Chapter XXIV: Introduction: General Characters and Terminology—brown Bodies—history (1)

OUTLINES OF CLASSIFICATION—MARINE POLYZOA—OCCURRENCE—FORMS OF COLONY AND OF ZOOECIA—OVICELLS—AVICULARIA—VIBRACULA—ENTOPROCTA.

The following pages[500] deal with animals whose very existence is hardly known to those who are not professed naturalists. There are but few Polyzoa which have earned the distinction of possessing a popular name, and most of such names as do exist cannot be found outside treatises on Natural History. It is true that many of the members of this group have been vaguely termed "Zoophytes"; but this term implies no more than that they possess a superficial resemblance to certain plants, and it must be remembered that this habit of growth is assumed by many animals which have nothing to do with the Polyzoa. The term "Coralline" is sometimes applied to those calcareous Polyzoa which grow into coral-like forms; and the Tertiary deposit known as the "Coralline Crag" is so called from the large number of fossil Polyzoa which it contains.

The Polyzoa are none the less a most attractive group. Let any one examine a dry piece of a brown paper-like substance (Fig. 232, A), which may be found thrown up on the beach on many parts of our coasts. Of this species (_Flustra foliacea_), the {466}so-called "sea-mat," an old writer says: "For curiosity and beauty, I have not, among all the plants or vegetables I have yet observed, seen any one comparable to this seaweed."[501] Viewed with the microscope, the frond is seen to consist of two layers, placed back to back, of oblong chambers, each of which is the dried body-wall of a single individual. The whole is obviously a _colony_, and to this fact the term Polyzoa refers.

The chambers just noticed are termed "zooecia." Each is rounded at one end, near which is the "orifice," through which the tentacles of the living animal can be pushed out. Two short, stiff spines usually occur on each side of the orifice; and the symmetry of this forest of spines fully justifies the above-quoted remark.

The upper part of some of the zooecia is somewhat swollen, these swellings representing the conspicuous "ovicells" of many other genera. In the early part of the year each ovicell protects an orange-coloured egg or embryo, and the larvae are readily liberated if the fresh colony be placed in clean sea-water. "At least ten thousand" were hatched out in three hours from a colony placed in a glass by Sir John Dalyell.[502] The larva swims freely in the water for a short time, and should it find a {467}suitable resting-place, it fixes itself and forms the starting-point of a colony, the number of whose individuals is continually increased by the production of buds at the growing edge. The "avicularia" of this species will be alluded to later (see p. 482).

_F. foliacea_ has long been known to possess in the fresh state a remarkable odour, which is described, according to the fancy of the observer, as a strong odour of fish, or as the smell of violets after a shower. Others have compared it to that of the orange or verbena, or to that of a mixture of roses and geranium.

_Flustrella hispida_, another of our commonest Polyzoa, which may be found between tide-marks on the stalks of _Fucus_, consists of a softish brown encrustation, about one-sixteenth of an inch thick, covered by numerous spines. If examined undisturbed in a rock-pool, or transferred to a glass of sea-water, the brown mass will be seen to become surrounded by a delicate bluish halo, which is about as thick as the encrusting mass itself, and consists of the tentacles of the numerous individuals of the colony. The microscope shows that each individual is provided with a circlet of some thirty or more long, delicate tentacles, which together form a graceful funnel (as in Fig. 233). At the bottom of the funnel is the mouth, to which Diatoms or other minute organic particles are conveyed by the cilia which fringe the tentacles. If the tentacles be touched with a needle, the whole funnel is retracted with great rapidity, and in this retracted condition we see no more than the body-walls of the animals. After an interval the tips of the tentacles are cautiously protruded; the tentacles are _gradually_ pushed out, at first in a close bundle, but finally separating from one another to form the funnel which we have already noticed.

There is hardly a more surprising spectacle in the whole animal kingdom than a living fragment of the genus _Bugula_. The colony grows in the shape of a small tree, whose height may amount to several inches; and is characterised, in many species, by a spiral arrangement of the branches, which makes the genus easy to recognise at first sight (Fig. 233, A). The stem and branches are composed of a single layer of zooecia, arranged two or more abreast. Each zooecium bears, on its outer side, a most singular body termed an avicularium, from its resemblance to a bird's head. Imagine a minute eagle's head attached by a short but flexible neck to the zooecium. Suppose further {468}that this structure moves backwards and forwards in a deliberate but determined fashion, its lower jaw usually widely open so as to be nearly 180° distant from its position when closed. Suppose that the lower jaw is moved by powerful muscles which can be distinctly seen inside the transparent head of the avicularium, and that every now and then it closes with a snap, seizing any unfortunate worm which may happen to be within reach with a grasp of iron. The above gives a very faint idea of the appearance of a living _Bugula_ colony, with its hundreds of swaying avicularia, and with its tentacular funnels protruding from their zooecia, and withdrawing themselves capriciously from time to time.

GENERAL CHARACTERS.—The Polyzoa are colonies, leaf-like or tree-like in form, and often strongly resembling seaweeds, or forming encrustations on the surface of stones and water-plants, or taking on other shapes. The units of the colony are complete individuals (Fig. 234). The zooecium or body-wall encloses a body-cavity, in which lies a digestive canal, with which are closely connected the central nervous system and the retractile, ciliated tentacles. The structures other than the zooecium constitute the "polypide." The mouth (_m_) leads into the ciliated pharynx (_ph_) which is followed by the oesophagus (_oe_) which again passes into the stomach (_s_), whose walls are coloured by a {469}characteristic yellowish pigment. The stomach gives off the intestine (_in_), which is lined by strong cilia, by means of which a rotatory movement is given to the faeces contained in it. This communicates by a narrow passage with the rectum (_r_), which opens by means of the anus (_a_).

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In the retracted condition the tentacles (_tn_) lie in a cavity {470}which opens to the exterior by the orifice (_o_). The cavity is bounded by a thin membrane termed the "tentacle-sheath" (_ts_), and it is incompletely subdivided, near its upper end, by a diaphragm (_d_), perforated by a circular hole through which the tentacles can be protruded. The diaphragm bears the thin folded collar characteristic of the Ctenostomata, the group to which the species figured belongs (see p. 477).

Fig. 238, B, shows the tentacles of _Bowerbankia_ in their fully expanded and partially expanded condition. Comparing this with Fig. 234, it will be clear that when protrusion is taking place, the tentacles are forced in a bundle, tips first, through the diaphragm and next through the orifice of the zooecium, the alimentary canal offering no resistance to this movement, owing to the length of the oesophagus. A moment's consideration will show that the bases of the tentacles, in passing through the orifice, will carry with them that part of the flexible tentacle-sheath to which they are attached; and it will further be clear that so much of the tentacle-sheath as is thus protruded will be turned inside out. This process of "evagination" continues until its further progress is stopped by the retractor-muscles (_r.m_), and by the parieto-vaginal muscles (_p.v_), which pass from the interior of the body-wall to the upper part of the tentacle-sheath. The latter has now become the delicate layer which connects the expanded tentacles with the zooecium; and the anus (Fig. 238, C, _a_) opens directly to the exterior. Since the name "tentacle-sheath" is thus descriptive of the condition of retraction only, the term "kamptoderm"[504] has been suggested as an alternative name.

The presence of a complete digestive canal and the ciliation of the tentacles in Polyzoa are conspicuous differences between these animals and the Hydroids, with some of which the Polyzoa may have a marked external similarity.

The outermost[505] layer of the body-wall is known as the "ectocyst" (Fig. 234, _e_). This may be densely calcareous, in which case the dried Polyzoon differs little in appearance from the living animal with its tentacles retracted; or it may be partially calcified, or it may consist entirely of a flexible cuticle, {471}as in Fig. 234. The ectocyst is prolonged through the orifice (_o_) as far as the diaphragm (_d_).

Forms with a calcareous ectocyst are commonly ornamented with ridges or other patterns, which are often of great beauty. The ectocyst in these cases is commonly interrupted at intervals by pores (Fig. 239, C), into which processes of the "endocyst"—the living, internal part of the body-wall—extend. These may appear as superficial pores, which apparently open to the exterior in the dried condition, or they may perforate the septa between adjacent individuals. This may be strikingly demonstrated by decalcifying a branch of _Crisia_ (Fig. 237), in which the zooecia then appear connected by numerous strands of tissue. In many marine forms the communications between the individuals are in the form of small sieve-like plates known as "rosette-plates."

The endocyst may consist of definite layers of ectoderm (_em_) and mesoderm (_mm_), as in Fig. 234, but the mesoderm is commonly in the form of a loose network, some of which is attached to the body-wall, some to the alimentary canal, some forming connecting strands between these two layers, and other cells floating about freely in the body-cavity. These mesodermic structures are often spoken of as the "funicular tissue," since one or more strands of it commonly take on the form of a definite "funiculus" (_f_). This structure may bear the ovary (_ov_), while the testes (_t_) are found, commonly in the same zooecium, attached to various parts of the body-wall. The eggs and spermatozoa, when ripe, break off and float freely in the body-cavity.

The funicular tissue was at one time described as a "colonial nervous system." The idea expressed by this term must be considered erroneous from the fact that no nervous co-ordination of the individuals is known to exist, in the vast majority of cases. The actual nervous system consists of a ganglion (_g_) placed between the mouth and anus of each polypide, and lying in a small circular canal (not shown in Fig. 234) which immediately surrounds the oesophagus. This canal is developed in the bud as a part of the body-cavity, from which it becomes completely separated in marine forms. The Polyzoa have no vascular system.

BROWN BODIES.—In the majority of cases, an extraordinary process of regeneration takes place periodically during the life of each zooecium. The tentacles, alimentary canal, and nervous system break down, and the tentacles cease to be capable of being {472}protruded (Fig. 235, 1). The degenerating organs become compacted into a rounded mass (Fig. 235, 2 and 3, _b.b_), known from its colour as the "brown body." This structure may readily be seen in a large proportion of the zooecia of transparent species. In active parts of the colony the body-wall next develops an internal bud-like structure (Fig. 235, 1, _b_), which rapidly acquires the form of a new polypide (Fig. 235, 2 and 3). This takes the place originally occupied by the old polypide, while the latter may either remain in the zooecium in the permanent form of a "brown body," or pass to the exterior. In _Flustra_ the young polypide-bud becomes connected with the "brown body" by a funiculus (Fig. 235, 1, 2). The apex of the blind pouch or "caecum" of the young stomach is guided by this strand to the "brown body," which it partially surrounds (3). The "brown body" then breaks up, and its fragments pass into the cavity of the stomach, from which they reach the exterior by means of the anus.

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There is some reason to believe[507] that these remarkable processes are connected with the removal of waste nitrogenous matters. The Marine Polyzoa are not known to be, in most cases, provided with definite excretory organs, although it is {473}possible that the intertentacular organ (Fig. 234, _i_) described on p. 508 may in some cases perform excretory functions. There can, however, be little doubt that some kind of excretion takes place in the Polyzoa; and in considering what organs could possibly perform this work, our attention is arrested by the alimentary canal. The digestive organs of the young bud are perfectly colourless. As growth proceeds, certain parts acquire a yellowish, and later a brown colour. The degeneration of the polypide is followed by the grouping of large numbers of the free cells of the body-cavity into a mass which closely surrounds the incipient "brown body." Under their action, the latter becomes considerably smaller, probably as the result of the absorption of matters of nutritive value into other tissues. The final result is the formation of the compact "brown body," whose colour is principally derived from the pigment formerly present in the alimentary canal. Experiments made by introducing into the tissues of the Polyzoa certain artificial pigments which are known to be excreted by the _kidneys_ when injected into the bodies of other animals, have given some reason for believing that the appearance of the brown pigment in the wall of the digestive organs is, in part, a normal process of excretion; although that process is not entirely carried out by the organs in question.

Little is known with regard to the duration of life of a single polypide; but some information bearing on this question may be obtained from a set of observations made on _Flustra papyrea_.[508] The table gives the number of days from the time at which the polypides were noticed to commence their degeneration:—

Days.
2 "Brown body" partially formed, the parts of the polypide being still
easily recognisable.
5 Tentacles still just recognisable: appearance of new polypide-bud.
8 Stage shown in Fig. 235, 2.
11 Union of apex of stomach with "brown body."
16 "Brown body" half surrounded by stomach, and preparing to break up
(Fig. 235, 3).
21 "Brown body" broken up into numerous fragments, contained in the
alimentary canal of the new polypide.
35 "Brown body" almost completely absorbed.[509]

{474}These results did not hold good for all the zooecia in a single colony. In some, the "brown body" was not completely got rid of at the end of sixty-eight days, the conclusion of the experiment.

So striking are the facts relating to the "brown bodies" that it has been believed[510] that what we have above described as the individual really consists of two kinds of individuals: firstly, the "polypide" or complex of tentacles and digestive organs; and secondly, the "zooecium," or house of the zooid or polypide, corresponding with what has been described above as the body-wall. The one individual, the zooecium, is on this view provided with successive generations of the second kind of individual, the polypide; and these latter function as the digestive organs of the two-fold organism. This view, though fascinating at first sight, is not borne out by an examination of all the facts of the case, especially when the Entoprocta are taken into account.

HISTORY.—The history of the Polyzoa, as far as 1856, has been fully treated by Allman in his great work on the Fresh-water Polyzoa;[511] but a few words may be said on this subject.

The Polyzoa attracted comparatively little attention before the beginning of the present century. Originally passed over as seaweeds, their real nature was established in connexion with the discovery of the animal nature of corals. So great a revolution could hardly be accepted without a struggle, and even Linnaeus went no further in this direction than to place them in a kind of half-way group of "zoophytes," whose nature was partly animal and partly vegetable. It is hardly necessary to point out that this view has now been abandoned by common consent; and indeed there is no more reason for regarding an animal as showing an approach to the plants because it grows in the external semblance of a seaweed than there would be for supposing a bee-orchid to be allied to the animal kingdom because of the form of its flowers.

But the claims of the Polyzoa to rank as a separate class were by no means admitted with the discovery that they were animals. They were still confounded with Hydroids, Alcyonarians, or Corals until their possession of a complete alimentary canal was recognised as a feature distinguishing them from those {475}animals. This was principally due to the observations of J. V. Thompson[512] in Ireland, who introduced the term POLYZOA; and of C. G. Ehrenberg[513] in Germany, who proposed the class-name BRYOZOA, or moss-like animals.

It is impossible to avoid all mention of the controversy which has raged with regard to these two rival terms. The controversy is for the present at rest, the name Polyzoa being employed by the majority of English writers, amongst whom must be mentioned Allman, Busk, Hincks, and Norman, admittedly authorities of the first rank; while Bryozoa is employed by practically all the Continental writers.

The priority of Thompson's name is unquestioned. While Ehrenberg, however, definitely introduced Bryozoa _as the name of a group_, Thompson was less precise in this respect, although he states[514] that his discovery "must be the cause of extensive alterations and dismemberments in the class with which they [the Polyzoa] have hitherto been associated." Thompson, in fact, clearly understood that the Polyzoa could no longer rank with the Hydroids. The controversy has been summarised by Hincks, in his _History of the British Marine Polyzoa_,[515] where references to other papers on the same subject are given.

The Polyzoa were associated by H. Milne-Edwards with the Tunicata in the group Molluscoidea (Molluscoïdes[516]), to which the Brachiopoda were afterwards added by Huxley.[517] A knowledge of the development of the Tunicata has, however, shown that these animals must be withdrawn from any association with the other two groups; while there is little real evidence that even the Brachiopods have anything to do with the Polyzoa.

CLASSIFICATION.—The Polyzoa are divided into two sub-classes:—I, the ENTOPROCTA; and II, the ECTOPROCTA.[518] Although the character referred to by these terms is merely the position of the anus with relation to the tentacles,[519] there can be no doubt that the two groups differ widely from one another in {476}many important respects. I do not, however, accept the view, maintained by some authors, that the Entoprocta and the Ectoprocta are two separate classes which are not nearly related.

The base from which the whole set of tentacles springs is known as the "lophophore."[520] In the Entoprocta (Fig. 236, 1) the lophophore is circular; the mouth is situated near the margin of the area surrounded by the tentacles; and the anus is found _within_ the circlet, near the end opposite to the mouth.

In (2) and (3), representing the Ectoprocta, the anus is _outside_ the series of tentacles. In the majority of cases, including all the marine Ectoprocta and one or two of the fresh-water forms, the lophophore is circular (2), the mouth occurring at the centre of the circle, and not being provided with a lip. These forms of Ectoprocta constitute the Order GYMNOLAEMATA,[521] the dominant group of the Polyzoa in respect of number of genera and species. The remaining Ectoprocta belong to the exclusively fresh-water Order PHYLACTOLAEMATA,[522] in which the mouth is protected by an overhanging lip or "epistome"; the ground-plan of the tentacles is, except in _Fredericella_, horse-shoe shaped (Fig. 236, 3), and the tentacles themselves are usually much more numerous than in the other cases.

The general characters of these divisions will be more easily understood by referring to the figures given of living representatives of the groups. The Entoprocta are illustrated by Figs. 243-245; the Gymnolaemata by Figs. 238, 240; and the Phylactolaemata by Figs. 247, 248.

{477}The Gymnolaemata include three Sub-Orders:—

1. CYCLOSTOMATA.[523]—Body-wall densely calcareous, the zooecia being
more or less tubular, usually with a _circular_ orifice (Fig. 237).

2. CHEILOSTOMATA.[524]—Body-wall of varying consistency. The orifice is
closed, in the retracted state of the polypide, by a chitinous _lip_ or
"operculum," which is more or less semicircular (Figs. 239, 241).

3. CTENOSTOMATA.[525]—Body-wall always soft. The cavity into which the
tentacles are retracted is closed by a frill-like membrane, the edges of
whose folds have some resemblance to the teeth of a _comb._ This
membrane, the "collar," is seen in different conditions of protrusion or
retraction in Figs. 234, 238. The stomach may, in this group, be preceded
by a muscular gizzard (Fig. 238, C, _g_).

OCCURRENCE.—By far the larger number of the Polyzoa are inhabitants of the sea. A recently published catalogue[526] of marine Polyzoa includes nearly 1700 living species; and of these, the great majority belong to the Gymnolaemata. This group is further known to include an enormous number of fossil forms. Not only do we find that in living Polyzoa the members of a single Order largely outnumber the remainder of the Polyzoa, but we may further notice that the Cheilostomata, one of the sub-Orders of the dominant group, are at present largely in excess of the whole of the rest of the Polyzoa taken together.

Polyzoa may be collected with ease on almost any part of our coasts. The fronds of the "sea-mat" (_Flustra foliacea_) are thrown up by the waves in thousands in places where the bottom is shallow and sandy. The bases of the larger seaweeds growing on rocks between tide-marks are nearly always thickly covered with encrustations of _Flustrella hispida_ or of species of _Alcyonidium_, in places where they are kept moist by being covered with a sufficiently thick layer of other algae. Rocks which are protected from the sun may be coated with calcareous Cheilostomes; and these are also found, in company with branching Polyzoa of various kinds, on the bases of the _Laminaria_ thrown up by gales or exposed at spring tides. The graceful spirals of _Bugula turbinata_ (Fig. 233, A) may be found hanging from the rocks at extreme low water; while colonies of _Scrupocellaria_, remarkable for their vibracula (see p. 484), are common in many places between tide-marks. Certain species affect the mouths of estuaries.

{478}_Membranipora membranacea_ commonly covers many square inches of the frond of _Laminaria_ with its delicate lace-like encrustation. Nitsche[527] has shown that this species has its calcareous matter deposited in plates, separated by intervals of uncalcified ectocyst. The effect of this arrangement is to make the colony flexible, and to enable it to adapt its shape to the movements of the _Laminaria_, which is swayed to and fro by the action of the waves. Many of the calcareous forms growing on _Laminaria_ have no special arrangement of this kind, and they accordingly grow in colonies whose area is so small that the greatest movements to which the seaweed is liable are not sufficient to crack or break the colony.

Many species show a decided, or even exclusive, preference for particular situations; as, for instance, species of _Triticella_, which are only found on certain Crustacea. Many encrusting forms prefer the inside of dead shells of _Pecten_, _Cyprina_, etc., to any other habitat. _Terebripora_[528] excavates tubular cavities in the substance of the shells of Molluscs. _Hypophorella_[529] inhabits passages which it forms in the walls of the tubes of the Polychaets, _Lanice_ and _Chaetopterus_. _Lepralia foliacea_, one of the Cheilostomata, forms masses which may reach a circumference of several feet, simulating a small coral-reef. Its contorted plates are a regular museum of Polyzoa, so numerous are the species which delight to find shelter in the quiet interstices of the colony. The exquisite little colonies of _Crisia eburnea_ are commonly found on red seaweeds, or on the branches of the Hydroid _Sertularia_.

The Polyzoa are found at all depths, certain Cheilostomes having been recorded from 3000 fathoms. The Cyclostomes dredged by the "Challenger" were all found in depths of 1600 fathoms or less, while the Ctenostomes are a distinctly shallow water group, most having been found at less than 40 fathoms, and only three at so great a depth as 150 fathoms.[530]

A few forms (_Membranipora pilosa_, _Scrupocellaria reptans_, etc.) are known to be phosphorescent;[531] but it is not known what is the purpose of this phenomenon.

{479}EXTERNAL FORM.—The Polyzoa may be roughly divided into (1) encrusting forms, usually calcareous, but sometimes soft; and (2) erect forms, which are either rigid or flexible. This flexibility can coexist with a highly calcified ectocyst, as in _Crisia_ (Fig. 237), _Cellaria_, and others in which the branches are interrupted at intervals by chitinous joints. The coral-like forms may assume the most exquisite shapes, pre-eminent among which are the lovely net-like colonies of _Retepora_. Polyzoa of this type are seldom found between tide-marks, where their brittle branches would be liable to be snapped off by the waves. The _erect_ species which occur in such positions are flexible, although flexible species are by no means restricted to the zone between tide-marks.

Although the form of the colony is very different in different Polyzoa, a pocket-lens will usually show whether a given specimen belongs to the group or not. The surface is nearly always more or less distinctly composed of zooecia, or at least shows their orifices. The entire colony may be built up of these zooecia; and this is by far the commonest arrangement, both in encrusting and in erect forms. In certain genera, however, and particularly in some Ctenostomes (Fig. 238), and in most of the Entoprocta, the {480}individuals grow out at intervals from a cylindrical stem or "stolon" (_st_), which is not composed of zooecia.

The Cyclostomata may assume an erect or encrusting habit. Their zooecia are always more or less cylindrical; the upper ends being often completely free, although in many cases the whole zooecium is closely adnate to its neighbours. In the breeding season the forms which belong to this group are provided with curious "ovicells," which contain the embryos. These may either be pear-shaped swellings on the branches (_Crisia_, Fig. 237), or they may form inflations of the surface, between the zooecia. The mature ovicell is provided with one or more openings, through which the larvae escape.

The Ctenostomata rarely have even the slightest trace of calcareous matter. _Alcyonidium_ and its allies form soft encrustations, or may even grow into erect masses six inches or more in height (_A. gelatinosum_). In this type the zooecia are often so closely united that it may be difficult or impossible to make out their limits in the living colony. Many of the dendritic or branching {481}Ctenostomes (Fig. 238) are characterised by an extreme delicacy of habit. The zooecia in these cases are sharply marked off from the stem. They are either cylindrical or ovoid, being commonly attached by a very narrow base, so that in some species they readily fall off, and may thus be completely absent in certain parts of the colony. In such forms as _Vesicularia spinosa_, it requires considerable experience to recognise a stem which has lost its zooecia as being part of a Polyzoon. In _Mimosella_ the zooecia possess a remarkable power of movement on the stem, similar to that possessed by the leaflets of the Sensitive Plant.[532] In certain forms (_Bowerbankia_, _Amathia_) the zooecia occur in groups separated by intervals which are devoid of zooecia, but in other cases they may have a more irregular arrangement. The collar to which this group owes its name is by no means a conspicuous feature. Its position when retracted has been shown in Fig. 234, while Fig. 238 further illustrates its relations.

The Cheilostomata grow in a great variety of forms, and also show a wide range of character in their zooecia. The orifice is commonly surrounded by stiff spines (Fig. 257, p. 524), which perhaps have the function of protecting the delicate polypides from the sudden impact of foreign bodies. These spines may attain an enormous development, as in _Bicellaria ciliata_, and some forms of _Electra_ (_Membranipora_) _pilosa_ (Fig. 256, A).

The operculum is usually, though by no means always, a conspicuous feature of the Cheilostome zooecium. It is invariably of chitinous consistency, and is more or less semicircular in outline, the straight edge forming a hinge on which the operculum opens. In some cases the orifice is surrounded by a raised margin or "peristome" (Fig. 255, B, C); the operculum is then situated at the bottom of a depression of the surface, and may be concealed from view. In others, in which the front wall of the zooecium is membranous (_Bugula_, Fig. 233), the operculum is merely a part of this membrane, and so is quite inconspicuous; and in cases of this kind the membranous wall may be protected by an arched spine, the "fornix," developed from one side of the zooecium (Fig. 254, _f_). The ovicells are commonly a conspicuous feature of this group, although they are believed to differ fundamentally from those of Cyclostomata. They have the form of a helmet-like covering overhanging the orifice (Figs. 240, 241), {482}and may be either prominent or more or less concealed by the growth of adjacent parts of the zooecia. The presence of ovicells of this description is perfectly distinctive of the Cheilostomata.

AVICULARIA AND VIBRACULA.—Most singular of the external appendages of the Cheilostomata are the extraordinary "avicularia" and "vibracula" of some genera.[533] By the comparison of a carefully selected series of genera, it has been established that the avicularium is a special modification of a zooecium. One of its least modified forms is found in _Flustra foliacea_ (Fig. 232), where the avicularia (_a_) are small zooecia with a conspicuously large operculum ("mandible"). Avicularia of a similar type occur in _Cellaria_ (Fig. 239, A), _Schizotheca_, etc., the avicularium occupying the place of an ordinary zooecium. These are the "vicarious" avicularia of Mr. Busk.[534]

In the next stage (Figs. 239, B, 256, B) the avicularian zooecium is further reduced; it has in most cases lost its place in the series of individuals, and is found instead seated on some part of an ordinary zooecium ("adventitious" avicularia). The avicularium now consists of a much reduced zooecium, bearing the well-developed operculum or mandible.

{483}Having arrived at this point, the avicularia seem to lose all sense of the propriety of remaining in the positions once occupied by zooecia. They have become degraded to the rank of appendages of the zooecia, and as such they may occur in an astonishing variety of positions. Sometimes one occurs on each zooecium in the middle line, or asymmetrically, or even on the top of the ovicell; in other cases the orifice is flanked by an avicularium on each side (Fig. 239, B). Sometimes (_Cellepora_) the avicularia are of more than one kind, some being large and some small, some having a pointed mandible and others a mandible with a rounded spoon-like end.

In the cases so far considered, the body of the avicularium is fixed. The highest differentiation acquired by these structures occurs in cases like _Bugula_, where they are borne on flexible stalks, which may even exceed the avicularia in length.[535]

In _Bugula turbinata_ (Fig. 240) each zooecium is provided with one of these appendages, attached to the base of the outer of the two spines which border its orifice. The avicularia of the two edges of the flattened branch are much larger than those of the more internal zooecia. The upper jaw is strengthened by a kind of buttress, or thickening of the ectocyst, which passes on each side across the avicularium to the hinge-line of its mandible. The upper part of the beak is strongly hooked, while the tip of the mandible bears a {484}prominent spike, which fits inside the upper beak when the jaw snaps. A great part of the head is filled with a strong muscle, whose fibres exhibit a distinct transverse striation, and converge into a median tendon. The latter is inserted into the middle of the mandible. The muscle serves to close the jaws, and is the representative of the muscles by which the operculum is closed in an ordinary zooecium. The lower jaw is opened by means of a pair of muscles which are situated immediately under the ectocyst of the avicularium, and pass into the mandible close to its hinge.

Within the jaws, in the region which we may term the palate, is a rounded knob, which bears a tuft of delicate sensory hairs, which doubtless enable the avicularium to recognise the presence of any foreign body. The closure of the mouth may, indeed, be instantaneously induced by touching it with the point of a needle. It has been suggested that a small mass of cells which bears these hairs may represent the rudiment of the polypide.

The "vibraculum" (Fig. 242) is regarded as an avicularium in which the mandible has become elongated, so as to form a {485}thin, chitinous "seta," which from time to time moves through the water. The part of the vibraculum which represents the zooecium commonly bears a tubular rootlet, used for attaching the colony to the substance on which it is growing (Fig. 254, p. 517).

In _Microporella ciliata_ (Fig. 241, A) the avicularia are very variable, and in some cases take on a "vibraculoid" character. But in the fully-developed vibraculum (Fig. 242) there is usually no such compromise of characters. It may, however, be noted that _Scrupocellaria scabra_ (Fig. 254), which belongs to a genus characterised by its highly differentiated vibracula, possesses structures (_v.z_) which could hardly be distinguished from avicularia were it not for the presence of the rootlet (_r_).

In the course of some observations which I had the opportunity of making on _Bugula calathus_ at Naples, a fine hair offered to a small colony was seized with such force by the avicularia that the entire colony was lifted out of the water by the hair. The same colony had captured (1) a small _Nereis_, which it held with several of its avicularia; (2) an Anisopod Crustacean, 2½ mm. long; and (3) a small Amphipod, which was held by one of its antennae. The Anisopod was held by the tip of one leg with one avicularium, and by the penultimate joint of one of its chelae with an avicularium of another branch. It was captured in such a way that its chela, the "hand" of which was about half as long as the avicularium, actually closed on to the avicularium without being able to effect its escape. A little later the other chela was caught by another avicularium. Curiously enough, however, an avicularium did not necessarily close even when part of a captured animal was actually in its mouth. The avicularia made no attempt to place themselves in an advantageous position for catching fresh parts of the _Nereis_, which they might easily have done. The avicularia which had captured prey remained motionless. The others moved backwards and forwards (cf. the various positions of the avicularia shown in Fig. 240) ten times in ¾ to 1 minute, snapping their jaws perhaps once in that time. The two Crustacea were still retained by the avicularia two days later. On the next day they had both disappeared; but the colony had again caught the _Nereis_, which had previously effected its escape with the loss of nearly all its tentacular cirri.

These observations, and others which have been recorded, do not, unfortunately, give any information as to the purpose of the {486}movements of the avicularia and vibracula. It is obvious that they may be defensive in character; and it cannot be doubted that the avicularia can prevent inquisitive worms from straying at will over the surface of the colony. There is no evidence to show that animals are discouraged from interfering with a _Bugula_ owing to the presence of its defensive weapons.

It is not, indeed, certain what are the enemies against which the Polyzoa have specially to guard. Sea-urchins and certain Molluscs are known to browse on Polyzoa. Fresh-water Polyzoa, in which avicularia and vibracula are absent, are attacked by the larvae of Insects, and by Triclad Planarians. I have found the latter with their long pharynx everted and completely buried in a _Cristatella_ colony. It is possible that some marine Cheilostomes may be saved from attacks of this kind owing to the existence of their armoury of avicularia and vibracula. It is also possible that these structures are of service by removing foreign particles which might otherwise settle on the colony, and tend to block up its orifices. It has further been suggested that animals seized by the avicularia may be held until they die, and that their disintegrating particles may then be carried to the mouths of the polypides by the ciliary currents of the tentacles; but proofs of this suggestion are {487}wanting, and it must be admitted that the subject needs further elucidation.

The vibracula ordinarily remain stationary for some little time, every now and then giving a sweep through the water. In the majority of cases these structures, like the avicularia, act perfectly independently of one another, so far as can be made out; but in _Caberea_ (Fig. 242) the vibracula move in unison, the simultaneous action of the whole series, after a period of quiet, being described as "positively startling."[536]

It has been stated by Busk[537] that the entire colony in _Selenaria_ and _Lunulites_ may be moved from place to place by the large vibracula which these forms possess.

ENTOPROCTA.—The Entoprocta, although a very small sub-class, deserve special consideration, if for no other reason, from the fact that many writers regard them as the most primitive group of Polyzoa, and consequently as the forms which show most affinity to other classes of animals.

Their most obvious characteristic is, as we have already seen,[538] the position of the anus within the circle of tentacles. The individuals formed by budding always remain more separate from one another than those of most Ectoprocta.

{488}The commonest Entoproctous genus is _Pedicellina_, a graceful little animal, which occurs on many parts of our coast. It may often be discovered by looking carefully on the pink, jointed, calcareous alga, _Corallina_, which may be found growing at the edges of deep and cool rock-pools not too far above low-water mark. Its creeping stem or "stolon" is firmly attached to the surface of the seaweed, and sends off vertical stems here and there.[539] Each stem bears a "calyx," which is practically an individual of the colony. The stolon terminates, at one or both ends, in a growing-point (_a_), from which new individuals are budded off. The stalks bend from time to time in a curious spasmodic manner, by which means the calyces are moved about with an irritable and angry air. A good idea of the way in which the tentacles are folded away when the animal is disturbed may be obtained by putting the two wrists together, with the fingers spread out to represent the tentacles, the retraction of which would be represented by turning the tips of the fingers down into the space, the "vestibule," between the two palms. A delicate fold of skin growing from the edge of the calyx closes over the retracted tentacles, owing to the contraction of a sphincter muscle present in its circular edge. The body-wall is not separated from the alimentary canal by a definite body-cavity, so that there is no obvious distinction between the polypide and the zooecium. The existence of the Entoprocta is in fact a strong reason for refusing to admit that these two terms correspond with two different kinds of individuals.

Let us now imagine the condition we should have if a large and continuous cavity were developed between the alimentary canal and the body-wall. The body-wall would clearly have the general relations of a zooecium, while the alimentary canal and tentacles would obviously correspond with the polypide. The existence of the body-cavity would make it possible for the animal to _retract_ its tentacles instead of merely turning them in. Regarded in this way, there is but little difficulty in comparing the Ectoprocta with the Entoprocta.

The calyces are deciduous, _i.e._ they are lost from time to time, the end of the stalk then producing a polypide-bud, which {489}forms the vestibule and alimentary canal of a new calyx. Hence the phenomenon which may so commonly be noticed in _Pedicellina_ of a "young head on old shoulders." The loss of the calyces may have some relation to the formation of the "brown bodies" in the Ectoprocta.

Another Entoproct, _Loxosoma_ (Fig. 245) is remarkable for being the only Polyzoon which is not colonial. The buds, which are formed in two lateral series, break off as soon as they are mature, and at once begin to lead an independent existence. _Loxosoma_ is further remarkable for being almost invariably found commensally with other animals, where it may occur in enormous numbers. _L. phascolosomatum_, common in the Channel Islands, is only found on the tip of the tail of _Phascolosoma_ (see p. 428), which inhabits the mud of _Zostera_-beds. Other species are found on the external surface of certain sponges (_Tethya_, _Euspongia_, _Cacospongia_); or on the outside of a compound Ascidian, _Leptoclinum_, which may itself be carried about as a detachable covering on the back of a crab (_Dromia_). Another species is found on the ventral surface of the Polychaet _Aphrodite_, and of its ally _Hermione_.

_L. annelidicola_, an interesting species recently investigated by Prouho,[540] was originally described in 1863 as a Trematode, under the name of _Cyclatella_. It escaped further notice until it was again found in the neighbourhood of Roscoff, in Brittany, on certain Polychaets belonging to the family Maldanidae (see p. 332). The calyx has a very flattened form, and is borne on a short stalk, which terminates in a large attaching disc, formerly mistaken for the sucker of a Trematode. The features in which this species differs from other members of the genus are shown by M. Prouho to be correlated with its mode of life. The animal has the habit of lying flat on its back, the disc at the end of its stalk being firmly attached to the skin of the worm, and its short stalk being bent round into a curve so {490}as to bring the calyx into a supine position, with its lophophore directed upwards. This habit, together with its flattened form, prevents it from being crushed between the worm and its tube. But without some further provision its position might be merely a source of danger. For supposing the calyx to be directed backwards in relation to the worm, a sudden backward movement of the latter into its tube might bring the _Loxosoma_ into fatal contact with the inner surface of the tube. There would obviously not be sufficient room to turn round in a _vertical_ plane, so as to bring the body into a position of safety, _i.e._ into a position in which it moves stalk first. But by a beautiful arrangement of the muscles of its stalk this movement is effected in a horizontal plane; on touching the _Loxosoma_ with the point of a needle it would swing round in this way through 180° with "une rapidité qui étonne."

_Urnatella_[541] is a beautiful form with a segmented stalk, the stalks usually arising in pairs from a common base. It has at present only been found in fresh water in the United States.

In _Pedicellina_ the plane of the lophophore is at right angles to the stalk, which is separated from its calyx by a marked constriction. In _Loxosoma_ the lophophore is set obliquely,[542] and there is no constriction at the base of the calyx. In _Urnatella_ we find an intermediate condition, the lophophore resembling that of _Loxosoma_, while the constriction at the base of the calyx is similar to that of _Pedicellina_. Since the latter is known to pass in its development[543] through a stage with an oblique lophophore, it may be presumed that _Loxosoma_ is a more archaic form than _Pedicellina_. In other respects, the structure of the Entoprocta is very constant, whatever the genus.

{491}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 of _Urnatella_, 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]

{492}CHAPTER XVIII

POLYZOA (_continued_)

FRESH-WATER POLYZOA—PHYLACTOLAEMATA—OCCURRENCE—STRUCTURE OF _CRISTATELLA_— 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.

_Alcyonidium_ and _Bowerbankia_ (Fig. 238) flourish in estuaries, while _Victorella_ and _Paludicella_ (Fig. 250) are only known as fresh or brackish water forms. _Victorella_ was 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 genera _Hislopia_ and _Norodonia_,[546] which have been described from fresh water of India and China respectively, is at present uncertain. The undoubted Cheilostome _Membranipora_ has, 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]

{493}The Entoprocta, as we have seen, are represented in fresh water by the genus _Urnatella_.

The PHYLACTOLAEMATA are 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 of _Plumatella_ on the shells preserved in the Berlin Museum, of species of the Mollusc _Aetheria_ from 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 of _Daphnia_—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 in {494}a 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.

_Fredericella_ is 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. _Paludicella_ is common at thirteen fathoms. These two genera, with _Plumatella_, 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 of _Plumatella_), or of gelatinous masses of varying size (_Lophopus_, _Cristatella_).

_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.

{495}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 of _Fredericella_, 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]

Around the edge of the _Cristatella_ is 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 of _Cristatella_ is 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. {496}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 by _Cristatella_ is 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 of _Cristatella_ and of _Pectinatella_.

The colonies of _Lophopus_ are 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. {497}After a time the narrow connection breaks, and two new colonies are formed.