CHAPTER XIII. POLYZOA, AND COMPOUND TUNICATA. AT the lower extremity of the great series of Molluscous animals we find two very remarkable groups, whose mode of life has much in common with Zoophytes, whilst their type of structure is conformable in all essential particulars to that of the true Mollusks. These animals are for the most part microscopie in their dimensions; and as some members of both these groups are found on almost every coast, and are most interesting objects for anatomical examination, as well as for observation in the living state, a brief general account of them will be here appropriate. 325. Polyzoa.-The group which is known under this name to British naturalists, corresponds with that which by Continental FIG. 245. zoologists is designated Bryozoa: the former name (though first used in the B singular instead of the plural number), having been introduced by Mr. J. V. Thompson in a memoir published in 1830, seems to have precedence in point of time over the latter, which was conferred by Prof. Ehrenberg in 1831 on a most heterogeneous group, wherein the Bryozoa, as now limited, were combined with the Foraminifera. As the history of the researches by which the Polyzoa have been raised from the class of Zoophytes (in which they were formerly ranked, for the most part in apposition with the Hydrozoa), to the Molluscan sub-kingdom, has already been sketched (p. 49), we may now proceed, without further preface, to a survey of the leading features of their organization. The Cells of Lepraliæ :-A, L. Hyndmanni ; B, L. figularis ; c, L. verrucosa. animals of the Polyzoa, in consequence of their universal ten dency to multiplication structure is FIG. 246. formed, closely corresponding with the polypidom of a Zoophyte, which has been appropriately designated the "polyzoary." The individual cells of the "polyzoary" are sometimes only connected with each other by their common relation to a creeping stem or "stolon," as in Laguncula (Fig. 246); but more frequently they bud forth directly, one from another, and extend themselves in different directions over plane surfaces, as is the case with Flustræ, Lepraliæ, &c. (Fig. 245); whilst not unfrequently the Polyzoary developes itself into an arborescent structure (Fig. cavity; c, valve separating this cavity from the œsopha 247) which may even present somewhat of the den Laguncula repens, as seen in its expanded state at A, and in its contracted state, in two different aspects, at B and c. The same references answer for each figure:a a, tentacula clothed with vibratile cilia; b, pharyngeal gus; e, the stomach, with f its pyloric valve, and g the circle of cilia surrounding that orifice; h, wall of the stomach with biliary follicles; i, the intestine, contain sity and massiveness of ing k excrementitious matter, and terminating at 7 the the stony Corals. Each anus; m, the testicle; n, the ovary; o, an ovum set free from the ovary; p, openings for the escape of the ova; individual is composed, spermatozoa freely moving in the cavity that surrounds externally of a sort of sac, of which the outer or tegumentary layer is either simply membranous, or is horny, or in some instances calcified, so as to form the cell; this investing sac is lined by a more delicate mem the viscera; r, retractor muscle of the angle of the aperture of the sheath; s, retractor of the sheath; t, retractor of the tentacular circle; u, retractor of the oesophagus; v, retractor of the stomach; w, principal extensor muscle; z, transverse wrinkles of the sheath; y, fibres of the sheath, themselves probably muscular; 2, muscles of the tentacula; a (at the base of the tentacular circle in A), nervous or œsophageal ganglion; ẞ, stem, -D, a portion of the tentacular circle shown sepa rately on a larger scale; a a, the tentacula clothed with cilia; bb, their internal canals; c, muscles of the tentacula; d, transverse muscles forming a ring at the base of the tentacula: e, muscles of the tentacular circle. brane, which closes its orifice, and which then becomes continuous with the wall of the alimentary canal; this lies freely in the visceral sac, floating (as it were) in the liquid which it contains. The further details of the anatomy will be best understood from the examination of a characteristic example, such as the Laguncula repens; which is shown in the state of expansion at A, Fig. 246, and in the state of contraction at B and c. The mouth is surrounded by a circle of tubular tentacula, which are clothed with vibratile cilia; these tentacula, in the species we are considering, vary from ten to twelve in number; but in some other instances they are more numerous. By the ciliary investment of their tentacula, the Polyzoa are at once distinguishable from those Hydraform polypes to which they bear a superficial resemblance, and with which they were at one time confounded; and accordingly, whilst still ranked among the Zoophytes, they were characterized as Ciliobrachiata. The tentacula are seated upon an annular disk, which is termed the "lophophore," and which forms the roof of the visceral or perigastric cavity; and this cavity extends itself into the interior of the tentacula, through perforations in the "lophophore." The mouth, situated in the centre of the "lophophore," leads to a funnel shaped cavity, or pharynx, b, which is separated from the œsophagus, d, by a valve at c; and this œsophagus opens into the stomach, e, which occupies a considerable part of the visceral cavity. In the Bowerbankia, and some other Polyzoa, a muscular stomach or gizzard, for the trituration of the food, intervenes between the oesophagus and the true digestive stomach. The walls of the stomach, h, have considerable thickness; and they are beset with minute follicles, which seem to have the character of a rudimentary liver. This, however, is more obvious in some other members of the group. The stomach is lined, especially at its upper part, with vibratile cilia, as seen at c, g; and by the action of these, the food is kept in a state of constant agitation during the digestive process. From the upper part of the stomach, which is (as it were) doubled upon itself, the intestine i opens, by a pyloric orifice ƒ, which is furnished with a regular valve; within the intestine are seen at k particles of excrementitious matter; which are discharged by the anal orifice at l. No circulating apparatus here exists; but the liquid which fills the cavity that surrounds the viscera, contains the nutritive matter which has been prepared by the digestive operation, and which has transuded through the walls of the alimentary canal; a few corpuscles of irregular size are seen to float in it. The visceral sacs of the different individuals put forth from the same stem, appear to communicate with each other. No other respiratory organs exist than the tentacula; into whose cavity the nutritive fluid is probably sent from the visceral cavity, for aeration by the current of water that is continually flowing over them. 326. The production of gemmæ may take place either from the bodies of the animals themselves, which is what always happens when the cells are in mutual apposition; or from the connecting stem or stolon, where the cells are detached from each other, as in Laguncula. There is first seen a bud-like protuberance of the horny external integument, into which the soft membranous lining prolongs itself; the cavity thus formed, however, is not to become (as in Hydra and its allies) the stomach of the new zooid; but it constitutes the chamber surrounding the digestive viscera, which organs have their origin in a thickening of the lining membrane, that projects from one side of the cavity into its interior, and gradually shapes itself into the alimentary canal with its tentacular appendages. Of the production of gemmæ from the zooids themselves, the best examples are furnished by the Flustra and their allies. From a single cell of a Flustra, five such buds may be sent off, which develope themselves into new zooids around it; and these, in their turn, produce buds from their unattached margins, so as rapidly to augment the number of cells to a very large amount. To this extension there seems no definite limit; and it often happens that the cells in the central portion of the leaf-like expansion of a Flustra are devoid of contents and have lost their vitality, whilst the edges are in a state of active growth. Independently of their propagation by gemmation, the Polyzoa have a true sexual generation; the sexes, however, being usually, if not invariably, united in the same individuals. The sperm-cells are developed in a glandular body, the testicle m, which lies beneath the base of the stomach; when mature, they rupture, and set free the spermatozoa qq, which swim freely in the liquid of the visceral cavity. The ova, on the other hand, are formed in an ovarium n, which is lodged in the membrane lining the tegumentary sheath, near its outlet; the ova, having escaped from this into the visceral cavity, as at o, are fertilized by the spermatozoa which they there meet with; and are finally discharged by an outlet at p, beneath the tentacular circle. 327. These creatures possess a considerable number of muscles, by which their bodies may be projected from their sheaths or drawn within them; of these muscles, r, s, t, u, v, w, x, the direction and points of attachment sufficiently indicate the uses; they are for the most part retractors, serving to draw in and double up the body, to fold together the circle of tentacula, and to close the aperture of the sheath, when the animal has been completely withdrawn into its interior. The projection and expansion of the animal, on the contrary, appear to be chiefly accomplished by a general pressure upon the sheath, which will tend to force out all that can be expelled from it. The tentacula themselves are furnished with distinct muscular fibres, by which their separate movements seem to be governed; the arrangement of these is seen at D. At the base of the tentacular circle, just above the anal orifice, is a small body (seen at A, a), which is a nervous ganglion; as yet no branches have been distinctly seen to be connected with it in this species; but its character is less doubtful in some other Polyzoa. 328. If we scrutinize the foregoing characters, we shall find that the most important of them are Molluscan, rather than Zoophytic. In the first place, all true Polypes use their tentacula to grasp their food and convey it to the mouth; and these tentacula are destitute of cilia; whilst, on the other hand, in all the Acephalous Mollusca, the nutritive matter is drawn in by a ciliary current, which also serves to aerate the fluids. Now the latter, as we have just seen, is the case with the Polyzoa; and thus, although their arms very commonly present a circular disposition around the mouth, they may be considered as representing, in their relation to the economy of the animal, the ciliated branchial sac of the Ascidians (§ 331). But they do not by any means constantly present this radial symmetry; thus, in the Plumatella, a beautiful fresh-water genus of Polyzoa, the ciliated arms are set upon two lobes or projections, one on either side of the mouth. The structure of the alimentary canal, again, removes the Polyzoa from the zoophytic series. In no true polype is there a separate intestine and anal orifice, nor does the whole apparatus hang freely in the visceral cavity; and the existence of a gizzard-like organ, and of a rudimentary liver (closely resembling that found in the lowest Tunicata), are also characters of elevation. The most important of all the single characters furnished by the anatomy of these animals, is their nervous system; which, as already pointed out, is distinctly Molluscan in its type. The absence of a heart and a distinct circulating system is, it is true, a Zoophytic character; but we shall presently find that even in the Tunicata, which are true Mollusks, the character of the circulating apparatus is extremely degraded. The propagation by gemmation, although formerly supposed to be a character exclusively Zoophytic, is known to belong also to the greater part of the "tunicated" Mollusks; and from this, therefore, no argument can be drawn in favor of the zoophytic nature of the Polyzoa. And although many of their composite fabrics have a stony density, and closely resemble the solid polypidoms of the helianthoid and asteroid Polypes, yet in others, especially amongst the freshwater species, we find a very close resemblance to the gelatinous bed or leathery crust in which the Compound Ascidians are lodged; and if we imagine calcareous matter to be deposited in this bed or crust, we should have a fabric closely resembling that of many stony polyzoaries. 329. Of all the Polyzoa of our own coasts, the Flustræ or “seamats" are the most common; these present flat expanded surfaces, resembling in form those of many sea-weeds (for which they are often mistaken), but exhibiting, when viewed, even with a low magnifying power, a most beautiful network, which at once indicates their real character. The cells are arranged on |