), or of gelatinous masses of varying size (Lophopus,Cristatella).
fig246Fig. 246.—A,Plumatella(Alcyonella)fungosaPall., Naples (fresh water), small part of a mass, natural size;B,Plumatella repensL., R. Yare, on the leaf of a water-lily, natural size.
Fig. 246.—A,Plumatella(Alcyonella)fungosaPall., Naples (fresh water), small part of a mass, natural size;B,Plumatella repensL., R. Yare, on the leaf of a water-lily, natural size.
Fig. 246.—A,Plumatella(Alcyonella)fungosaPall., Naples (fresh water), small part of a mass, natural size;B,Plumatella repensL., R. Yare, on the leaf of a water-lily, natural size.
Cristatella mucedo(Fig. 247) is remarkable for its power of moving from place to place; it consists of an elongated mass of greenish, gelatinous substance, which, in its fully developed state, may reach a length of eight inches or more, with a transverse diameter of three-eighths of an inch. It has a flattened sole on which it crawls, while the graceful plumes of its numerous polypides protrude as a delicate fringe from its upper side.
The tentacles are about eighty to ninety in number, and they are, as in other Phylactolaemata, united at their bases by a delicate web. The lophophore is horse-shoe-shaped (Fig. 236, 3) throughout the group, with the exception ofFredericella, in which genus it is circular.
In some Phylactolaemata the polypide has been observed to interlace its tentacles, so that the plume becomes a kind of cage, in which the more active Infusoria are imprisoned until their struggles have so far weakened them that they are swept into the mouth by the action of the cilia of the tentacles.[552]
fig247Fig. 247.—Cristatella mucedoCuv. (a small colony), R. Yare, above Norwich, × 24.
Fig. 247.—Cristatella mucedoCuv. (a small colony), R. Yare, above Norwich, × 24.
Fig. 247.—Cristatella mucedoCuv. (a small colony), R. Yare, above Norwich, × 24.
Around the edge of theCristatellais found a zone of budding tissue, which gives rise continuously to new individuals. Now, whereas in Gymnolaemata the growing edge gives rise to zooecia, whose cavities become completely cut off from that of the older ones; in Phylactolaemata the partitions between the zooecia are never completed. The body-cavity ofCristatellais thus a continuous space, interrupted at the margin only by vertical septa (see Fig. 247), which represent the partitions between the zooecia of other forms.
The body-wall consists of two epithelial layers of ectoderm and mesoderm, between which is a layer of muscular fibres.Parts of the epithelium lining the body-cavity are ciliated. Into the common body-cavity hang the polypide-buds at the edge of the colony, and the mature polypides in the more central regions. There are usually three rows of polypides on either side of the middle line, in the neighbourhood of which is an area devoid of polypides, but containing "brown bodies" and statoblasts. The polypides nearest to the middle line pass in succession into the condition of "brown bodies," while young buds near the margin grow up coincidently to form new polypides.
The movement of the colony is in the direction of the long axis, although either end may go first. Sir John Dalyell records an observation[553]on a specimen (about one inch long) which was artificially divided into two halves. The two halves "receded from each other as if by common consent," and were nearly an inch apart in twenty hours.
An observation made at Cambridge on a small colony of about 7 mm. in greatest length gave the following results. The colony moved 13 mm. (nearly twice its own length) in 8¼ hours: in the next 40 hours it moved 20 mm. (⅘ inch); while in the following 24 hours it moved only 6 mm. Large colonies change their place only with reluctance.
The locomotive power possessed byCristatellais not unique among Phylactolaemata.Lophopus, the first fresh-water Polyzoon of which any description was published, was originally described by Trembley in 1744 under the name of the "Polype à pannache." Trembley observed the spontaneous division of the colony,followed by the gradual separation from one another of the daughter-colonies.[554]The power of dividing spontaneously is also possessed by colonies ofCristatellaand ofPectinatella.
The colonies ofLophopusare surrounded by an excessively hyaline ectocyst, and are usually triangular, as shown by Fig. 248. When division is about to occur, the base of the triangle becomes indented, and the indentation travels towards the apex in such a way as to bisect the triangle. The two halves diverge from one another during the process, so that before division is complete, they are looking, in some cases, in opposite directions.After a time the narrow connection breaks, and two new colonies are formed.
Fig. 248 shows a colony shortly after division has taken place. The colony had moved forwards, in a direction away from its apex, for three days in a nearly straight line, the distances moved in each day being respectively 6, 8½, 8½ mm. These observations, for which I am indebted to Mr. Lister, show a considerably higher speed than in those recorded by Trembley, who observed no colony which moved more than half an inch (12.5 mm.) in eight days.
The genusPectinatellaalso has some power of locomotion. This magnificent Polyzoon occurs in masses several feet in length (as much as six feet inP. gelatinosafrom Japan[555]), and four to eight inches in thickness. The greater part ofP. magnifica[556]consists of a thick, opaline, and gelatinous ectocyst, the upper surface of which is covered by hundreds of rosette-like colonies, which increase in number by division. The masses are thus aggregations of colonies, which secrete a common basal ectocyst. The latter decays in the autumn; and the separate rosettes, or groups of them, may thus be set free, being found as floating masses, which may again attach themselves to a solid object till the time of their death.Pectinatellahas not yet been recorded in England, although, considering the ease with which statoblasts are transported, it is by no means improbable that it will eventually be recorded as a British genus. It is at present known to inhabit America, Japan, and Hamburg.
fig248Fig. 248.—Lophopus crystallinusPall., Cambridge, showing the rate of movement. The colony and the distances moved are × 2.
Fig. 248.—Lophopus crystallinusPall., Cambridge, showing the rate of movement. The colony and the distances moved are × 2.
Fig. 248.—Lophopus crystallinusPall., Cambridge, showing the rate of movement. The colony and the distances moved are × 2.
It is by no means certain what is the mechanism by whichmovement takes place in the above cases. The ectocyst ofCristatellais confined to the base of the colony, and there forms a thin slimy film, which lubricates the surface over which the animal moves. It has been stated[557]that progression is produced in the following way. The polypides are withdrawn by means of retractor muscles, which originate from the septa and inner surface of the sole. Thus at each retraction of any polypide, the muscle pulls on a portion of the sole. Should the expanded polypides place themselves in a suitable position, the movement will be in the direction of the resultant of the forces due to the separate retractor muscles; while it is probable that their cilia assist in the onward movement. It should be noted that it is definitely stated that a colony in which all the polypides are retracted can alter its position,[558]although even then the retractor muscles might still contract to some extent.
The movement probably depends on several causes. It must probably be conceded that the sole itself has some effect on this process. Its outer cells are contractile, and have the power of raising themselves from the underlying ectocyst. They may then again attach themselves, and this new attachment does not always take place in exactly the same place as the former one. Any movement of the muscles of the sole, or of the retractor muscles, will thus shift the skin to a new place.[559]
Protrusion of the Polypide.—While it is perfectly clear that retraction is principally performed by the great retractor muscles acting directly on the polypide, it is less easy to explain the converse movement. There can, however, be little doubt that protrusion is effected by the pressure of the fluid of the body-cavity, caused in large part by contractions of the common body-wall.
Now since, inCristatella, the body-cavity is a continuous space, any pressure on the fluid must act uniformly on all its contents. The cause which determines the protrusion of a polypide is thus to a large extent the relaxation of the sphincter-muscle which surrounds its orifice, aided by special muscles which dilate the orifice. Any polypide which is retracted while the pressure of the fluid in the body-cavity is sufficient to keep other polypides protruded, must therefore keep either itsretractor-muscles or its sphincter in a state of contraction in order to remain in that position. And as a matter of fact,CristatellaandLophopusdiffer from most other Polyzoa in the readiness with which they expand their tentacles, after they have been induced to retract themselves by mechanical irritation.
Plumatellaand other forms have a chitinous ectocyst, which, however, is sticky when it is first formed. By virtue of this property, the branches become attached to the leaf on which the colony is growing, and may have their natural transparency obscured by taking up foreign bodies. The stiffness of the ectocyst naturally involves some modification of the process by which the polypides are protruded. In some cases, this is effected by the separation of the endocyst from the ectocyst in the lower parts of the tube. The muscles of the body-wall can thus press on the fluid of the body-cavity without being restrained by the inflexible ectocyst. In other cases, the tube of ectocyst is rendered flexible by the presence of a thin line along one side where the chitin is deficient.
fig249Fig. 249.—Plumatella repensL., R. Yare, × 30.a, Anus;b, polypide-bud;c, caecum of stomach;d, duplicature;e, epistome (see p.476);f, funiculus;g, ganglion;m, retractor muscle;p, parieto-vaginal muscles;ph, pharynx;s, statoblasts attached tof.
Fig. 249.—Plumatella repensL., R. Yare, × 30.a, Anus;b, polypide-bud;c, caecum of stomach;d, duplicature;e, epistome (see p.476);f, funiculus;g, ganglion;m, retractor muscle;p, parieto-vaginal muscles;ph, pharynx;s, statoblasts attached tof.
Fig. 249.—Plumatella repensL., R. Yare, × 30.a, Anus;b, polypide-bud;c, caecum of stomach;d, duplicature;e, epistome (see p.476);f, funiculus;g, ganglion;m, retractor muscle;p, parieto-vaginal muscles;ph, pharynx;s, statoblasts attached tof.
The upper end of the retracted tentacle-sheath is connected with the body-wall by bands known as the parieto-vaginal muscles (Fig. 249,p). Theseserve not only to dilate the orifice when protrusion is commencing, but also to prevent the polypide from being forced out too far. They are arranged in such a way that a circular fold, the duplicature (d), is never turned inside out, even in the state of complete protrusion of the polypide.
The mechanism of the protrusion of the polypide in the Gymnolaemata is in many cases obscure. The body-wall is not muscular in this group, in some forms of which, however, short strands known as the parietal muscles (Fig. 234,p) pass across the body-cavity from one point to another of the zooecium. As doubts have been thrown on the function of these muscles in causing protrusion, it will be worth while to refer to the detailed and convincing statements of Farre,[560]relating to this point.
Farre's observations were made on certain transparent Ctenostomes (BowerbankiaandFarrella). He states that the parietal muscles "were distinctly seen to contract whenever the protrusion of the animal took place, and to become relaxed again upon its retiring into its cell." Their contraction may indent the outline of the ectocyst, or may cause the separation of the endocyst from the ectocyst. The endocyst is then drawn into longitudinal lines at the origin and insertion of these fibres. It is further suggested that some part is played in the process by the muscular walls of the alimentary canal, which is a good deal bent in the retracted condition. The effort to straighten itself is believed to have some share in forcing out the polypide. The flexible, membranous character of the "aperture" (see p.524) inMembranipora(Fig. 256, A) is said by Nitsche[561]to be an arrangement for the protrusion of the polypides; the parietal muscles passing from the lateral walls of the zooecium to the upper membranous wall, which is accordingly depressed by their contraction.
Although it is hardly possible to doubt the accuracy of Farre's observations, which have, moreover, been confirmed by Hincks, it is by no means certain that this is the whole explanation in all cases. Oka,[562]for instance, states that protrusion of the polypide in Phylactolaemata can be effected in a branch whose body-wall has been cut open. Pergens[563]believes that the diaphragm (Fig.234,d) acts as a pump, introducing water from the tentacle-sheath into the body-cavity, into which it is said by him to open, and so forcing out the polypide. It is probable that many of the forms which have a stiff, unyielding ectocyst possess special arrangements for introducing water in some way into the space bounded by the ectocyst,[564]and so forcing out the polypide. Such, for instance, may be the median pore which occurs beneath the orifice inMicroporella(Fig. 241, A,mp), and in certain other cases.
Reproduction of Phylactolaemata.—Sexual reproduction takes place inCristatellafrom June to August. The spermatozoa are ordinarily produced on the funiculus. The ovaries usually occur on the inner side of the common wall of the colony, not far below the orifice of a polypide. Each ovary matures a single egg, which developsin situ, the free larva leaving the colony by the orifice of one of the degenerated polypides.
A second method of reproduction takes place by means of the statoblasts, which are developed on the funiculus (Fig. 249). According to Verworn,[565]each statoblast arises from a single cell of the funiculus; and on this view, the statoblast is, as supposed by the earlier observers, a special kind of winter-egg. According to more recent researches,[566]the funiculus consists of a central axis, formed from the ectoderm, and of an outer sheath of mesoderm-cells; the statoblast is developed from the two kinds of cells of which the funiculus is composed, and is consequently comparable in its mode of origin to an ordinary bud. Its special peculiarities are: its origin as an internal bud, its possession of a chitinous shell, and the fact that it is destined to leave the parent colony, and to develop, after a period of rest, into a new colony. Germination takes place by the formation of a polypide-bud inside the statoblast, which finally splits along its equator into two halves. The contents emerge as a young colony which possesses at least one fully-formed polypide.
Remarkable structures known as "hibernacula" occur in the fresh-water Ctenostomes,PaludicellaandVictorella. These bodies are in the former (Fig. 250, B) specially modifiedexternalbuds, which persist through the winter when the rest of the colony dies down. At the close of winter the shell splits into twohalves, exactly as takes place in the statoblasts, and a young colony emerges. It is possible that the statoblasts may have been evolved from a hibernaculum, which was at first produced externally, but has become modified in such a way as to acquire an internal mode of origin.[567]
The simplest known statoblast is that ofFredericella(Fig. 251, A), which differs from that of other Phylactolaemata in having no ring of air-cells. InPlumatella, the statoblast (Fig. 251, B) has a broad equatorial ring of air-cells, which enable it to float at the surface of the water on the decay of the parent tubes. In some species, certain statoblasts which are produced in the adherent parts of the colony remain attached to the substratum. These "sessile statoblasts" may have no trace of the ring of air-cells; but the fact that many sessile statoblasts have rudiments of this structure suggests that they are a secondary modification of the floating statoblast. InLophopus(Fig. 251, C) the ring of air-cells is very broad, and is pointed at each end; while inCristatella(Fig. 251, D) and inPectinatellathe statoblast is circular, and possesses an armature of hooked spines. That ofCristatella, measures about .75 mm. in its greatest length.
fig250Fig. 250.—Paludicella ehrenbergivan Beneden, × about 3.A, Part of a colony with expanded polypides;B, remains of part of a colony which has produced hibernacula or winter-buds (h);z, zooecium. (From Kraepelin.)
Fig. 250.—Paludicella ehrenbergivan Beneden, × about 3.A, Part of a colony with expanded polypides;B, remains of part of a colony which has produced hibernacula or winter-buds (h);z, zooecium. (From Kraepelin.)
Fig. 250.—Paludicella ehrenbergivan Beneden, × about 3.A, Part of a colony with expanded polypides;B, remains of part of a colony which has produced hibernacula or winter-buds (h);z, zooecium. (From Kraepelin.)
Kraepelin has suggested that the above order of increasing complexity of the statoblasts corresponds with the order in which the genera to which they respectively belong would be placed, on the assumption that the Phylactolaemata have been derived from the Ctenostomata. Thus, inFredericella, the form of the lophophore is circular, as in the Gymnolaemata. The number of the tentacles is comparatively small (20-24). The arborescent form of the colony resembles that of many Ctenostomes, and the zooecia are more or less cut off from one another by incomplete septa.
InPlumatella, the lophophore has become horse-shoe-shaped, and the tentacles are more numerous (38-60). In general form and in the arrangement of the septa this genus resemblesFredericella, with which it may easily be confused.
InCristatellawe have the most highly modified of all the Phylactolaemata. The individuality of the zooecium is here subordinated to that of the colony as a whole. The branched arrangement of the zooecia is greatly obscured. The body-cavities have become completely confluent, although rudiments of the septa still exist. The ectocyst has been lost, with the exception of the basal layer of the colony. The tentacles are more numerous (80-90); and in accordance with the increase in the elaboration of the genus, its statoblasts belong to the most complicated type known.
fig251Fig. 251.—Statoblasts of Phylactolaemata.A,Fredericella sultanaBlum., × 38;B,Plumatella repensL., × 38;C,Lophopus crystallinusPall., × 28;D,Cristatella mucedoCuv., × 28. (A, from Allman;B-D, from Kraepelin.)
Fig. 251.—Statoblasts of Phylactolaemata.A,Fredericella sultanaBlum., × 38;B,Plumatella repensL., × 38;C,Lophopus crystallinusPall., × 28;D,Cristatella mucedoCuv., × 28. (A, from Allman;B-D, from Kraepelin.)
Fig. 251.—Statoblasts of Phylactolaemata.A,Fredericella sultanaBlum., × 38;B,Plumatella repensL., × 38;C,Lophopus crystallinusPall., × 28;D,Cristatella mucedoCuv., × 28. (A, from Allman;B-D, from Kraepelin.)
The production offloatingstatoblasts may seem a strange adaptation to the conditions of fresh-water life, since it might be assumed,a priori, that these structures would be specially liable to be frozen during the winter. The following experiments made by Braem[568]show, however, that the germinating power of the statoblasts is improved by a certain amount of frost. A number of statoblasts were taken; half of these were placed in water, which was then frozen; and these were found to germinate readily when afterwards exposed to suitable conditions. The other half were not subjected to the action of frost; and these could not be made to germinate, even although the water had been cooled to a point slightly above the freezing point. It thus appears that the buoyancy, so far from being a risk, is a means of exposing the statoblast to the conditions which are most favourable to its later development.
Braem supposes that the beneficial action of frost is due to a lowering of the vital energy of the statoblast. As in the case of reproductive bodies known in many other fresh-water organisms, the statoblast germinates only after a period of rest. Although this period is often shortened by a lowering of the temperature, it can also be induced by the exclusion of air, as in an experiment during which the statoblasts were enclosed in airtight tubes. The respiratory processes were thereby lessened, and the germinating power was materially improved.
Since the development of the statoblasts depends largely on the temperature, the first warm weather in early spring will probably induce the germination of those which are floating; and the young colony, leaving the protection of the statoblast, will become susceptible to frost. But even if the first-formed colonies are killed off by a subsequent frost, other statoblasts which have remained in the mud during the winter are disentangled from time to time, and germinate on reaching the surface.
Distribution.—The protective value of the shell is also shown by the fact that the statoblast may be kept for some months in a dry condition without losing its power of germination. There can be little doubt that the capability of withstanding desiccation enables the species to enlarge its area of distribution. It is asserted that fresh-water Polyzoa decrease in abundance in proportion to the distance from the mouth of the river in which they are found. The current will naturally tend to bring together the statoblasts from the Polyzoa growing in the upper waters.
Nothing is more surprising than the wide geographical distribution of the Phylactolaemata. The European genera are all recorded from North America.Fredericella,Plumatella, andLophopusare further recorded from Australia; whilePlumatellais known to occur also in Malacca, the Philippine Islands, India, Japan, Africa, and South America, It is even stated that some of the Australianspeciesare identical with those found in Europe.
Some of the fresh-water Polyzoa are extremely variable, and observers are by no means agreed in deciding whether certain well-known forms are to be regarded as varieties or as species. While certain genera, such asCristatellaandLophopus, are comparatively constant in their form,Plumatellais excessively variable.Plumatellahas a number of species greater than that of any other form, and the genus has a wider distribution than anyother. This greater variation of species of the dominant genus is in complete accordance with the general law enunciated by Darwin that "wide-ranging, much diffused, and common species vary most."
While the ordinary forms ofPlumatellaconsist of branching colonies, which are either completely adherent to their substratum, or grow in a more or less erect manner, another habit which is assumed by this genus is so different from the first that it has been considered to mark a distinct genus,Alcyonella. The Alcyonelloid form (Fig. 246, A) consists of closely packed tubes which stand more or less at right angles to their substratum, which they may cover with a dense mass an inch thick, and with a superficial area of several square inches. But in spite of this difference, it is possible thatA. fungosais only a variety of an ordinaryPlumatellaform. Whether this is so or not, a typicalPlumatellamay in places take on an Alcyonelloid habit; and parts of anAlcyonellamay become so lax in growth as to resemble aPlumatella.
The British genera of fresh-water Polyzoa may be distinguished from one another by means of the following table:—
Zooecia perfectly distinct from one another. Lophophore circular. Statoblasts absent
2
Colony formed of branching tubes composed of confluent zooecia
3
Colony gelatinous, not obviously formed of branching tubes. Lophophore horse-shoe shaped
4
Colony consisting of a stolon from which new zooecia originate. These may give rise to new stolons, or directly to new zooecia
Victorella
Branches composed entirely of club-shaped zooecia, each of which may give off two zooecia near its upper end
Paludicella(Fig. 250)
(b) Tubes parallel with one another
Alcyonellaform ofPlumatella(Fig. 246, A)
Tubes cylindrical, usually dark brown. Statoblasts (Fig. 251, A) few, without air-cells. Lophophore circular
Fredericella
Colony hyaline, usually divided into three or four short lobes. Ectocyst thick. Statoblasts (Fig. 251, C) pointed at each end, with a broad ring of air-cells
Lophopus(Fig. 248)
Colony slug-shaped, crawling on a flattened sole. Ectocyst rudimentary. Statoblasts (Fig. 251, D) circular, with marginal hooks
Cristatella(Fig. 247)
Colonies consisting of small rosettes, many of which are attached to a thick basal layer of hyaline ectocyst. Statoblasts circular, with marginal hooks. (Not recorded as British)
Pectinatella
Reproductive Processes of Polyzoa in general.
In studying the reproductive processes of Polyzoa, we have to deal with two very distinct phenomena; firstly, with the development of eggs; and secondly, with the formation of buds.
The process of budding usually does no more than increase the number of individuals in a colony which already exists, and is seldom responsible for the commencement of a new colony. InLoxosoma, however, the buds break off and lead an independent existence; and in the Phylactolaemata a large proportion of the colonies have their origin in the statoblasts. In certain cases, again, new colonies may be formed by the detachment of parts of an old one, as by the fission ofCristatellaandLophopus, or by the breaking up of a richly-branched species into several colonies by the decay of the proximal parts.
We may then in the majority of cases look to an embryo for the foundation of a new colony. The embryo develops into a larva, which, after a period in which it swims freely, settles down, and is metamorphosed into the first zooecium. This primary individual forms the starting-point of a colony, and often differs to a considerable extent from the other zooecia which arise from it. In Cyclostomata, for instance, the proximal end of the primary zooecium permanently retains the disc-like shape assumed by the young larva when it first fixed itself. The primary zooecium may be recognised with equal ease in many Cheilostomata, and may differ from its successors by possessing a richer development of marginal spines, or in other respects.
Reproductive Organs.—Eggs and spermatozoa are commonly found in the same colony, either in different individuals, or else in the same zooecium (see Fig. 234, p.469). In some cases, the zooecium first develops spermatozoa, and later eggs. The Entoprocta have a more marked separation of the sexes than obtains in other Polyzoa. The genusLoxosomais perhaps always dioecious (i.e.with separate sexes).Pedicellinais sometimes found with ovaries and testes in the same individual, sometimes with these organs in different individuals; and it is not clear whether a given species always behaves alike in these respects.
The reproductive organs of the Entoprocta open by ducts of their own into the vestibule. In the Ectoprocta they are developed in the body-cavity, and they have no ducts.
The fate of the ripe egg differs widely in different cases. In the Entoprocta it develops in a kind of brood-pouch formed from part of the vestibule. The fact that inPedicellina(Fig. 243) the embryos grow largely during their development, shows that nutritive material must be supplied to them from the parent. There is reason to believe that the epithelium of the brood-pouch is responsible for this process. The eggs are also known to develop at the expense of nutritive substances prepared by the parent in the ovicells of the Cyclostomata. In other cases, as in some species ofAlcyonidium, the egg is large, and its copious yolk doubtless supplies a large part of the material required for development.
In the Ectoprocta, development takes place in a variety of places. In most Cheilostomata a single egg passes into the globular ovicell, which is formed above the orifice of many of the zooecia. In certain Ctenostomata,[569]Phylactolaemata,[570]and Cyclostomata,[571]the ripe egg is taken up by a rudimentary polypide-bud, which is specially formed for the purpose. In the Ctenostomata and in the fresh-water Polyzoa these buds, if present, are found in ordinary zooecia which do not become modified externally in any special way. In the Cyclostomata (Crisia), on the contrary, the formation of the polypide-bud is intimately bound up with the development of the ovicell. The number of the zooecia which produce eggs that are capable of development is greatly restricted in this group. The ovicell, which contains numerous embryos, is not merely a portion of a zooecium, as in the Cheilostomata; but it is probably to be regarded as a modification of the entire fertile zooecium or zooecia. These take on an appearance widely differing from that of the ordinary zooecia, and in course of time give rise to the ovicells (see Fig. 237).
In all these cases the egg develops inside the parent, and it was hardly known, before the publication of the interesting researches of M. Prouho,[572]that some of the Polyzoa lay eggs which develop externally. In these cases a considerable number of eggs are produced simultaneously by a single zooecium.M. Prouho further throws light on a much contested subject; namely, the nature of the so-called "intertentacular organ" (i, Fig. 234, p.469), described so long ago as 1837 by Farre,[573]but looked for in vain by the majority of later observers.
The failure to find this organ, even in species which possess it,in certain individuals, according to Farre's statements, is now satisfactorily explained by M. Prouho, who shows that while it is absent in a large number of polypides, it is normally present in those individuals which possess an ovary, and in those only; and that its primary function is that of an oviduct.
The intertentacular organ is an unpaired ciliated tube, which is situated between the two tentacles which are nearest to the ganglion. In the retracted condition of the polypide, it opens from the body-cavity into the tentacle-sheath; and in the expanded condition, directly to the exterior.
In the remarkable case ofAlcyonidium duplex, each zooecium normally possesses two sexual polypides. The first of these produces a testis and then becomes a "brown body." The second is meanwhile developed, and produces an ovary and an intertentacular organ, a structure which was not present in the male polypide. The eggs pass through the intertentacular organ into the tentacle-sheath, and attach themselves to the diaphragm (d, Fig. 234), where they remain during their development.
Although the intertentacular organ has been found by Prouho in female polypides only, it would perhaps be going too far to assert that it is confined to polypides of that sex. Hincks[574]has observed the passage of spermatozoa in enormous numbers through the organ, although it may be noted that there is no sufficient proof that eggs were not present as well in these zooecia. It further appears that in some cases waste matters may be removed from the body-cavity through the same passage.
It may be presumed that the egg is normally fertilised by a spermatozoon, although this is at present largely a matter of inference. It is believed by Joliet[575]that fertilisation is reciprocal, although Prouho has come to the opposite conclusion. Joliet has, however, very justly pointed out that the enormous number of spermatozoa developed by a single individual would be disproportionately large, if their function were merely to fertilise theovum in the same zooecium. According to his view, the egg is fertilised by a spermatozoon after it has passed into the tentacle-sheath or ovicell, or some other place where it is in free communication with the outside water.
Development and Affinities.—Few parts of the history of the Polyzoa are more fascinating than that which deals with their development; and it is probable that no other is capable of giving so much insight into the affinities of the several groups to one another and to other groups of the animal kingdom.