Chapter XXV: Introduction: General Characters and Terminology—brown Bodies—history (2)

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 genus _Pectinatella_ also has some power of locomotion. This magnificent Polyzoon occurs in masses several feet in length (as much as six feet in _P. gelatinosa_ from Japan[555]), and four to eight inches in thickness. The greater part of _P. 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. _Pectinatella_ has 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.

It is by no means certain what is the mechanism by which {498}movement takes place in the above cases. The ectocyst of _Cristatella_ is 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 The polypides are withdrawn by means of retractor muscles, which originate from the septa and inner surface of the sole.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, in _Cristatella_, 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 its {499}retractor-muscles or its sphincter in a state of contraction in order to remain in that position. And as a matter of fact, _Cristatella_ and _Lophopus_ differ from most other Polyzoa in the readiness with which they expand their tentacles, after they have been induced to retract themselves by mechanical irritation.

_Plumatella_ and 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.

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_). These {500}serve 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 (_Bowerbankia_ and _Farrella_). 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) in _Membranipora_ (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. {501}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 in _Microporella_ (Fig. 241, A, _mp_), and in certain other cases.

REPRODUCTION OF PHYLACTOLAEMATA.—Sexual reproduction takes place in _Cristatella_ from 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 develops _in 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, _Paludicella_ and _Victorella_. These bodies are in the former (Fig. 250, B) specially modified _external_ buds, which persist through the winter when the rest of the colony dies down. At the close of winter the shell splits into two {502}halves, 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 of _Fredericella_ (Fig. 251, A), which differs from that of other Phylactolaemata in having no ring of air-cells. In _Plumatella_, 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. In _Lophopus_ (Fig. 251, C) the ring of air-cells is very broad, and is pointed at each end; while in _Cristatella_ (Fig. 251, D) and in _Pectinatella_ the statoblast is circular, and possesses an armature of hooked spines. That of _Cristatella_, measures about .75 mm. in its greatest length.

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, in _Fredericella_, 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.

{503}In _Plumatella_, 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 resembles _Fredericella_, with which it may easily be confused.

In _Cristatella_ we 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.

The production of _floating_ statoblasts 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.

{504}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_, and _Lophopus_ are further recorded from Australia; while _Plumatella_ is known to occur also in Malacca, the Philippine Islands, India, Japan, Africa, and South America, It is even stated that some of the Australian _species_ are 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 as _Cristatella_ and _Lophopus_, are comparatively constant in their form, _Plumatella_ is excessively variable. _Plumatella_ has a number of species greater than that of any other form, and the genus has a wider distribution than any {505}other. 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 of _Plumatella_ consist 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 that _A. fungosa_ is only a variety of an ordinary _Plumatella_ form. Whether this is so or not, a typical _Plumatella_ may in places take on an Alcyonelloid habit; and parts of an _Alcyonella_ may become so lax in growth as to resemble a _Plumatella_.

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
1. { 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_
2. { Branches composed entirely of club-shaped zooecia, each of which
{ may give off two zooecia near its upper end
{ _Paludicella_ (Fig. 250)

{ Tubes hyaline or opaque, usually containing numerous oval
{ statoblasts (Fig. 251, B), most of which have a ring of
{ air-cells. Lophophore horse-shoe shaped.
{ (_a_) Tubes divergent _Plumatella_ (Fig. 246, B)
3. { (_b_) Tubes parallel with one another _Alcyonella_ form of
{ _Plumatella_ (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
4. { 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_

{506}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. In _Loxosoma_, 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 of _Cristatella_ and _Lophopus_, 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 genus _Loxosoma_ is perhaps always dioecious (_i.e._ with separate sexes). _Pedicellina_ is 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.

{507}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 in _Pedicellina_ (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 of _Alcyonidium_, 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. {508}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 of _Alcyonidium 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 the {509}ovum 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.

The comparative study of the larvae of the Polyzoa may be said to date from 1877, when J. Barrois published an elaborate Monograph[576] on this subject. Although some of Barrois' earlier opinions have been subsequently modified, this work still gives the best figures of the external form of the beautiful larvae of many genera. A detailed account of the larval forms of Polyzoa must be omitted from want of space; and the general conclusions only can be given.

{510}The larvae of the Entoprocta (Fig. 252, A) resemble the so-called "Trochosphere" of Polychaeta (see p. 274). The common characters shared by the larvae of Chaetopoda, Echiuroid Gephyrea, Mollusca, and Polyzoa, and by adult Rotifera, may well point to the derivation of these groups from a common ancestor. On this assumption, it is possible that the Polyzoa have been derived from forms which existed long ages ago, which combined the common characters of these groups, and the structure of which we can picture to ourselves only so far as the "Trochosphere" larva can be taken to represent it in a much simplified condition. Such a view harmonises well with the great antiquity of the Polyzoa. Certain Ectoproct forms have a larva, known as _Cyphonautes_ (Fig. 252, B), which closely resembles the larval form of the Entoprocta; and it is a fact which probably has considerable significance that this type of larva is known to occur only in those species of _Membranipora_ (_Electra_), _Alcyonidium_, and _Hypophorella_, which lay eggs.[577] This may perhaps be regarded as a primitive form of development which has been lost in species in which development takes place inside the parent. _Cyphonautes compressus_ (Fig. 252, B), one of the commonest objects taken in the surface-net off our own coasts, is the larva of _Membranipora_ (_Electra_) _pilosa_. Whilst this larva is provided with a well-developed alimentary canal, those of most other Ectoprocta possess a mere rudiment of this structure, and depend for their nutrition either on yolk present in the egg or on material supplied by the parent. In most cases the mature larva has no recognisable trace of a digestive system; and, although it has a free-swimming period, it does not become truly pelagic.

The alimentary canal of the larva of _Pedicellina_ is known to persist in the primary individual of the colony. In all other known cases, even in that of _Cyphonautes_, the larva at fixation loses practically all its internal organs, and becomes a mere body-wall containing a mass of degenerated larval tissues. It is in fact a zooecium containing a "brown body." A polypide-bud is now developed, the body-cavity appears as the result of the shrinkage of the "brown body," and the primary individual of the colony is thereby established.

The larvae of the Ectoprocta form a tolerably complete series, starting from _Cyphonautes_, itself allied to the larva of the {511}Entoprocta, and ending with the Phylactolaemata. _Alcyonidium_ (Fig. 253, B) possesses a rudimentary alimentary canal,[578] although the most conspicuous structures are those connected with the fixation and other phenomena of larval life. The larvae of many of the encrusting Cheilostomes (Fig. 253, A) resemble that of _Alcyonidium_, while those of _Bugula_, _Scrupocellaria_, etc., belong to a type easily derivable from that of the encrusting forms. The branching Ctenostomes (_Bowerbankia_, etc.) have a larva which may be regarded as derived, along slightly different lines, from that of _Alcyonidium_. The Cyclostomata and the Phylactolaemata have the most modified forms of larva. That of the former group may owe some of its peculiarities to the occurrence of a remarkable process of embryonic fission, which takes place in the ovicell, and as the result of which each egg gives rise to a large number of larvae.[579] The Phylactolaemata have a larva which is not unlike that of _Bowerbankia_.

We have seen that the larva at fixation becomes a zooecium, {512}which in the Gymnolaemata forms a polypide-bud after fixation. The peculiarities of the Phylactolaematous larva may be explained by assuming that it becomes a zooecium while it is still free-swimming. Thus the larva of _Plumatella_ develops one or sometimes two polypides, which actually reach maturity before fixation takes place. That of _Cristatella_ develops from two to twenty[580] polypides or polypide-buds at the corresponding period, and it is in fact a young colony while still free-swimming.

Now in most colonial animals, such as Coelenterates and Ascidians, the larva metamorphoses itself into a temporarily solitary animal, which then gives rise to the remainder of the colony by budding. The majority of the Gymnolaemata behave in this way; while the Phylactolaemata may not only develop a multiplicity of polypides in their larval stage, but the individuality of the zooecia is then just as much obscured as in the adult state. These facts are more easily explained if we assume that _Cristatella_ is the end-point in a series than if we suppose it to be a starting-point.

On the view maintained by many authorities, that the Polyzoa are related, through _Phoronis_, with the Gephyrea and the Brachiopoda, we should expect to find in those Polyzoa which most closely resemble _Phoronis_ in their adult state—that is to say in the Phylactolaemata—some indications of affinity to that animal in their development. This is emphatically not the case. The hypothesis that the Phylactolaemata are related to _Phoronis_ leads, moreover, to the improbable conclusion that the similarities between the Entoproct-larva and _Cyphonautes_, on the one hand, and the Trochosphere larva of Polychaeta, on the other hand, is entirely superficial and meaningless. In spite, therefore, of the similarity between _Phoronis_ and a single individual of the Phylactolaemata, and in spite of the marked resemblance between its nephridia and structures which have been described in _Cristatella_[581] and _Pectinatella_[582] the comparative study of the development appears to indicate that the resemblances between _Phoronis_ and the Phylactolaemata are the result of a coincidence rather than of any close relationship.

A few points connected with the metamorphosis of the {513}Polyzoa deserve more special notice. There is generally great difficulty in persuading larvae to fix themselves when kept in a small quantity of water, which becomes over-heated in the air of a laboratory. The difficulty may be surmounted by placing colonies containing embryos, together with some clean pieces of the seaweed on which the adults are habitually found, in a vessel closed by a piece of fine muslin, and by leaving the vessel attached to a buoy or in a deep tide-pool. The larvae being without an alimentary canal, fix themselves, after a very short free life, on the seaweed.

It is probable that a great struggle for existence normally takes place at the commencement of the metamorphosis. Any one who will examine, in June or July, rocks covered by _Fucus_ on which _Flustrella hispida_ is growing, will probably find numerous young fronds of _Fucus_, from half an inch to an inch or two in length, growing under the shelter of the older fronds. The bivalve larvae of _Flustrella_ show a marked preference for fixing on these young fronds—perhaps in order that the duration of life of the colony may coincide with that of the _Fucus_—and these young fronds are commonly covered by very numerous recently-fixed larvae, and by young colonies of various ages. Or, it is easy to observe, by placing pregnant colonies of _Bowerbankia_ in a vessel of water, that the larvae, which are hatched out in thousands, fix themselves in dense masses on certain parts of the wall of the vessel. It is clear that but a small proportion of these larvae will find room for further development.

Next with regard to the mode of fixation. Attachment always takes place by the surface on which the mouth or its rudiment is situated, and the permanent alimentary canal opens on the opposite surface. In _Pedicellina_, the one case in which the larval digestive organs are known to become those of the first adult individual, this presupposes a rotation of the alimentary canal, in order to bring it into its new position.

It is well known that the larvae of other fixed animals may undergo a somewhat similar change. Thus those of Ascidians and of Barnacles fix themselves by their anterior end, and ultimately reach their adult form by performing a kind of a somersault. The process may perhaps be explained by supposing that some part of the anterior end or of the oral surface is specially sensitive, and that the larva fixes itself by that portion of its {514}body which is best fitted for ascertaining which is the proper substance on which to fix.

BUDDING.—The formation of a new individual may take place by the outgrowth of part of the body-wall, as in _Pedicellina_ (Fig. 243, p. 487) and in _Bowerbankia_ (Fig. 238, p. 480). In _Pedicellina_ a young stalk is formed by an outgrowth near one of the growing points, and the upper part of this outgrowth becomes constricted off to form the calyx. In other cases (cf. the growing ends of the branches in Fig. 237) a partition grows across the body-cavity at the growing edge of the colony, and so cuts off a part destined to become a new zooecium.

The zooecium formed in one of these ways acquires an alimentary canal by the formation of a polypide-bud, some stages in the growth of which are shown in Fig. 235 (p. 472). Contrary to what happens in Coelenterates and Tunicates, in which the endoderm takes part in the budding, there is good reason for believing that in Polyzoa the polypide-bud is developed entirely from ectoderm and mesoderm.[583] The bud is a two-layered vesicle, attached to the inner side of the body-wall. Its inner layer is derived from the ectoderm, which at first projects into the body-cavity in the form of a solid knob surrounded by mesoderm-cells. A cavity appears in the inner, ectodermic mass, and the upper part of the vesicle so developed becomes excessively thin, forming the tentacle-sheath, which is always developed in the condition of retraction. The lower part becomes thicker; its inner layer gives rise to the lining of the alimentary canal, to the nervous system, and to the outer epithelium of the tentacles, which grow out into the tentacle-sheath (cf. Fig. 235). The outer layer gives rise to the mesodermic structures, such as the muscles, connective tissue, and generative organs.

These processes are fundamentally similar, whether in the metamorphosed larva, in a young zooecium, in an old zooecium after the formation of a "brown body," or in the germinating statoblast of the Phylactolaemata.

{515}CHAPTER XIX

POLYZOA (_continued_)

CLASSIFICATION—GEOGRAPHICAL DISTRIBUTION—PALAEONTOLOGY—METHODS FOR THE EXAMINATION OF SPECIFIC CHARACTERS—TERMINOLOGY—KEY FOR THE DETERMINATION OF THE GENERA OF BRITISH MARINE POLYZOA

Our account of the Polyzoa would be manifestly incomplete without some reference to the systematic arrangement of these animals. An outline of the principal groups has been given on p. 475. So far, the classification is easy, but it is otherwise when we attempt to subdivide most of the groups any further.

Systems of classification which depend exclusively upon the external characters of animals have been repeatedly shown to be unsatisfactory. Now with regard to the Polyzoa, not only is it the case that the great majority of forms are only known in their external characteristics, but current systems of classification cannot be regarded as final, because it is not yet certain which of the external features have most systematic value. Two obvious points can be at once selected—namely, the character of the zooecium and the character of the entire colony. One or two instances will serve to show what different results are obtained by depending exclusively on either of these characters by itself.

According to the older writers, the habit of the colony was taken as the most important generic character; and there can indeed be no doubt that this feature has great importance within certain limits. Any one who has examined different species of such genera as _Flustra_, _Cellaria_, _Bugula_, _Retepora_, etc., must feel that the form of the colony goes for a good deal. But a consideration of other cases shows that there is great risk in the {516}indiscriminate use of this method of arranging the Polyzoa. The old genus _Eschara_, composed of forms with an erect coral-like habit,[584] included species which are now placed in such different genera as _Lepralia_, _Porella_, _Microporella_, etc. The older works on Polyzoa include all encrusting forms of Cheilostomata, with a completely calcareous front wall, in the genus _Lepralia_, the members of which are now distributed in numerous widely separated genera.

As an instance of the converse arrangement—essential similarity of the zooecia with great differences of the general habit—may be mentioned the common _Membranipora_ (_Electra_) _pilosa_.[585] Ordinarily growing in the form of close encrustations on seaweeds, this species may take on entirely different habits of growth. The zooecia are now dissociated, growing in single lines over the substratum; now forming erect tufts, composed of single lines of zooecia or of several rows. The erect, branching habit appears to be induced in the first instance by the character of the seaweed on which the colony begins life. Thus colonies which encrust the thin branches of _Corallina_ may have impressed on them something of the mode of growth of the seaweed, so that when they extend beyond the tips of the branches of the _Corallina_, they continue to grow in delicate branches, which still retain more or less the same diameter as those which form their base. An extreme variation results in the beautiful form known as _Electra verticillata_, in which the zooecia are arranged with great regularity in whorls, which together form erect branches.[586] But with all these variations, the zooecia are so much alike that it is hardly possible to regard the extreme forms as more than varieties of a single species. A careful examination of this case would convince most observers that the characters of the zooecium are a more trustworthy guide to classification than those of the entire colony, a result which was first clearly stated by Smitt, and amply confirmed by Hincks.[587]

The avicularia of the Cheilostomata afford useful help in classifying this group; but while certain genera are always provided with avicularia, others include some species with these organs, and other species without them. Again, while the species {517}of some genera (e.g. _Cellepora_) possess a great variety of forms of avicularia, the same pattern of avicularium may characterise several widely different genera. Further, the _position_ of the avicularium may be very different in species which are apparently closely related. Well-developed vibracula, although constant in their occurrence in such forms as _Scrupocellaria_ (Fig. 254) and _Caberea_ (Fig. 242), occur here and there in species of encrusting forms which are ordinarily placed in very different families.

Now although some of these discrepancies are perhaps due to errors in classification, whereby species which are really allied have been wrongly placed in distinct genera, this explanation would not prove satisfactory in all cases. Thus in _Bugula_, a genus which is specially characterised by the high development of its avicularia, these organs are normally absent in _B. neritina_. The fact that this species was rightly placed in the genus has been confirmed by the discovery made by Waters[588] that avicularia occur in specimens which are believed to be identical with that species.

1. The Cyclostomata appear to fall naturally into two main groups, (A) the ARTICULATA, including the Crisiidae (Fig. 237), distinguished by their erect branches, divided at intervals by chitinous joints; and (B) the INARTICULATA, which include the remaining families, whether erect or encrusting, agreeing in the negative character of being unjointed.

{518}2. The Cheilostomata consist of (A) the CELLULARINA, including the flexible, erect forms, such as _Bugula_ (Fig. 233) and _Scrupocellaria_ (Fig. 254); (B) the FLUSTRINA, to which belong _Flustra_ (Fig. 232), _Membranipora_ (Fig. 256, A, B), _Micropora_ (Fig. 256, C), and other forms in which the front wall of the zooecium is either membranous, or depressed and marked off by a ridge-like margin; (C) the ESCHARINA, including the great majority of forms, in which no part of the front wall remains membranous, the wall of the zooecium being wholly calcified.

3. The Ctenostomata comprise (A) the ALCYONELLEA or encrusting forms; and (B) the VESICULARINA or branching forms. The zooecia in the latter subdivision (Fig. 238) are given off from a tubular stem or stolon, which is usually erect and branching.

We thus have the following arrangement of recent forms. The genera mentioned are for the most part those which have already been alluded to in the preceding account:—

Sub-class I. ENTOPROCTA.
_Loxosoma_, _Pedicellina_, _Urnatella_.

Sub-class II. ECTOPROCTA.
Order 1. Gymnolaemata.
Sub-order 1. Cyclostomata.
A. Articulata. _Crisia_.
B. Inarticulata. _Hornera_, _Idmonea_, _Tubulipora_, _Stomatopora_,
_Diastopora_, _Entalophora_, _Lichenopora_.
Sub-order 2. Cheilostomata.
A. Cellularina. _Aetea_, _Eucratea_,[589] _Catenicella_,
_Cellularia_, _Gemellaria_, _Menipea_, _Scrupocellaria_,
_Caberea_, _Notamia_ (= _Epistomia_), _Bicellaria_, _Bugula_,
_Beania_.
B. Flustrina. _Cellaria_, _Flustra_, _Membranipora_, _Electra_,
_Lunulites_, _Membraniporella_, _Cribrilina_, _Micropora_,
_Selenaria_.
C. Escharina. _Retepora_, _Microporella_, _Lepralia_, _Porella_,
_Smittia_, _Mucronella_, _Schizoporella_, _Schizotheca_,
_Mastigophora_, _Porina_, _Cellepora_.
Sub-order 3. Ctenostomata.
A. Alcyonellea. _Alcyonidium_, _Flustrella_.
B. Vesicularina. _Vesicularia_, _Amathia_, _Bowerbankia_,
_Farrella_, _Hypophorella_, _Triticella_, _Mimosella_,
_Victorella_, _Paludicella_.
Order 2. Phylactolaemata.
_Fredericella_, _Plumatella_ (including _Alcyonella_), _Lophopus_,
_Cristatella_, _Pectinatella_.

{519}Even this classification, which deals only with the larger groups, must not be made use of without a word of warning. The division of the Cheilostomata is a matter of great difficulty; and no scheme which has yet been suggested can be regarded as more than tentative. The great number of forms included in this group makes its subdivision extremely desirable from the point of view of convenience; but a further knowledge of the anatomy and of the development of many of the forms of doubtful systematic position is probably necessary before any scheme which is likely to be permanent is put forward. Those who desire to make a further study of the classification of the Polyzoa should refer to the works of Hincks,[590] Busk,[591] MacGillivray,[592] and Gregory.[593]

The Polyzoa do not appear to lend any valuable assistance towards settling the disputed problems of Geographical Distribution. They are not in any case terrestrial, while the fresh-water species do not always respect the limits between the great zoogeographical regions. It has already been pointed out (p. 504) that _Plumatella_, _Fredericella_, and _Lophopus_ are believed to occur in Australia, and the first-named genus is practically world-wide in its distribution.

Many marine forms also have a surprisingly wide distribution. Thus among the British species which are described by Mr. Hincks as occurring from Norway to New Zealand are _Membranipora pilosa_, _Scrupocellaria scruposa_, _Cellaria fistulosa_, _Microporella ciliata_, and _M_. _malusii_. Even if it should be proved that specific differences do exist between the southern forms and our own, there can be no doubt of the wide distribution of certain species. It was pointed out by D'Orbigny that _Bugula neritina_ has the habit of attaching itself to the bottoms of ships, a fact which may possibly account for the wide distribution of this species; although it would not be safe to assume this explanation of the facts in all cases. Other Polyzoa, on the contrary, have a more restricted range. Thus _Catenicella_ is specially characteristic of the Australian region.

It is perhaps surprising that marine Polyzoa should in so many cases have so wide a range. Even though it is the rule {520}for Polyzoa to have free larvae, the period during which these larvae are free-swimming is, so far as is known, a short one in most cases. _Cyphonautes_ is a common pelagic form (see p. 510), and probably remains for a considerable period in the larval condition. Other Polyzoon-larvae appear to fix themselves very soon after their birth; and this would not appear to give much time for them to be carried to great distances by ocean-currents. It may, however, be suggested that it does not follow that because we know that a larva may, under favourable conditions fix itself a few minutes after it becomes free, we should be justified in assuming that that larva would not retain for a long period the power of undergoing a normal metamorphosis should it be drifted away from suitable fixing-grounds.

PALAEONTOLOGY.[594]—The number of fossil Polyzoa is enormous. D'Orbigny devoted two hundred plates and more than a thousand octavo pages[595] to a Monograph on the Cretaceous Polyzoa of France. Many of the fossil forms are extraordinarily well preserved, and there is often no difficulty in recognising the identity between certain fossil species belonging to the more recent formations and living forms. It thus becomes necessary to consult Palaeontological memoirs in working at recent Polyzoa.

While the great majority of fossil Polyzoa do not differ in any essential particular from recent species, this is not altogether the case with the Palaeozoic forms. Leaving out of account the Stromatoporoids, which have been variously referred to the Sponges, Hydrozoa, and Foraminifera, as well as to the Polyzoa, the Palaeozoic strata contain large numbers of peculiar Cyclostomata, together with members of the Trepostomata, a fourth Sub-order of Gymnolaemata, allied to the Cyclostomata. The Trepostomata are for the most part Palaeozoic, but a few survived as late as the Jurassic period.[596] These, with the other Polyzoa from the same formations, are considered by Dr. Gregory in his recently published _Catalogue of the Fossil Bryozoa in the British Museum_ (1896).

The number of Polyzoa recorded from the earlier secondary strata is small. The majority of the known Jurassic forms {521}belong to the Cyclostomata; and one or two Cheilostomes are recorded from the same period. Recent papers by Walford[597] on Jurassic Polyzoa contain the description of genera which are believed to be intermediate between the Cyclostomata and Cheilostomata, particularly with regard to the characters of their ovicells. Although it is not impossible there may be a connection between the ovicells of these two groups, it has yet to be proved that the two sets of structures are homologous.

The Cretaceous period marks the commencement of a large number of Cheilostome genera, although the Cyclostomes still remain numerous.

In the Tertiary formations the Cyclostomes gradually become less numerous, and although in earlier geological periods they far outnumbered the Cheilostomes, these relations are now reversed. Certain Tertiary strata, and particularly the Coralline Crag (Pliocene), are remarkable for the extremely large number of Polyzoa they contain. It will be noticed that no mention has been made of the Entoprocta, the Ctenostomata, and the Phylactolaemata. Their absence in the fossil condition[598] need not, however, be a matter for surprise, as none of these forms are so well suited for being fossilised as are the calcareous Cyclostomata and Cheilostomata. There is consequently no adequate reason for assuming that the absence of a palaeontological record implies that these groups have been recently evolved.

DETERMINATION OF GENERA OF MARINE POLYZOA.—The species to which a Polyzoon belongs can only be determined, in most cases, with the assistance of the low powers of a microscope. There are very great advantages in the use of a binocular instrument, by means of which a microscopic preparation appears with its parts standing up in proper relief.

In the case of the calcareous forms, the external characters may be more readily made out in a dry preparation than in any other way. For this purpose, the colony should be washed with fresh water, in order to remove the salts, which otherwise crystallise out on drying and obscure the surface. Preparations of this kind must be looked at with the aid of reflected light. Canada-balsam or glycerine preparations are also valuable, whether {522}stained or unstained; and are essential for the examination of the softer forms. In the case of erect species, both surfaces of the branch should be looked at. The opercula, avicularia, and rosette-plates afford important systematic characters in the case of the Cheilostomata.

It must not be forgotten to take account of the condition of the zooecia at different ages. The old zooecia often become entirely altered in form, by the deposition of additional calcareous matter, or by the loss of certain parts present in the younger zooecia. Thus the marginal spines may be entirely lost in the older individuals, while in those forms which develop a "peristome" (see Fig. 255 and p. 524), the characters of the orifice can often be determined in the young zooecia only. It is thus essential to examine the growing ends of the branches or the rim of the colony, as the case may be.

In order to make preparations with the tentacles expanded, hydrochlorate of cocaine, chloral hydrate or spirit should be added gradually to the water. When the animals are completely anaesthetised they may be killed by means of a 7-10 p.c. solution of sulphate of copper (best made in distilled water or in rain water). This method gives admirable results in the case of both {523}fresh-water and marine Polyzoa. The use of formaline (see p. 229) may be strongly recommended for the Vesicularina.

The only recent work dealing with all the marine BRITISH FORMS is Mr. Hincks' invaluable _History of the British Marine Polyzoa_.[599] As the use of this book, unaided by any artificial help, is by no means easy to the beginner, the following key has been compiled as an index to the genera. The Entoproct forms, _Loxosoma_ and _Pedicellina_ (see pp. 488-491), are not included in the table.

In order to facilitate the use of the table here given in conjunction with Mr. Hincks' work, the nomenclature there adopted has been followed throughout. References to other descriptions of the species may be obtained by consulting Miss Jelly's admirable _Synonymic Catalogue of the Recent Marine Bryozoa_.[600]

TERMINOLOGY.—A few technical terms must of necessity be employed. The colony is _adherent_ when its zooecia are attached to the object on which the colony is growing. The _zooecium_ is the body-wall of a single individual; and, except in transparent species, is the only part which can be seen from the outside in the retracted condition of the _polypide_ or tentacles with the alimentary canal. The outermost layer of the zooecium is known as the _ectocyst_; it may be simply membranous, or calcified, or may be rendered opaque by foreign bodies; its surface in {524}calcareous forms is often marked by _pores_ (Fig. 239, C, _p_), which are vacuities in the calcareous wall, closed externally by membrane. A special _median pore_ (Fig. 241, A, _m.p_) may occur, and is in some cases at least a complete perforation through the body-wall.

The tentacles are protruded through the _orifice_, which in Cheilostomata is usually guarded by a movable chitinous lid, or _operculum_ (Fig. 256, A, _o_). Should the ectocyst be thickened or raised into a ridge surrounding the orifice, a tubular passage results, known as the _secondary orifice_ (Fig. 255), at the deeper end of which is the true orifice. The _peristome_ (Fig. 255, C, _pr_) is the raised or thickened part which gives rise to the secondary orifice. Should the zooecium be outlined by a raised ridge, the part so enclosed is known as the _area_ (Fig. 256, C, _a_), if calcareous. The _aperture_ or _opesia_ (Fig. 256, A, B, _ap_) is a membranous part of the front surface; and may consist of the whole or part of the area. The orifice or the aperture is commonly provided with _spines_ (Fig. 256, B, _s_).

The _avicularium_ and the _vibraculum_ are specially modified zooecia (see p. 482), which occur in a great variety of forms, in certain Cheilostomata only. The operculum of the ordinary zooecium is represented by the _mandible_ (Fig. 239, B, _m_) in the avicularium, and by the _seta_ (Fig. 242, _s_) in the vibraculum. The representative of the zooecium itself is known as the _avicularian_ (Fig. 239, A, _a.z_) or _vibracular zooecium_ (Fig. 242, _v.z_).

{525}An _ovicell_ is a swelling in which the embryo develops, in certain Cyclostomata (Fig. 237) and Cheilostomata (Fig. 241, A, _o_). A _stolon_ (Fig. 238, B, _st_) is a stem, not formed of fused zooecia, from which new individuals originate. An _internode_, in a jointed colony, is the part between any two joints. The _fornix_ or _scutum_ (Fig. 254, A, _f_) is a modified spine which in some Cheilostomata overhangs the aperture. A _mucro_ (Fig. 255, A, _mu_) is a spike or protuberance developed just below the orifice. A _sinus_ (Fig. 239, B, _s_) is a slight bay on the lower margin of the orifice.

The orifice opens at the _upper_ end of the zooecium, on its _front_ surface. The _length_ of the zooecium is the distance from the upper to the lower ends, and the _width_ the distance between its sides.