The Polyzoa appear to be related to the Sipunculoid Gephyraean worms (Gephyraea inermia) more nearly than to any other class of the animal kingdom. The study and interpretation of the facts of their ontogeny (growth from the egg) presents such extreme difficulty that in the present state of our knowledge it is necessary to regard them ad interim as forming with the Brachiopoda and Sipunculoids an isolated group, to which the name “Podaxonia” may be applied, pending the decision of their affinities by the increase of our knowledge of the embryology of important members of the group.

The forms included at the present day in Thompson’s class of “Polyzoa” may then be thus classified: [table]

We shall most readily arrive at a conception ot the essential structure of a Polyzoon, and of the variations to which that essential structure is subject within the class, by first examining one member of the group in detail and subsequently reviewing the characters presented by the divergent sub-classes, orders, &c., above indicated.

The most convenient form for our purpose is Paludicella Ehrenbergii (fig. 2, A), belonging to the typical section of the class (the Eupolyzoa) and to the order Gymnolaema. The organism occurs as minute trec-like growths (figs. 2, A and 1, E) attached to stones in freshwater streams and canals. The branches of the little tree are rarely more than an inch in length, and are regularly swollen and jointed at intervals. Each of the very numerous joints is about one-fifth of an inch long, and is in reality a tubular horny box attached above and below to the preceding and succeeding joints, and having on one side of it a spout-like aperture from which a crown of tentacles can be protruded. Each joint is thus inhabited by a distinct animal which is more or less completely shut off from the one in front of it and the one behind it, although it originated from the hinder and has given rise to the fore-lying individual by a process of budding, and retains a continuity of substance with both. A single cell or joint with its contained animal is represented in fig. 2, A.

Paludicella produces an arboriform colony, the main trunk or stolon being adherent to some stone or piece of wood. The substance of the wall of the cells is formed by a chemical body allied to chitin. Other Polyzoa may form mat-like expansions—the cells being placed in one plane, side by side (fig. 1, C, D, F, G), as well as in linear series; others again form solid masses, whilst many agree with Paludicella in the simple linear arrangement of their units. Phoronis and Loxosoma, on the other hand, do not form colonies at all—the former because it does not bud, the latter because the buds become detached from their parent as soon as formed, as do the buds of the Hydrozoon Hydra.

On the whole Paludicella presents us with a very simple form of Polyzoon-colony (technically termed a “zoarium”), in which the aggregate of budded persons, each of which is called a “polypide,” does not exhibit any marked individuation, but is irregular and tree-like. But, just as in the Hydrozoa we find the Siphonophora presenting us with a very definite shape and individuality of the aggregate or colonv. so in the Polvzoa we find instances of high indι-

viduation of the zoarium of a similar kind. The most remarkable example is afforded by the locomotive zoarium or colony of Cristatella (fig. 3); and another very striking instance is that of the stalked zoaria of Kinetoskias (fig. 14) and Adeona.

The horny consistence of the cells which are produced by Paludicella is very usual in other Polyzoa; but we find frequently that the substance which forms the cells is gelatinous and soft instead of being horny, or again may be strongly calcareous. The term coenoeciunι is applied to the mass of cells belonging to a colony or zoarium when considered apart from the living polyp ides which form it. Often such coenoecia are found retaining form and structure when the soft living polypides have decomposed and disappeared. A single cell of the coenoecium, corresponding to a single polypide, is called by the special students of the Polyzoa a zooecium.

If we examine a single cell or zooecium of Paludicella more carefully whilst its polypide is alive, we discover that the horny cell is nothing more than the cuticle of the polypide itself, to which it is absolutely adherent. At the so-called “mouth” or spout of the cell the cuticle suddenly changes its character and becomes a very delicate and soft pellicle instead of being thickandhorny. There is no real discontinuity of the cuticle at this region, but merely a change in its qualities. This gives to that portion of the body of the polypide which lies beyond the spout a mobility and capacity for folding and pleating which is entirely denied to that part where the cuticle is more dense (fig. 2, A). Accordingly we find that the anterior portion of the body of the polypide can be pulled into the hinder part as the finger of a glove may be tucked into the hand. It is, in fact, an “introvert” (for the use of this term see Mollusca, vol. xvi. p. 652). This arrangement is universal in the Ectoproctous Eupolyzoa, but does not obtain either in the Entoprocta, the Pterobranchia, or the Vermiformia. In Phoronis, Rhabdopleura, and Cephalodiscus the anterior part of the body can not be tucked or telescoped into the hinder part as it can in typical Eupolyzoa. On the other hand it is very important to note that the Sipunculoid Gephyraeans are all pre-eminently characterized by possessing identically this arrangement. The introversion is effected in Paludicella (as in other Eupolyzoa) by a series of long detached retractor muscles of considerable power (fig. 2, A, mr, r', m) ; the same is true of Sipunculus.

The view has been advanced by Allman (4) that the retractile part of the polypide is to be considered as a distinct individual budded from the basal portion, which is regarded as an equivalent individual. It does not appear to the present writer that such a theoretical conception tends to facilitate the understanding of the structure and relations of these animals.

An “ectocyst" and “endocyst” have also been distinguished in former treatises, and these terms form part of a special “polyzoarial” nomenclature, but do not appear to be any longer needful. Equally undesirable is the misapplied term “endosarc” lately introduced by Jolliet (5) to denote a certain portion of the Polyzoon structure which will not be referred to here by that name.

The retractile or introversible portion of the body of the polypide of Paludicella is terminated by a crown of sixteen stiff non-contractile tentacles (fig. 2, A, br) which form a circle around a central aperture—the animal’s mouth. These tentacles are hollow and beset with vibratile cilia. The beating of the cilia causes a powerful current in the water by which food is brought to the animal’s mouth. Each tentacle is also muscular, and can be bent and straightened at will. The tentacles not only serve to bring food into the mouth, but they are efficient as gillfilaments, being possibly homologous with (as well as functionally similar to) the gill-filaments of Lamellibranch Molluscs. They also serve as delicate tactile organs, and are the only sense organs possessed by the Eupolyzoa.

In Paludicella the platform around the mouth from which the tentacles arise, or lophophore, as it is termed, is circular. This is the case in all members of the large group of Gymnolaema and in the Entoprocta. But in the Phylactolaema the lophophore is drawn out on each side, right and left, so as to present a horse-shoe shape (fig. 2, B), and in some forms, notably Lophopus and Alcyonella, the two arms or diverging rami of the horseshoe are very strongly developed.

In the Pterobranchia the tentacles are confined in one genus (Rhabdopleura) to the two arm-like outgrowths of the lophophore, and are not simply hollow but contain a well-developed cartilaginoid skeleton (fig. 7). In the allied genus Cephalodiscus there are not merely a single pair of such arm-like processes, each bearing two rows of tentacles, but the lophophore is developed into twelve arm-like processes (fig. 9), which form a dense tuft of filaments around the anterior extremity of the animal.

In the Vermiformia (Phoronis) we again meet with a very perfect horse-shoe-shaped lophophore (fig. 4). The tentacles upon the crescentic or otherwise lobed circumoral region of the Sipunculoids are the representatives of the tentacles of the Polyzoa; whilst the tentaculiferous “arms” of the Brachiopoda appear to be the equivalents of the Polyzoon’s lophophore much drawn out and in most cases spirally rolled.

Just below the circular crown of tentacles in Paludicella we find an aperture which the study of internal anatomy proves to be the anus. In all Polyzoa the anus has this position near the mouth; and in this respect we again note an agreement with Sipunculus and the other so-called Gephyraea inermia. In one division of the Polyzoa alone is there any noteworthy variation in the position of the anus, namely, in the Entoprocta (sub-class of the section Eupolyzoa). In these forms the anus, instead of lying just below the lophophore or platform from which the tentacles spring, is included like the mouth within its area (fig. 15, C).

Passing now to the deeper structure of Paludicella, we find that it is a Coelomate animal; that is to say, there exists between the body-wall and the wall of the alimentary tract a distinct space termed “perigastric space,” body-cavity,” or “coelom.” This is true of all Polyzoa, though it has been erroneously stated by G. O. Sars that Rhabdopleura does not possess such a coelom. In Eupolyzoa (excepting the Entoprocta) the coelom is very capacious; it is occupied by a coagulable haemolymph in which float cellular corpuscles, and also the generative products, detached, as is usual in Coelomata, from definite “gonads” developed on its lining membrane (fig. 2, A, o, t). This lining membrane or “coelomic epithelium” is ciliated in the Phylactohema, but its characters appear not to have been definitely determined in other Eupolyzoa. The coelomic space and the tissues bounding it are continuous throughout the colony or zoarium of a Polyzoon—either directly without any constriction marking off one polypide from another, or through perforate septum-like structures as in Paludicella (see right-hand upper process of fig. 2, A), which form incomplete barriers between juxtaposed zooecia, and are termed “rosette-plates” or “communication-plates.” The coelomic cavity is continued in Paludicella and probably in all Polyzoa into the tentacles, so that these organs expose the haemolymph fluid to a respiratory action, and hence may be called branchial.

The body-wall of Paludicella consists, alike in the anterior introversible region and in the posterior region, of an outer cuticle which has already been spoken of as thickened around the base of the polypide so as to become there the hard tube-like zooecium. Beneath this is the delicate layer of living epidermic cells which are the mother-cells or matrix of that cuticle. Beneath this again are a few scattered annuli of muscular fibre-cells arranged ring - wise around the cylindrical body; more deeply placed than these are five large bundles of longitudinally placed muscular fibre-cells which are attached at three different levels to the soft introversible portion of the body, and by their retraction pull it in three folds or telescopic joints into the capacious hinder part of the body. In some Polyzoa the muscular fibre-cells present transverse striations. These folds are shown in fig. 2, A; but when the longitudinal muscles are completely contracted the tentacular crown would be pulled down far out of sight into the midst of the body by the great longitudinal muscle mr. Deeper than the longitudinal muscles, and clothing them and everything else which projects into the coelom, is the coelomic epithelium, not easily observed, and sufficiently known only in the Phylactohema. Part of it gives rise to the generative products (fig. 2 A, o, t). Other Eupolyzoa have a similar but not identical arrangement of the longitudinal muscles—acting essentially as retractors of the “introvert” or soft anterior region of the body—and a similar structure of the body-wall which is in essential features identical with that of the Sipunculoid worms, the Chaetopod worms, and other typical Coelomate animals.

The alimentary canal of Paludicella forms a closely compressed U-shaped loop depending from the closely approximated mouth and anus into the capacious coelom. It is clothed on its coelomic surface (in Phylactolaema at any rate) with coelomic epithelium, and beneath this are extremely delicate muscular layers. Within it is lined, except in the immediate region of the mouth (which is lined by the in-pushed outer cell-layer), by the enteric celllayer— the digestive cells derived from the archenteron of the embryo. We can distinguish in Paludicella a contractile pharyngo-oesophagus (fig. 2, A, oe), a digestive stomach V (the lining cells of which have a yellow colour), and an intestine which forms that arm of the loop connected with the anus. This simple form of alimentary canal is uniformly present in Polyzoa. In Boτverbankia and its allies a muscular gizzard with horny teeth is interposed between oesophagus and digestive stomach.

The alimentary canal of Paludicella does not hang quite freely in the coelomic cavity, but, as is usually the case in other classes where the coelom is large, mesenteries are present in the form of fibrous (muscular Î) bands clothed with coelomic epithelium and suspending the gut to the body-wall. In Paludicella there are two of these mesenteries, an anterior (√) and a posterior (x). The presence of two mesenteric bands is exceptional. Usually in the Eupolyzoa we find one such mesentery only, corresponding to the hinder of the two in Paludicella. The special name funiculus (Huxley) is applied to this mesenteric band, and it is noteworthy that the cells of the coelomic epithelium, either upon its surface or at its point of insertion into the body-wall, are modified as reproductive elements, forming either the testis or ovary; in the Phylactolaema they form here also special asexual reproductive bodies, the statoblasts. The nervous tissue and organs of Paludicella have not been specially investigated, but in many Eùpolyzoa an oval mass of nerve-ganglion cells is found lying between the mouth and anus, and there is no doubt that it is present in this case. In Plumatella nerve-fibres have been traced from this ganglion to the tentacles and other parts around the mouth (fig. 11, w, x i y). A “colonial nervous system” was described some years ago by Fr. Müller in Serialaria; but modern histologists do not admit that the tissue so named by Müller is nerve-tissue. The ganglion above mentioned is the only nervous tissue at present known in Polyzoa (but see fig. 17, x).

No heart or blood-vessels of any kind exist in Paludicella nor in any of the Eupolyzoa or Pterobranchia. On the other hand the isolated vermiform genus Phoronis presents a closed contractile system of longitudinal vessels (dorsal and ventral) which contain nucleated corpuscles coloured red by haemoglobin (figs. 4, 5).

No excretory organs (nephridia) or genital ducts have been observed in Paludicella, nor have such organs been detected in the majority of the Polyzoa which have been studied. In the Entoprocta, however, a pair of minute ciliated canals are found in the nearly obliterated bodycavity opening to the exterior near the tentacular crown in both Pedicellina and Loxosoma, which represent the cephalic nephridia of worms. A definite pair of nephridia occur in Phoronis. A similar significance is perhaps to be attributed to the “intertentacular organ” of Farre—a ciliated passage opening between two tentacles of the lophophore in Membranipora, Alcyonidium, and other forms—through which Hincks has observed the spermatozoa to escape in large numbers. This organ occurs equally in female specimens of Membranipora, and is not therefore simply a spermatic duct.

Paludicella, as we have seen, develops both ova and spermatozoa in one and the same polypide. The details of impregnation and development have not been followed in this instance, but in some of the marine Eupolyzoa (Gymnolaema) remarkable bud-like structures termed o<xcia are developed for the special reception of the ova, and in these organs fertilization takes place. In the Entoprocta there is a peculiar brood-pouch. The spermatozoa of one polypide probably in all cases fertilize the ova of another, but we have not yet in many cases a knowledge of how the spermatozoa get to the eggs, or how the eggs escape from the body-cavity of the parent. In the hippocrepian freshwater Polyzoa (Phylactolaema) the ova appear to be fertilized and undergo the early stages of development within the body-cavity of the parent or in a hernia-like protrusion of it. Probably in such cases the embryos escape by the death of the parent and rupture of the parental tissues, as do also the peculiar asexual internal buds or statoblasts of these forms.

The embryo Polyzoon or “larva” swims freely in its early condition by means of cilia, and is in this condition a single polypide or “person.” The forms assumed by these ciliated larvae in different Polyzoa are very various and exceedingly difficult of interpretation. We shall have more to say with regard to them below (see figs. 19, 20, 21). The ciliated larva then fixes itself and commences to produce polypides by a process of budding, the buds remaining not merely in contact but in organic continuity, and increasing continually in number so as to form a large colony or zoarium. In Paludicella we have seen that this colony has a simple tree-like form. The new buds form as wart-like growths, usually one, sometimes two in number, at the free end of a cell or zooecium near the spout-like process from which the tentacular crown is everted. In Paludicella all the polypides of a colony are alike; there is no differentiation of form or distribution of function amongst the members of the colony. In many Eupolyzoa this simplicity is by no means maintained, but a great variety of form and function is assumed by various members of the aggregate. The only approach to a differentiation of the polypides in Paludicella is in the arrest of growth of some of the buds of a colony in autumn, which, instead of advancing to maturity, become conical and invested with a dark-coloured cuticle. They are termed hybernacula. Should the rest of the polypides die down in winter, these arrested buds survive and go on to complete development on the return of spring.

In Paludicella we have thus seen a fairly simple and central example of Polyzoon structure and life-history. The variations upon this theme presented in different groups of Polyzoa have been to some small extent noted in the preceding account, but we shall now be able to indicate them more precisely by considering the various groups of Polyzoa in succession. The limit assigned to this article necessitates very large omissions. The reader who wishes to have the fullest information on the many difficult and uncertain matters connected with this subject is referred to Allman, Freshwater Polyzoa (Ray Society, 1856); Hincks, British Marine Polyzoa (Van Voorst, 1880); Haddon, “Budding in Polyzoa,” Quart. Joum. Mier. Sei., 1883; Balfour, Embryology, vol. i. p. 242; and the original memoirs cited by these writers.

THE VERMIFORMIA.

The first section of the Polyzoa comprises but a single genus, Phoronis. It differs from all other Polyzoa first in its greater size (species 2 inches long are known) and elaboration of organization, and correlatively with that in the fact that it does not produce buds. Further, it does not produce a closely adherent cuticular zooecium as do Paludicella and the Eupolyzoa generally, but a leathery tube in which the animal freely moves, resembling that of some Chaetopods (Sabella). Like some Sabellae, Phoronis forms closely packed aggregates of individuals not brought together by any process of budding, but each separately developed from an egg. Phoronis has an elongate, wormlike, unsegmented body, with a conical posterior termination (like Sipunculus), and anteriorly provided with a horse - shoe - shaped crown of tentacles surrounding the mouth (figs. 4, 5). There is an intertentacular “web” between the bases of the tentacles as in the Phylactolaema. Caldwell (6) has recently shown that the tentacles are supported by a mesoblastic skeleton, as is also the case in Rhabdopleura, but apparently not the case in any other Polyzoa. Close to the mouth, as in all Polyzoa, is placed the anus, outside the horse-shoe-shaped lophophore or tentacular platform (fig.

11, i). The tentacular crown is not intro versible; in this point Phoronis differs from Paludicella and the Ectoproctous Eupolyzoa, and agrees with the Entoprocta and the Pterobranchia. Overhanging the mouth is a small praeoral lobe or “epistome” (figs. 4, 5, c). This organ is aborted in Paludicella, and indeed in all the Gymnolaema, but is present in the other Polyzoa, and is especially large and well developed in Rhabdopleura and Cephalodiscus. It has been compared to the Molluscan foot, but undoubtedly in Phoronis it is the persistent representative of the prae-oral lobe of the larva (fig. 6), and therefore cannot be compared to the Molluscan foot. If we are right in associating Phoronis with the Polyzoa, this fact is sufficient to show that the epistome of the Phylactolaema (fig. 11, e) and the buccal shield of Rhabdopleura (fig. 7, <Z) and of Cephalodiscus (fig. 9, ό) are also cephalic in nature, and cannot rightly be identified with the post-oral and ventral muscular lobe known as the foot in Mollusca. A circum-oral nerve ring occurs at the base of the tentacles and sends off a cord which runs along the left side of the body. The alimentary canal presents the same general form and regions as in Paludicella. It hangs in the body-cavity, to the walls of which it is suspended by definite mesenteries.Phoronis presents a closed contractile vascular system containing red-coloured blood-corpuscles (figs. 4, 5, f, g, h). A pair of ciliated canals acting as genital pores is found near the anus; these have been shown by Caldwell to be typical nephridia.The development of Phoronis is remarkable. The egg gives rise (after the usual phases of cleavage and gastrulation) to the larval form knoλvn as Actinotrocha (fig. 6). This larva possesses a hood-like region overhanging the mouth and a number of ciliated post-oral processes or tentacles. The anus is placed at the extremity of the elongate body opposite to that bearing the mouth and prae-oral hood. The prae-oral hood becomes the epistome, and the tentacles, by further development (new tentacles replacing the larval ones), become the horse-shoe-shaped group of tentacles of the adult. A very curious process of growth changes the long axis of the body and results in the anus assuming its permanent position near the mouth. An invagination appears on the ventral face of the larva between the anus and mouth, and attains considerable size. At a definite moment in the course of growth this invagination is suddenly everted, carrying with it in its cavity the intestine in the form of a loop. Thus a new long axis is suddenly established at right angles to the original oro-anal axis, and continues to develop as the main portion of the body. The short area extending from the prae-oral hood to the anus is thus the true dorsal surface of Phoronis, whilst the elongated body is an outgrowth of the ventral surface perpendicular to the primary oro-anal axis, as conversely in many Mollusca we find a short ventral area (the foot) between mouth and anus, and an outgrowth of the dorsal surface (the visceral hump) perpendicular to the primary oro-anal axis, forming the chief body of the animal. In these relations Phoronis (and with it the other Polyzoa) agrees with Sipunculus. On the other hand Echiurus, the Chaetopods, Nemertine worms, and some other groups which start from a simple larval form not unlike that of Phoronis, present a continual elongation of the original oro-anal axis, and no transference of the long axis by the perpendicular or angular growth of either the ventral or the dorsal surface of the larva.

Phoronis was discovered originally in the Firth of Forth by Dr Strethill Wright. It occurs in the Mediterranean and in Australian seas (Port Jackson).

THE PTEROBRANCHIA.

This section of the Polyzoa also comprises forms which differ very widely from Paludicella. Inasmuch as their development from the egg is at present quite unknown, it may possibly prove that they have other affinities. Only two genera are known, Rhabdopleura (Allman) and Cephalodiscus (MTntosh), the former dredged by Dr Norman in deep water off the Shetlands (and subsequently in Norway), the latter taken by the “Challenger” expedition in 250 fathoms off the coast of Patagonia.

The Pterobranchia have the mouth and anus closely approximated, and immediately below the mouth are given off a series of ciliated tentacles, but these do not form a complete circle as in Paludicella, nor is the lophophore (the platform of their origin) horse-shoe-shaped as in Phoronis. The lophophore is drawn out into a right and a left arm in Rhabdopleura (fig. 7), upon each of which are two rows of ciliated tentacles; no tentacles are developed centrally in the region between the two arms, so that the mouth is not completely surrounded by these processes. The horseshoe-shaped lophophore of Phoronis could be modified so as to represent the tentaculiferous arms of Rhabdopleura by suppressing both rows of tentacles at the curve of the horse-shoe, and leaving only those which occur on the arms or rami of the horse shoe (see fig. 4). The lophophore of Cephalodiscus presents us with twelve processes, each carrying two rows of ciliated tentacles; in fact we have six pairs of tantaculiferous arms instead of a single pair, and each of these arms is precisely similar to one of the arms of Rhabdopleura (fig. 9), excepting that it terminates in a knob instead of tapering. There is no arrangement for introverting the anterior portion of the body into the hinder portion in the Pterobranchia.

The little epistome or prae-oral lobe of Phoronis is represented in the Pterobranchia by a large muscular shield or disk-like structure (fig. 7, d and fig. 9, δ) which overhangs the mouth and has an actively secreting glandular surface by which the tube or case (tubarium) in which the polypide is enclosed is secreted.Both Rhabdopleura and Cephalodiscus produce colonies by budding; but the colonies of the former are large, definite, and arborescent, whilst those of Cephalodiscus are remarkable for the fact that the buds do not remain long in organic continuity with their parent, but become detached and nevertheless continue to be enclosed by the same common envelope or secretion. The bud-formation of Rhabdopleura recalls that of Paludicella in the fact that it leads to the formation of continuous arboriform communities. That of Cephalodiscus resembles the budding of Loxosoma, since no two fully-formed individuals remain in organic continuity. Both Rhabdopleura and Cephalodiscus (like Phoronis) produce cases or investments in which they dwell. These are free secretions of the organism, and are not, like the coenoecia of Eupolyzoa, cuticular structures adherent to and part of the polypide’s integument. The dwelling of Rhabdopleura is a branched system of annulated tubes of a delicate membranous consistency, each tube corresponding to a single polypide, the rings of which it is built being successively produced at the termination of the tube by the secreting activity of the prae-oral disk (fig. 7, E). The polypides freely ascend and descend in these tubes owing to the contractility of their stalks. On the other hand the dwelling of Cephalodiscus is a gelatinous, irregularly branched, and fimbriated mass (fig. 8), excavated by numerous cavities which communicate with the exterior. In these cavities are found the numerous detached small colonies of Cephalodiscus (fig