fig235Fig. 235.—Flustra papyreaPall. Naples. × 50. Illustrating the development of a new polypide after the formation of a "brown body." In 1,a, two masses formed from the alimentary canal;b, young polypide-bud;b.b, degenerating tentacles;c, connective tissue: 2, another zooecium, later stage;b.b, brown body;r.m, retractor muscles;s, stomach;t, tentacles of new polypide;t.s, tentacle-sheath: 3, the same zooecium, 191 hours later; letters as in 2. 1 and 2 are seen from the front, 3 from the back.[506]
Fig. 235.—Flustra papyreaPall. Naples. × 50. Illustrating the development of a new polypide after the formation of a "brown body." In 1,a, two masses formed from the alimentary canal;b, young polypide-bud;b.b, degenerating tentacles;c, connective tissue: 2, another zooecium, later stage;b.b, brown body;r.m, retractor muscles;s, stomach;t, tentacles of new polypide;t.s, tentacle-sheath: 3, the same zooecium, 191 hours later; letters as in 2. 1 and 2 are seen from the front, 3 from the back.[506]
Fig. 235.—Flustra papyreaPall. Naples. × 50. Illustrating the development of a new polypide after the formation of a "brown body." In 1,a, two masses formed from the alimentary canal;b, young polypide-bud;b.b, degenerating tentacles;c, connective tissue: 2, another zooecium, later stage;b.b, brown body;r.m, retractor muscles;s, stomach;t, tentacles of new polypide;t.s, tentacle-sheath: 3, the same zooecium, 191 hours later; letters as in 2. 1 and 2 are seen from the front, 3 from the back.[506]
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 ispossible that the intertentacular organ (Fig. 234,i) described on p.508may 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 thekidneyswhen 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 onFlustra papyrea.[508]The table gives the number of days from the time at which the polypides were noticed to commence their degeneration:—
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 thoseanimals. This was principally due to the observations of J. V. Thompson[512]in Ireland, who introduced the termPolyzoa; and of C. G. Ehrenberg[513]in Germany, who proposed the class-nameBryozoa, 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 Bryozoaas 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 hisHistory 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, theEntoprocta; and II, theEctoprocta.[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 inmany 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 foundwithinthe circlet, near the end opposite to the mouth.
In (2) and (3), representing the Ectoprocta, the anus isoutsidethe 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 OrderGymnolaemata,[521]the dominant group of the Polyzoa in respect of number of genera and species. The remaining Ectoprocta belong to the exclusively fresh-water OrderPhylactolaemata,[522]in which the mouth is protected by an overhanging lip or "epistome"; the ground-plan of the tentacles is, except inFredericella, horse-shoe shaped (Fig. 236, 3), and the tentacles themselves are usually much more numerous than in the other cases.
fig236Fig. 236.—Ground-plan of the lophophore in (1) Entoprocta, (2) Gymnolaemata, (3) Phylactolaemata:a, anus;ep, epistome;m, mouth. The tentacles are represented by shaded circles.
Fig. 236.—Ground-plan of the lophophore in (1) Entoprocta, (2) Gymnolaemata, (3) Phylactolaemata:a, anus;ep, epistome;m, mouth. The tentacles are represented by shaded circles.
Fig. 236.—Ground-plan of the lophophore in (1) Entoprocta, (2) Gymnolaemata, (3) Phylactolaemata:a, anus;ep, epistome;m, mouth. The tentacles are represented by shaded circles.
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.
The Gymnolaemata include three Sub-Orders:—
1.Cyclostomata.[523]—Body-wall densely calcareous, the zooecia being more or less tubular, usually with acircularorifice (Fig. 237).2.Cheilostomata.[524]—Body-wall of varying consistency. The orifice is closed, in the retracted state of the polypide, by a chitinouslipor "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 acomb.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).
1.Cyclostomata.[523]—Body-wall densely calcareous, the zooecia being more or less tubular, usually with acircularorifice (Fig. 237).
2.Cheilostomata.[524]—Body-wall of varying consistency. The orifice is closed, in the retracted state of the polypide, by a chitinouslipor "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 acomb.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 ofFlustrella hispidaor of species ofAlcyonidium, 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 theLaminariathrown up by gales or exposed at spring tides. The graceful spirals ofBugula turbinata(Fig. 233, A) may be found hanging from the rocks at extreme low water; while colonies ofScrupocellaria, remarkable for their vibracula (see p.484), are common in many places between tide-marks. Certain species affect the mouths of estuaries.
Membranipora membranaceacommonly covers many square inches of the frond ofLaminariawith 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 theLaminaria, which is swayed to and fro by the action of the waves. Many of the calcareous forms growing onLaminariahave 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 ofTriticella, which are only found on certain Crustacea. Many encrusting forms prefer the inside of dead shells ofPecten,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,LaniceandChaetopterus.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 ofCrisia eburneaare commonly found on red seaweeds, or on the branches of the HydroidSertularia.
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.
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 inCrisia(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 ofRetepora. Polyzoa of this type are seldom found between tide-marks, where their brittle branches would be liable to be snapped off by the waves. Theerectspecies which occur in such positions are flexible, although flexible species are by no means restricted to the zone between tide-marks.
fig237Fig. 237.—Crisia ramosaHarmer, Plymouth.A, End of a branch, × 1;B, another branch, × 20, showing the chitinous joints, the tubular zooecia characteristic of Cyclostomata, and the pear-shaped ovicell with a funnel-shaped orifice at its upper end.
Fig. 237.—Crisia ramosaHarmer, Plymouth.A, End of a branch, × 1;B, another branch, × 20, showing the chitinous joints, the tubular zooecia characteristic of Cyclostomata, and the pear-shaped ovicell with a funnel-shaped orifice at its upper end.
Fig. 237.—Crisia ramosaHarmer, Plymouth.A, End of a branch, × 1;B, another branch, × 20, showing the chitinous joints, the tubular zooecia characteristic of Cyclostomata, and the pear-shaped ovicell with a funnel-shaped orifice at its upper end.
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, theindividuals 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.
fig238Fig. 238.—Bowerbankia pustulosaEll. and Sol., Plymouth.A, Fragment of a colony, natural size, showing the branching stem, bearing tufts of zooecia:B, one of these tufts, with the growing apex of the stem (st), × 27;b, young zooecia (buds);c, the "collar" characteristic of Ctenostomata;t, tentacles;C, a single zooecium, with expanded tentacles, more highly magnified;a, anus;c, collar;g, gizzard;i, intestine;o, oesophagus;s, stomach.
Fig. 238.—Bowerbankia pustulosaEll. and Sol., Plymouth.A, Fragment of a colony, natural size, showing the branching stem, bearing tufts of zooecia:B, one of these tufts, with the growing apex of the stem (st), × 27;b, young zooecia (buds);c, the "collar" characteristic of Ctenostomata;t, tentacles;C, a single zooecium, with expanded tentacles, more highly magnified;a, anus;c, collar;g, gizzard;i, intestine;o, oesophagus;s, stomach.
Fig. 238.—Bowerbankia pustulosaEll. and Sol., Plymouth.A, Fragment of a colony, natural size, showing the branching stem, bearing tufts of zooecia:B, one of these tufts, with the growing apex of the stem (st), × 27;b, young zooecia (buds);c, the "collar" characteristic of Ctenostomata;t, tentacles;C, a single zooecium, with expanded tentacles, more highly magnified;a, anus;c, collar;g, gizzard;i, intestine;o, oesophagus;s, stomach.
The Ctenostomata rarely have even the slightest trace of calcareous matter.Alcyonidiumand 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 branchingCtenostomes (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 asVesicularia spinosa, it requires considerable experience to recognise a stem which has lost its zooecia as being part of a Polyzoon. InMimosellathe 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 inBicellaria ciliata, and some forms ofElectra(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),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 inFlustra foliacea(Fig. 232), where the avicularia (a) are small zooecia with a conspicuously large operculum ("mandible"). Avicularia of a similar type occur inCellaria(Fig. 239, A),Schizotheca, etc., the avicularium occupying the place of an ordinary zooecium. These are the "vicarious" avicularia of Mr. Busk.[534]
fig239Fig. 239.—Forms of avicularia.A,Cellaria fistulosaL., Plymouth, × 43;a.z, avicularian zooecium, with closed mandible;o, operculum of zooecium:B,Schizoporella unicornisJohnst., Scilly Is., × 43; zooecium bearing two avicularia;m, opened mandible of avicularium;s, sinus of orifice:C, zooecium ofSmittia landsboroviiJohnst., Plymouth, × 43; the operculum is seen at the bottom of a depression surrounded by a thin collar or "peristome," in an emargination of which is seen an avicularian zooecium (a.z);m, mandible (opened);p, pores;t, tooth.
Fig. 239.—Forms of avicularia.A,Cellaria fistulosaL., Plymouth, × 43;a.z, avicularian zooecium, with closed mandible;o, operculum of zooecium:B,Schizoporella unicornisJohnst., Scilly Is., × 43; zooecium bearing two avicularia;m, opened mandible of avicularium;s, sinus of orifice:C, zooecium ofSmittia landsboroviiJohnst., Plymouth, × 43; the operculum is seen at the bottom of a depression surrounded by a thin collar or "peristome," in an emargination of which is seen an avicularian zooecium (a.z);m, mandible (opened);p, pores;t, tooth.
Fig. 239.—Forms of avicularia.A,Cellaria fistulosaL., Plymouth, × 43;a.z, avicularian zooecium, with closed mandible;o, operculum of zooecium:B,Schizoporella unicornisJohnst., Scilly Is., × 43; zooecium bearing two avicularia;m, opened mandible of avicularium;s, sinus of orifice:C, zooecium ofSmittia landsboroviiJohnst., Plymouth, × 43; the operculum is seen at the bottom of a depression surrounded by a thin collar or "peristome," in an emargination of which is seen an avicularian zooecium (a.z);m, mandible (opened);p, pores;t, tooth.
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.
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 likeBugula, where they are borne on flexible stalks, which may even exceed the avicularia in length.[535]
fig240Fig. 240.—Bugula turbinata, showing avicularia (a,a'). The figure is explained on p.468.
Fig. 240.—Bugula turbinata, showing avicularia (a,a'). The figure is explained on p.468.
Fig. 240.—Bugula turbinata, showing avicularia (a,a'). The figure is explained on p.468.
InBugula 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 aprominent 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.
fig241Fig. 241.—Illustrating the transition from avicularia to vibracula.A,Microporella ciliataPall., Scilly Is., × 62;a, avicularium with short mandible (closed);a', avicularium with vibraculoid mandible (open);m.p, median pore;o, ovicell:B,Mastigophora dutertreiAud., Shetland Is., × 47;s, sinus of orifice;v, seta of vibraculum (or vibraculoid avicularium).
Fig. 241.—Illustrating the transition from avicularia to vibracula.A,Microporella ciliataPall., Scilly Is., × 62;a, avicularium with short mandible (closed);a', avicularium with vibraculoid mandible (open);m.p, median pore;o, ovicell:B,Mastigophora dutertreiAud., Shetland Is., × 47;s, sinus of orifice;v, seta of vibraculum (or vibraculoid avicularium).
Fig. 241.—Illustrating the transition from avicularia to vibracula.A,Microporella ciliataPall., Scilly Is., × 62;a, avicularium with short mandible (closed);a', avicularium with vibraculoid mandible (open);m.p, median pore;o, ovicell:B,Mastigophora dutertreiAud., Shetland Is., × 47;s, sinus of orifice;v, seta of vibraculum (or vibraculoid avicularium).
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 athin, 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).
InMicroporella 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 thatScrupocellaria 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 onBugula calathusat 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 smallNereis, 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 theNereis, 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 theNereis, 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 themovements 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 aBugulaowing to the presence of its defensive weapons.
fig242Fig. 242.—Caberea ellisiiFlem., Norway. × 40. Back view of part of a branch. The large vibracular zooecia (v.z) occupy nearly the whole of the surface.s, Seta of vibraculum;z, zooecia.
Fig. 242.—Caberea ellisiiFlem., Norway. × 40. Back view of part of a branch. The large vibracular zooecia (v.z) occupy nearly the whole of the surface.s, Seta of vibraculum;z, zooecia.
Fig. 242.—Caberea ellisiiFlem., Norway. × 40. Back view of part of a branch. The large vibracular zooecia (v.z) occupy nearly the whole of the surface.s, Seta of vibraculum;z, zooecia.
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 aCristatellacolony. 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 arewanting, 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 inCaberea(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 inSelenariaandLunulitesmay be moved from place to place by the large vibracula which these forms possess.
fig243Fig. 243.—Pedicellina cernuaPall., Guernsey. Entire colony. × 27. The colony has three growing ends,a; 1-8, individuals of colony; 1 and 8 are quite immature; and 7 (tentacles retracted) is still young; 2, is seen in longitudinal section;g, generative organ, and below it the ganglion;m, mouth;r, rectum;s, stomach; betweengandrare three embryos in the brood-pouch; the tentacles are retracted; in 5 and 6 the tentacles are expanded; in 6 two embryos are seen within the circle of the tentacles, to the left of them is the rectum, and to the right the mouth; 3 is in the act of losing its calyx, and has already developed the beginning of a new polypide-bud; in 4 the primary calyx has been lost, and the new calyx is clearly marked off from the stalk.
Fig. 243.—Pedicellina cernuaPall., Guernsey. Entire colony. × 27. The colony has three growing ends,a; 1-8, individuals of colony; 1 and 8 are quite immature; and 7 (tentacles retracted) is still young; 2, is seen in longitudinal section;g, generative organ, and below it the ganglion;m, mouth;r, rectum;s, stomach; betweengandrare three embryos in the brood-pouch; the tentacles are retracted; in 5 and 6 the tentacles are expanded; in 6 two embryos are seen within the circle of the tentacles, to the left of them is the rectum, and to the right the mouth; 3 is in the act of losing its calyx, and has already developed the beginning of a new polypide-bud; in 4 the primary calyx has been lost, and the new calyx is clearly marked off from the stalk.
Fig. 243.—Pedicellina cernuaPall., Guernsey. Entire colony. × 27. The colony has three growing ends,a; 1-8, individuals of colony; 1 and 8 are quite immature; and 7 (tentacles retracted) is still young; 2, is seen in longitudinal section;g, generative organ, and below it the ganglion;m, mouth;r, rectum;s, stomach; betweengandrare three embryos in the brood-pouch; the tentacles are retracted; in 5 and 6 the tentacles are expanded; in 6 two embryos are seen within the circle of the tentacles, to the left of them is the rectum, and to the right the mouth; 3 is in the act of losing its calyx, and has already developed the beginning of a new polypide-bud; in 4 the primary calyx has been lost, and the new calyx is clearly marked off from the stalk.
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.
The commonest Entoproctous genus isPedicellina, 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 toretractits 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, whichforms the vestibule and alimentary canal of a new calyx. Hence the phenomenon which may so commonly be noticed inPedicellinaof 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.Loxosomais 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 ofPhascolosoma(see p.428), which inhabits the mud ofZostera-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 PolychaetAphrodite, and of its allyHermione.
fig244Fig. 244.—Side view ofLoxosoma annelidicolaVan Ben. and Hesse. × 50. (From Prouho.)
Fig. 244.—Side view ofLoxosoma annelidicolaVan Ben. and Hesse. × 50. (From Prouho.)
Fig. 244.—Side view ofLoxosoma annelidicolaVan Ben. and Hesse. × 50. (From Prouho.)
L. annelidicola, an interesting species recently investigated by Prouho,[540]was originally described in 1863 as a Trematode, under the name ofCyclatella. 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 soas 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 theLoxosomainto fatal contact with the inner surface of the tube. There would obviously not be sufficient room to turn round in averticalplane, 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 theLoxosomawith 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.