both sides; and it has been calculated by Dr. Grant, that as a single square inch of an ordinary Flustra contains 1800 such cells, and as an average specimen presents about 10 square inches of surface, it will consist of no fewer than 18,000 zooids. The want of transparency in the cell-wall, however, and the infrequency with which the animal projects its body far beyond the mouth of the cell, renders the Polyzoa of this genus less favorable subjects for microscopic examination, than are those of the Bowerbankia, a Polyzoon with a trailing stem and separated cells like those of Laguncula, which is very commonly found clustering around the bases of Flustræ. It was in this, that many of the details of the organization of the interesting group we are considering, were first studied by Dr. A. Farre, who discovered it in 1837, and subjected it to a far more minute examination than any Polyzoon had previously received; and it is one of the best adapted of all the marine forms yet known, for the display of the beauties and wonders of this type of organization. The Halodactylus (formerly called Alcyonidium), however, is among the most remarkable of all the marine forms, for the comparatively large size of the tentacular crowns; these, when expanded, being very distinctly visible to the naked eye, and presenting a spectacle of the greatest beauty when viewed under a sufficient magnifying power. The polyzoary of this genus has a spongy aspect and texture, very much resembling that of the Alcyonian Zoophytes, for which it might readily be mistaken when its contained animals are all withdrawn into their cells; when these are expanded, however, the aspect of the two is altogether different, as the minute plumose tufts which then issue from the surface of the Halodactylus, making it look as if it were covered with the most delicate downy film, are in striking contrast with the larger, solid-looking polypes of the Alcyonium. The opacity of the polyzoary of the Halodactylus renders it quite unsuitable for the examination of anything more than the tentacular crown and the œsophagus which it surmounts; the stomach and the remainder of the visceral apparatus being always retained within the cell. Several of the fresh-water Polyzoa are peculiarly interesting subjects for microscopic examination; alike on account of the remarkable distinctness with which the various parts of their organization may be seen, and the very beautiful manner in which their ciliated tentacula are arranged upon a deeply crescentic or horseshoe-shaped "lophophore." By this peculiarity, the fresh-water Polyzoa are separated as a distinct sub-class from the marine; the former being designated as Hippocrepia (horseshoe-like), while the latter are termed Infundibulata (funnel-like).

330. The Infundibulata or Marine Polyzoa, constituting by far the most numerous division of the class, are divided into four

See his Memoir in the " Philosophical Transactions," for that year.

orders, as follows;-I. Cheilostomata, in which the mouth of the cell is sub-terminal, or not quite at its extremity (Fig. 245), is somewhat crescentic in form, and is furnished with a movable (generally membranous) lip, which closes it when the animal retreats. This includes a large part of the species that most abound on our own coasts, notwithstanding their wide differences in form and habit. Thus the polyzoaries of some (as Flustra) are horny and flexible, whilst those of others (as Eschara and Retepora) are so penetrated with calcareous matter as to be quite rigid; some grow as independent plant-like structures (as Bugula and Gemellaria), whilst others, having a like arborescent form, creep over the surfaces of rocks or stone (as Hippothoa), and others, again, have their cells in close apposition, and form crusts which possess no definite figure (as is the case with Lepralia and Membranipora). A large proportion of the Polyzoa of this order are furnished with very peculiar motile appendages, which are of two kinds, avicularia and vibracula. The "avicularia," or "bird's-head processes," are so named from the striking resemblance they present to the head and jaws of a bird (Fig. 247, B). They are generally "sessile" upon the angles or margins of the

FIG. 247.

cells, that is, are attached at once to them, without the intervention of a stalk, as in Fig. 247, A, being either projecting" or "immersed;" but in the genera Bugula and Bicellaria, where they are present at all, they are "pedunculate" or mounted on footstalks (B). Under one form or the other, they are wanting in but few of the genera belonging to this order; and their presence or absence furnishes valuable characters for the discrimination of species. Each avicularium has two "mandibles," of which one is fixed, like the upper jaw of a bird, the other movable like its lower jaw; the letter is opened and closed ▲, Portion of Cellularia ciliata, enlarged; B, one of the by two sets of muscles "bird's-head processes” of Bugula avicularia, more highly which are seen in the interior of the "head;" and between them is a peculiar body, furnished with a pencil of

magnified, and seen in the act of grasping another.

bristles, which is probably a tactile organ, being brought forwards when the mouth is open, so that the bristles project beyond it, and being drawn back when the mandible closes. The "avicularia" keep up a continual snapping action, during the life of the polyzoary; and they may often be observed to lay hold of minute worms or other bodies, sometimes even closing upon the beaks of adjacent organs of the same kind, as shown in Fig. 247, B. In the pedunculate forms, besides the snapping action, there is a continual rhythmical nodding of the head upon the stalk; and few spectacles are more curious than a portion of the polyzoary of Bugula avicularia (a very common British species) in a state of active vitality, when viewed under a power sufficiently low to allow a number of these bodies to be in sight at once. It is still very doubtful what is their precise function in the economy of the animal; whether it is to retain bodies that may serve as food within the reach of the ciliary current, or whether it is, like the "pedicellaria" of Echini (§ 314), to remove extraneous particles that may be in contact with the surface of the polyzoary. The latter would seem to be the function of the vibracula, which are long bristle-shaped organs, each one springing at its base out of a sort of cup (Fig. 245, A), that contains muscles by which it is kept in almost constant motion, sweeping slowly and carefully over the surface of the polyzoary, and removing what might be injurious to the delicate inhabitants of the cells when their tentacula are protruded. Out of 191 species of Cheilostomatous Polyzoa described by Mr. Busk, no fewer than 126 are furnished either with "avicularia," or with "vibracula," or with both of these organs. II. The second order, Cyclostomata, consists of those Polyzoa which have the mouth at the termination of tubular calcareous cells, without any movable appendage or lip. or lip. This includes a comparatively small number of genera, of which Crisia and Tubulipora contain the largest proportion of the species that occur on our own coasts. III. The distinguishing character of the third order, Ctenosomata, is derived from the presence of a comb-like circular fringe of bristles, connected by a delicate membrane, around the mouth of the cell, when the animal is projected from it; this fringe being drawn in when the animal is retracted. The polyzoaries of this group are very various in character, the cells being sometimes horny and separate (as in Laguncula and Bowerbankia), sometimes fleshy and coalescent (as in Halodactylus). IV. In the fourth order, Pedicellineæ, which includes only a single genus, Pedicellina, the lophophore is produced upwards on the back of the tentacles, uniting them at their base in a sort of muscular calyx, and giving to the animal when expanded somewhat the form of an inverted bell, like that of

1 See Mr. G. Busk's "Remarks on the Structure and Function of the Avicularian and Vibracular Organs of Polyzoa," in "Transact. of Microscop. Soc." Ser. II, vol. ii, p. 26.

Vorticella (Fig. 196). Among the Hippocrepia may be noticed, as exceptional forms, the Cristatella, whose polyzoary is unat tached, so as to be capable of moving freely through the water, and the Fredericella, the lophophore of which is rather circular than crescentic, the prolongation being so slight as only to be discernible on a careful examination. Generally speaking, the cells are lodged in a sort of gelatinous substratum, which spreads over the leaves of aquatic plants, sometimes forming masses of considerable size. As the animals of this group altogether resemble the true Zoophytes in their habits, and are found in the same localities, it is not requisite to add anything to what has already been said (§ 305) respecting the collection, examination, and mounting, of this very interesting class of objects.'

331. Compound Tunicata.-The Tunicated Mollusca are so named from the enclosure of their bodies in a "tunic" which is sometimes leathery or even cartilaginous in its texture, and which very commonly includes calcareous spicules, whose forms are often very beautiful. They present a strong resemblance to the Polyzoa, not merely in their general plan of conformation, but also in their tendency to produce composite structures by gemmation; they are differentiated from them, however, by the absence of the ciliated tentacula, which form so conspicuous a feature in the external aspect of the Polyzoa, by the presence of a distinct circulating apparatus, and by their peculiar respiratory apparatus, which may be regarded as a dilatation of their pharynx. In their habits, too, they are more inactive, exhibiting scarcely anything comparable to those rapid movements of expansion and retraction, which it is so interesting to watch among the Polyzoa; whilst, with the exception of the Salpida and other floating species which are chiefly found in seas warmer than those that surround our coast, they are rooted to one spot during all but the earliest period of their lives. The larger forms of the Ascidian group, which constitutes the bulk of the class, are always solitary; either not propagating by gemmation at all, or, if this process does take place, the gemmæ being detached before they have advanced far in their development. Since these cannot be considered as Microscopic objects (although no part of their organization can be properly made out without the assistance of that instrument), our attention will be confined to those "Compound Ascidians," the small size and transparency of whose bodies, when detached from the mass in which they are imbedded, not only enables their structure to be clearly

For a more detailed account of the Structure and Classification of this group, see Prof. Allman's "Report on the Fresh-water Polyzoa" in the "Transactions of the British Association" for 1850; Prof. Van Beneden's “ Recherches sur les Bryozoaires de la Côte d'Ostende," in "Mem. de l'Acad. Roy. de Bruxelles," tom. xvii; Mr. G. Busk's "Catalogue of the Marine Polyoza in the Collection of the British Museum ;" and Dr. G. Johnston's "History of British Zoophytes."

A

FIG. 248.

discerned without dissection, but allows many of their living actions to be watched. Of these we have a characteristic example in Amaroucium proliferum; of which the form of the composite mass and the anatomy of a single individual are displayed in Fig. 248. The clusters of these animals present themselves on rocks, sea-weeds, &c., very commonly between the tide-marks. They appear almost completely inanimate, exhibiting no very obvious movements when irritated; but if they be placed when fresh in sea water, a slight pouting of the orifices will soon be perceptible, and a constant and energetic series of m currents will be found to enter by one set and to be ejected by the other, indicating that all B the machinery of active life is going on within these apathetic bodies. In the tribe of Polyelinians, to which this genus belongs, the body is elongated, and may be divided into three regions, the thorax (A), which is chiefly occupied by the respiratory sac, the abdomen (B), which contains the digestive apparatus, and the postabdomen (c), in which the heart and generative organs are lodged. At the summit of the thorax is seen the oral orifice, c, which leads to the branchial sac, e; this is perforated by an immense number of slits, which allow part of the water to pass into the space between the branchial sac and the muscu

C

lar mantle, where it is especially collected in the thoracic sinus, f. At k is seen the œsophagus, which is continuous with the lower part of the pharyngeal cavity; this leads to the stomach, 7, which is surrounded by biliary tubuli; and from this passes off the intestine, m, which terminates at n in the cloaca. The long post-abdomen is principally occupied by the large ovarium, which contains ova in various stages of development. These, when matured and set free, find their way into the cloaca; where two large ova are seen (one marked p and the other immediately below

Compound mass of Amaroucium proliferum, with the anatomy of a single zooid:-A, thorax; B, abdomen; c, post-abdomen:-e, oral orifice; e, branchial sac; f, thoracic sinus; anal orifice; ', projection overhanging it; j, nervous ganglion; k, œsophagus; 1, stomach surrounded by biliary tubuli; m, intestine; n. termination of intestine in cloaca; o, heart; o, pericardium; p, ova

rium; p', egg ready to escape; q, testis; 7, spermatic canal: r', termination of this canal in the cloaca.