CHAPTER XIX

fig252Fig. 252.—Diagrams of larvae.A,Loxosoma, × 208;a, anus;b, brain, with left eye and ciliated pit;c, ciliated ring;ep, epistome;m, mouth;o, oesophagus;st, stomach;x, aboral adhesive organ:B,Cyphonauteslarva ofMembranipora(Electra)pilosa, × about 90;a,m,o,stas inA;c, anterior part, andc', posterior part of the ciliated ring;e, epidermis;ms, adductor muscle of shells;p, pyriform organ, of unknown function;sh, shell;v, vestibule; the "internal sac" or sucker, by which fixation is effected, is seen betweenaandms. (B, after Prouho.)

Fig. 252.—Diagrams of larvae.A,Loxosoma, × 208;a, anus;b, brain, with left eye and ciliated pit;c, ciliated ring;ep, epistome;m, mouth;o, oesophagus;st, stomach;x, aboral adhesive organ:B,Cyphonauteslarva ofMembranipora(Electra)pilosa, × about 90;a,m,o,stas inA;c, anterior part, andc', posterior part of the ciliated ring;e, epidermis;ms, adductor muscle of shells;p, pyriform organ, of unknown function;sh, shell;v, vestibule; the "internal sac" or sucker, by which fixation is effected, is seen betweenaandms. (B, after Prouho.)

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

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

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

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

fig253Fig. 253.—A, Aboral view of free larva ofLepralia foliaceaEll. and Sol.;a, long cilia of pyriform organ;g, aboral groove:B, longitudinal section of embryo ofAlcyonidium, × 135;c, ciliated ring;g, aboral groove;m, mouth;n, nervous system;p, "pyriform organ," of unknown function;s, "internal sac" or "sucker," by which fixation is effected;st, stomach.

Fig. 253.—A, Aboral view of free larva ofLepralia foliaceaEll. and Sol.;a, long cilia of pyriform organ;g, aboral groove:B, longitudinal section of embryo ofAlcyonidium, × 135;c, ciliated ring;g, aboral groove;m, mouth;n, nervous system;p, "pyriform organ," of unknown function;s, "internal sac" or "sucker," by which fixation is effected;st, stomach.

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

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

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

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

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

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

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

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

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

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

POLYZOA (continued)

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

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

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

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

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

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

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

fig254Fig. 254.—A, Front view, andB, back view of part of a branch ofScrupocellaria scabra, Van Ben., Durham Coast, × 43;a, lateral avicularium;a', smaller median avicularium;ap, membranous aperture;f, fornix;r, rootlet;s, seta of vibraculum;v.z, vibracular zooecium.

Fig. 254.—A, Front view, andB, back view of part of a branch ofScrupocellaria scabra, Van Ben., Durham Coast, × 43;a, lateral avicularium;a', smaller median avicularium;ap, membranous aperture;f, fornix;r, rootlet;s, seta of vibraculum;v.z, vibracular zooecium.

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

2. The Cheilostomata consist of (A) theCellularina, including the flexible, erect forms, such asBugula(Fig. 233) andScrupocellaria(Fig. 254); (B) theFlustrina, to which belongFlustra(Fig. 232),Membranipora(Fig. 256, A, B),Micropora(Fig. 256, C), and other forms in which the front wall of the zooecium is either membranous, or depressed and marked off by a ridge-like margin; (C) theEscharina, including the great majority of forms, in which no part of the front wall remains membranous, the wall of the zooecium being wholly calcified.

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

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

Sub-class I.Entoprocta.Loxosoma,Pedicellina,Urnatella.Sub-class II.Ectoprocta.Order 1. Gymnolaemata.Sub-order 1. Cyclostomata.A. Articulata.Crisia.B. Inarticulata.Hornera,Idmonea,Tubulipora,Stomatopora,Diastopora,Entalophora,Lichenopora.Sub-order 2. Cheilostomata.A. Cellularina.Aetea,Eucratea,[589]Catenicella,Cellularia,Gemellaria,Menipea,Scrupocellaria,Caberea,Notamia(=Epistomia),Bicellaria,Bugula,Beania.B. Flustrina.Cellaria,Flustra,Membranipora,Electra,Lunulites,Membraniporella,Cribrilina,Micropora,Selenaria.C. Escharina.Retepora,Microporella,Lepralia,Porella,Smittia,Mucronella,Schizoporella,Schizotheca,Mastigophora,Porina,Cellepora.Sub-order 3. Ctenostomata.A. Alcyonellea.Alcyonidium,Flustrella.B. Vesicularina.Vesicularia,Amathia,Bowerbankia,Farrella,Hypophorella,Triticella,Mimosella,Victorella,Paludicella.Order 2. Phylactolaemata.Fredericella,Plumatella(includingAlcyonella),Lophopus,Cristatella,Pectinatella.

Sub-class I.Entoprocta.

Loxosoma,Pedicellina,Urnatella.

Sub-class II.Ectoprocta.

Order 1. Gymnolaemata.

Sub-order 1. Cyclostomata.

A. Articulata.Crisia.

B. Inarticulata.Hornera,Idmonea,Tubulipora,Stomatopora,

Diastopora,Entalophora,Lichenopora.

Sub-order 2. Cheilostomata.

A. Cellularina.Aetea,Eucratea,[589]Catenicella,

Cellularia,Gemellaria,Menipea,Scrupocellaria,

Caberea,Notamia(=Epistomia),Bicellaria,Bugula,

Beania.

B. Flustrina.Cellaria,Flustra,Membranipora,Electra,

Lunulites,Membraniporella,Cribrilina,Micropora,

Selenaria.

C. Escharina.Retepora,Microporella,Lepralia,Porella,

Smittia,Mucronella,Schizoporella,Schizotheca,

Mastigophora,Porina,Cellepora.

Sub-order 3. Ctenostomata.

A. Alcyonellea.Alcyonidium,Flustrella.

B. Vesicularina.Vesicularia,Amathia,Bowerbankia,

Farrella,Hypophorella,Triticella,Mimosella,

Victorella,Paludicella.

Order 2. Phylactolaemata.

Fredericella,Plumatella(includingAlcyonella),Lophopus,

Cristatella,Pectinatella.

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

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

Many marine forms also have a surprisingly wide distribution. Thus among the British species which are described by Mr. Hincks as occurring from Norway to New Zealand areMembranipora pilosa,Scrupocellaria scruposa,Cellaria fistulosa,Microporella ciliata, andM.malusii. Even if it should be proved that specific differences do exist between the southern forms and our own, there can be no doubt of the wide distribution of certain species. It was pointed out by D'Orbigny thatBugula neritinahas the habit of attaching itself to the bottoms of ships, a fact which may possibly account for the wide distribution of this species; although it would not be safe to assume this explanation of the facts in all cases. Other Polyzoa, on the contrary, have a more restricted range. ThusCatenicellais specially characteristic of the Australian region.

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

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

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

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

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

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

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

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

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

fig255Fig. 255.—Illustrating the nature of a secondary orifice (Cheilostomata).A,Mucronella coccineaAbildg., Scilly Is., × 40. The ovicell (o) overhangs the primary orifice, which is concealed by the great development of the peristome, produced into the mucro (mu);t, the three teeth (denticles) within the secondary orifice;a, avicularium.B,Porella compressaSowb., Norway, × 40;p.o.primary orifice, above which is a concave lamina, the beginning of the ovicell. In the lower zooecium the ovicell (o) is further grown. The primary orifice is still visible, but it is partially concealed by the growth of the peristome, which encloses a minute avicularium;m, mandible of avicularium.C, Older part of the same colony;pr, peristome;s.o, secondary orifice;o', adult ovicell;p, pores.

Fig. 255.—Illustrating the nature of a secondary orifice (Cheilostomata).A,Mucronella coccineaAbildg., Scilly Is., × 40. The ovicell (o) overhangs the primary orifice, which is concealed by the great development of the peristome, produced into the mucro (mu);t, the three teeth (denticles) within the secondary orifice;a, avicularium.B,Porella compressaSowb., Norway, × 40;p.o.primary orifice, above which is a concave lamina, the beginning of the ovicell. In the lower zooecium the ovicell (o) is further grown. The primary orifice is still visible, but it is partially concealed by the growth of the peristome, which encloses a minute avicularium;m, mandible of avicularium.C, Older part of the same colony;pr, peristome;s.o, secondary orifice;o', adult ovicell;p, pores.

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

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

fig256Fig. 256.—Illustrating the terminology of the front surface of the zooecium (Cheilostomata).A,Membranipora(Electra)pilosaL., Cromer, × 47;ap, the membranous "aperture;"o, orifice.B,Membranipora flemingiiBusk, Plymouth, × 60;ap, the aperture, enclosed in a calcareous "area" (a);av, avicularium;s, marginal spines.C,Micropora coriaceaEsper, Plymouth, × 43;a, area (calcareous);o, operculum;ov, ovicell.

Fig. 256.—Illustrating the terminology of the front surface of the zooecium (Cheilostomata).A,Membranipora(Electra)pilosaL., Cromer, × 47;ap, the membranous "aperture;"o, orifice.B,Membranipora flemingiiBusk, Plymouth, × 60;ap, the aperture, enclosed in a calcareous "area" (a);av, avicularium;s, marginal spines.C,Micropora coriaceaEsper, Plymouth, × 43;a, area (calcareous);o, operculum;ov, ovicell.

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

Terminology.—A few technical terms must of necessity be employed. The colony isadherentwhen its zooecia are attached to the object on which the colony is growing. Thezooeciumis the body-wall of a single individual; and, except in transparent species, is the only part which can be seen from the outside in the retracted condition of thepolypideor tentacles with the alimentary canal. The outermost layer of the zooecium is known as theectocyst; it may be simply membranous, or calcified, or may be rendered opaque by foreign bodies; its surface incalcareous forms is often marked bypores(Fig. 239, C,p), which are vacuities in the calcareous wall, closed externally by membrane. A specialmedian pore(Fig. 241, A,m.p) may occur, and is in some cases at least a complete perforation through the body-wall.

The tentacles are protruded through theorifice, which in Cheilostomata is usually guarded by a movable chitinous lid, oroperculum(Fig. 256, A,o). Should the ectocyst be thickened or raised into a ridge surrounding the orifice, a tubular passage results, known as thesecondary orifice(Fig. 255), at the deeper end of which is the true orifice. Theperistome(Fig. 255, C,pr) is the raised or thickened part which gives rise to the secondary orifice. Should the zooecium be outlined by a raised ridge, the part so enclosed is known as thearea(Fig. 256, C,a), if calcareous. Theapertureoropesia(Fig. 256, A, B,ap) is a membranous part of the front surface; and may consist of the whole or part of the area. The orifice or the aperture is commonly provided withspines(Fig. 256, B,s).

fig257Fig. 257.—A,Cribrilina annulataFabr., Norway, × 33;c, calcareous bars concealing the membranous aperture:B,Membraniporella nitidaJohnst., Plymouth, × 45;a, calcareous bars growing up round the margin of the aperture;b, the same, further developed;c, the same, completely formed (as inA);av, avicularium;o, immature, ando', mature, ovicell;s, marginal spines.

Fig. 257.—A,Cribrilina annulataFabr., Norway, × 33;c, calcareous bars concealing the membranous aperture:B,Membraniporella nitidaJohnst., Plymouth, × 45;a, calcareous bars growing up round the margin of the aperture;b, the same, further developed;c, the same, completely formed (as inA);av, avicularium;o, immature, ando', mature, ovicell;s, marginal spines.

Theaviculariumand thevibraculumare specially modified zooecia (see p.482), which occur in a great variety of forms, in certain Cheilostomata only. The operculum of the ordinary zooecium is represented by themandible(Fig. 239, B,m) in the avicularium, and by theseta(Fig. 242,s) in the vibraculum. The representative of the zooecium itself is known as theavicularian(Fig. 239, A,a.z) orvibracular zooecium(Fig. 242,v.z).

Anovicellis a swelling in which the embryo develops, in certain Cyclostomata (Fig. 237) and Cheilostomata (Fig. 241, A,o). Astolon(Fig. 238, B,st) is a stem, not formed of fused zooecia, from which new individuals originate. Aninternode, in a jointed colony, is the part between any two joints. Thefornixorscutum(Fig. 254, A,f) is a modified spine which in some Cheilostomata overhangs the aperture. Amucro(Fig. 255, A,mu) is a spike or protuberance developed just below the orifice. Asinus(Fig. 239, B,s) is a slight bay on the lower margin of the orifice.

The orifice opens at theupperend of the zooecium, on itsfrontsurface. Thelengthof the zooecium is the distance from the upper to the lower ends, and thewidththe distance between its sides.

One or more of the following characters: orifice provided with an operculum; avicularia or vibracula present; a globular ovicell above the orifice of certain zooecia (Cheilostomata)

Opercula, avicularia, vibracula, and ovicells completely absent, or inconspicuous. Calcareous or non-calcareous. If calcareous, the orifice is not at the end of a free cylindrical portion

Calcareous; zooecia cylindrical, often united for the greater part of their length, but usually ending in a free cylindrical portion, which bears the terminal orifice. The zooecia may be much obscured by calcifications surrounding their basal parts

Zooecia long, tubular, with a lateral membranous region at the upper end, given off quite separately from a creeping stolon

Aetea

Zooecia more or less united to one another, orifice without chitinous operculum (Cyclostomata[601])

Zooecia without marginal spines; arising from a branching axis, which is not formed of zooecia

Colony adherent;orerect, fleshy and slightly branched;orerect, encrusted with earthy matter and repeatedly branched

Zooecia minute, boat-shaped, united by a delicate tube. Aperture large, with marginal spines

Beania mirabilis

Zooecia uniserial; with marginal spines. Branches arising from the top of a zooecium

Brettia

Zooecia uniserial; branches arising just below the large aperture. An ovicell may be developed above the orifice of a modified zooecium

Eucratea chelata

Zooecia somewhat pear-shaped; orifice small, semicircular

Huxleya fragilis

Colony entirely adherent,[602]the zooecia usually in a single layer

Erect Cheilostomata.

Branches cylindrical, calcareous, divided by chitinous joints. Orifices arranged all round the branch

Cellaria(Fig. 239, A)

Branches flexible, jointed or unjointed. Orifices not arranged all round the branch.

Avicularia resembling birds' heads, movable

Bugula(Fig. 233)

Avicularia not resembling birds' heads, unstalked;orabsent. Colony broadly leaf-shaped, composed of a single layer or of two layers of zooecia

Flustra(Fig. 232)

Branches numerous, straight. Zooecia back to back, with an oblique aperture. No avicularia

Gemellaria loricata

Branches delicate, curved. A pair of stalked avicularia between each two pairs of zooecia

Notamia(= Epistomia)bursaria

Avicularia resembling birds' heads, movable. Vibracula absent

Avicularia large, unstalked. Vibracula present or absent

Avicularia inconspicuous. Setae of the vibracula large, very conspicuous, on oblique vibracular zooecia, which almost cover the backs of the branches

Caberea(Fig. 242)

Zooecia in two series, alternate, with one or several conspicuously long marginal spines

Bicellaria

Zooecia in two or more series. Aperture occupying most of the front of the zooecium. Colony often spiral. Avicularia usually large

Bugula(Fig. 233)

Zooecia long, narrow below, commonly in triplets, with two lateral avicularia to each triplet. Fornix present.

Menipea ternata

Zooecia biserial, a considerable number forming an internode separated by a joint (often inconspicuous) from the next internode. Lateral avicularia usually large. Vibracular zooecia on the back or sides of the branches

Scrupocellaria(Fig. 254)

Characters as inScrupocellaria(No. 16), but with inconspicuous avicularia. A branched fornix

Scrupocellaria reptans

Zooecia biserial. Aperture large, the semicircular orifice at its upper end, where there is commonly a short spine

Cellularia peachii

Zooecia in one or two series. Branches originating from the backs of the zooecia, and facing in the opposite direction to the parent branch. Aperture small

Scruparia clavata

One or more conspicuously long marginal spines. Avicularia present or absent

Bicellaria

Zooecia biserial, in short internodes. An inconspicuous avicularium below the aperture. Fornix present

Menipea jeffreysii

Colony consisting of a network of narrow branches, the zooecia opening only on one of their surfaces

Retepora

Colony large, brittle, composed of contorted plates, uniting irregularly, usually composed of two layers of zooecia. Orifice large, indented laterally

Lepralia foliacea

Zooecia in more than four regular, longitudinal rows. Peristome raised, and, with the ovicell, forming a swelling on the surface of the branch

Escharoides quincuncialis

Orifice circular. A row of pores round the margin of the zooecium. A median pore resembling a small orifice below the true orifice. Small lateral avicularia

Porina borealis

Orifice surrounded by a peristome, produced into a mucro beneath the orifice. No pores

Palmicellaria elegans

Zooecia irregularly heaped, their long axes often perpendicular to the surface of the colony. Mucro largely developed, concealing the form of the orifice, and bearing an avicularium

Cellepora

Orifice with a sinus;orperistome interrupted or extended below into a sinus-like outgrowth, which usually includes a small avicularium

Neither median sinus nor interrupted or extended peristome

Branches of various forms. Surface of the older parts very even. Secondary orifice rather long, usually wider above, enclosing a small avicularium below, and appearing as a hole in the even surface of the branch

Porella(Fig. 255, B, C)

A prominent tooth projects into the orifice from its lower side. Zooecia with thin walls

Smittia landsborovii(Fig. 239, C)

No tooth in the orifice, at the side of which is a small avicularium. Old zooecia with thick walls. Colony composed of a short stem and flattened branches

Escharoides rosacea

A tooth projects from the lower side into the large, subcircular orifice, on each side of which is a small oval avicularium (colony erect or encrusting)

Mucronella pavonella

Branches cylindrical. Old zooecia with thick walls. Orifice in young zooecia longer than broad; beneath it a median pore, and in some cases a lateral avicularium with vibraculoid mandible

Diporula verrucosa

A distinct mucro, which may bear an avicularium above

Palmicellaria

Encrusting Cheilostomata.

Usually growing on a small univalve shell. Orifice longer than broad, indented laterally. Mucro present

Lepralia edax

One or two conspicuous processes, each bearing an avicularium, near the orifice, which is often concealed. Avicularia in many cases found on other parts of the colony

Cellepora

A tubular process below the aperture, in some cases: zooecia very narrow below

Eucratea chelata, var.repens

Zooecia minute, much narrowed below. Orifice small, usually with a sinus

Hippothoa

Zooecia not narrowed below. Orifice with a sinus

Schizoporella

Zooecia partly separated by a thin calcareous crust. Colonies small

Zooecia pear-shaped. Orifice with a sinus

Hippothoa expansa

Zooecia ovoid. Orifice subcircular, with a tubular peristome

Lagenipora socialis

Orifice close to the upper end of the zooecium (unless crowned by an ovicell). Front of the zooecium marked by transverse or radiating furrows or lines. The very young zooecium may possess a membranous area, which becomes roofed in by the union of two lateral series of converging bars (Fig. 257)

Furrows with uniserial rows of pores (often minute), which are rarely irregular

Cribrilina(Fig. 257, A)

No rows of pores. Distinct transverse lines or spaces and a median longitudinal suture between the bars

Membraniporella(Fig. 257, B)

Zooecia irregularly[603]heaped together (cf. No. 30)

Cellepora

Primary orifice conspicuous; with a sinus, or with a peristome extended or interrupted below, and sometimes simulating a sinus

Surface of the old zooecia much thickened, so that the secondary orifice does not project beyond the most prominent parts of the zooecium. Secondary orifice concealing the primary orifice, wider above, enclosing a small avicularium below

Porella(Fig. 255, B, C)

A prominent tooth projects into the orifice from its lower side. Peristome interrupted or with a sinus. Surface of the old zooecia not much thickened

Smittia

Orifice with a sinus and long spines. Peristome interrupted. Ovicell with a wedge-shaped or linear longitudinal fissure. Avicularia generally present, the avicularian zooecium conspicuous.

Schizotheca

Orifice semicircular. Vibracula present, near the orifice.

Mastigophora(Fig. 241, B)

Orifice semicircular or subcircular. No vibracula; avicularia with vibraculoid mandibles may occur

Schizoporella(Fig. 239, B)

Zooecium with a median pore;orcompletely tubular above

Zooecium with no median pore. The orifice may be partially surrounded by a collar-like development of the peristome, but it is not completely tubular

Orifice not tubular. A median pore

Microporella(Fig. 241, A)

Orifice markedly tubular. Median pore conspicuous

Porina tubulosa

Colony very small. Zooecia irregularly arranged, with no median pore

Celleporella

Zooecia very convex, with a granular surface; ovicells set far back. Orifice wider than long

Mucronella microstoma

Young zooecia with stellate pores. A minute avicularium, or merely a pore, on the upper and lower sides of the orifice in some zooecia.

Anarthropora monodon

Front of zooecium with an elevated margin, enclosing an area

Avicularian or vibracular zooecia replacing an ordinary zooecium, or at least situated between the zooecia

Avicularia and vibracula absent, or if present not replacing a zooecium

Vibracula present. Colonies small. A pair of longitudinal slits within the area

Setosella vulnerata

Very large avicularia present. Ovicell closed by a movable lid. Orifice subcircular, with a minute lateral tooth on each side.

Thalamoporella[605](Steganoporella)smittii

Orifice semicircular, quite at the upper end of the zooecium; usually with a knob on each side

Micropora coriacea[606](Fig. 256, C)

A transverse chitinous plate lies immediately below the operculum. A vibraculoid spine may occur

Megapora ringens

Area entirely membranous, usually bordered by spines.

Membranipora(includingElectra[607]) (Fig. 256, A)

Membranous portion reduced to a small portion, which may be variously lobed, enclosing the orifice.

Membranipora(other species) (Fig. 256, B)

Peristome collar-like, much raised below and at the sides of the orifice, deficient above. No avicularia

Phylactella

Orifice large, longer than broad. Peristome not deficient above the orifice

Lepralia

A minute avicularium above the orifice, or where an ovicell is present, situated at the summit of that structure. Zooecia not quite contiguous. Mucro sometimes present.

Chorizopora brongniartii

No avicularia. Ovicells on rudimentary zooecia, lying in a plane superficial to that of the rest of the colony. Zooecia long.

Schizoporella hyalina

Mucro rarely present. Orifice nearly always longer than broad, or nearly circular, usually large, and slightly indented laterally.

Lepralia

Colony glistening. Orifice much obscured by the mucro and by stout spines developed from the peristome. Tooth (concealed in old zooecia) large, strongly curved to one side.

Rhynchopora(Rhynchozoon[608])bispinosa

Tooth of the lower margin of the orifice symmetrical, sometimes bifid. Avicularia may be present laterally, but are not developed on the mucro

Mucronella(Fig. 255, A)

Orifice at least half the width of the zooecium, bordered below by a well-developed prominence or "umbo." Surface of the zooecium strongly areolated round the margin

Umbonula[609]

Orifice considerably less than half the width of the zooecium.

Schizoporella(Fig. 239, B)

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