The nervous system in all the Polyzoa consists of a single small ganglion (fig. 181, 2), placed upon one side of the cesophagus, between it and the anal aperture. Besides the single ganglion which belongs to each polypide, there is also in some of the Polyzoa, a "colonial nervous system;" that is to say, there is a well-developed nervous system, which unites together the various zooids composing the colony, and brings them into relation with one another. It is probably in virtue of this system that the avicularia are enabled to continue their movements and retain their irritability after the death of the polypides.

The muscular system is well developed, and consists of various muscular bands, with special functions attaching to each. The most important fasciculi are the retractor muscles (fig. 181, 2, g), which retract the upper portion of the polypide within the cell. These muscles arise from the inner surface of the endocyst near the bottom of the cell, and are inserted into the upper part of the oesophagus. The polypide, when retracted, is again exserted, chiefly by the action of the "parietal muscles," which are in the form of circular bundles runring transversely round the cell.

As far as is known, all the Polyzoa are hermaphrodite, cach polypide containing an ovary and testis (fig. 181, 2). The ovary is situated near the summit of the cell, and is attached to the inner surface of the endocyst. The testis is situated at the bottom of the cell, and a curious cylindrical appendage, called the "funiculus," usually passes from it to the fundus of the stomach. There are no efferent ducts to the reproductive organs; and the products of generation-i.e., the spermatozoa and ovaare discharged into the perigastric space, where fecundation takes place; and the impregnated ova escape by special openings in the body-wall, by dehiscence of the cell, or in some manner not as yet understood.

As already mentioned, continuous gemmation occurs in all the Polyzoa, the fresh zooids thus produced remaining attached to the organism from which they were budded forth, and thus giving rise to a compound growth.

A form of discontinuous gemmation, however, occurs in many of the Polyzoa, in which certain singular bodies, called "statoblasts," are developed in the interior of the polypide. The statoblasts are found in certain seasons lying loose in the perigastric cavity. In form "they may be generally described as lenticular bodies, varying, according to the species, from an orbicular to an elongated-oval figure, and enclosed in a horny shell, which consists of two concavo-convex discs united by their margins, where they are further strengthened by a ring which runs round the entire margin, and is of different structure from the discs. . . . When the stato

blasts are placed under circumstances favouring their development, they open by the separation from one another of the two faces, and there then escapes from them a young Polyzoön, already in an advanced stage of development, and in all essential respects resembling the adult individual in whose cell the statoblasts were produced" (Allman). The statoblasts are formed as buds upon the "funiculus "-the cord already alluded to as extending from the testis to the stomach-upon which they may usually be seen in different stages of growth. They do not appear to be set free from the perigastric space prior to the death of the adult, and when liberated they are enabled to float near the surface of the water, in consequence of the cells of the marginal ring, or "annulus," being spongy and filled with air. They must be looked upon as gemma peculiarly encysted, and destined to remain for a period in a quiescent or pupa-like state" (Allman).

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As regards the development of the Polyzoa, the embryo upon its emergence from the ovum presents itself as a ciliated, free-swimming, sac-like body, from which the polypide is subsequently produced by a process of gemmation. In the singular Rhabdopleura the primitive bud is enclosed between two fleshy lobes or valve-like plates, attached along their dorsal margin, and giving exit in front to the rudimentary lophophore. As the development proceeds, these plates cease to keep pace in their growth with the rest of the bud; till ultimately they appear as a peculiar shield-like organ on the hæmal side of the lophophore. These lobes have been compared by Dr Allman with the mantle-lobes of the Lamellibranchiata.

DIVISIONS OF THE POLYZOA.-The Polyzoa are divided into two divisions or orders-the Phylactolæmata (fig. 182, 3), distinguished by the possession of a bilateral horse-shoe-shaped lophophore, and of an "epistome" arching over the mouth; and the Gymnolæmata (fig. 182, 2), in which the lophophore is orbicular, and there is no epistome.

The order of the Phylactolæmata includes most of the fresh-water Polyzoa, forming the families Plumatellida (or Lophopea) and Cristatellida. Two marine genera-viz., Pedicellina and Rhabdopleura-the types of two distinct sub-orders are also comprised in the order. In the latter of these the branched tubular concecium is furnished with a correspondingly divided chitinous rod, to which the polypites are attached by their funiculi, on its adherent side. Loxosoma has been sometimes placed near Pedicellina, but is an anomalous form, with affinities to the Hydroids.

The order of the Gymnolamata includes the fresh-water genera Paludicella and Urnatella, and the vast majority of the marine Polyzoa. Of these latter, the sub-order of the Cheilostomata is the most important, as embracing the greater number of the common forms. In these, the opening of the cell is sub-terminal, and is generally closed by a movable lip or shutter. On the other hand, in the sub-order Cyclostomata, the cells are tubular, the orifices terminal, of the same diameter as the cell itself, and without any movable apparatus for closure. Lastly, in the singular group of the Ctenostomata (including Vesicularia, Alcyonidium, and Valkeria), the

cells arise from a common tube, and their mouths are terminal, and furnished with a setose fringe for their closure.

AFFINITIES OF THE POLYZOA.-By many zoologists the Polyzoa are now regarded as being an anomalous class of Worms, closely related to the true Annelides. That there are points of relationship between these apparently diverse groups cannot be doubted; but these do not seem sufficient to outweigh the points of divergence, such, for example, as the absence of segmentation in the former, and the totally different form of the nervous system. The Polyzoa have also striking affinities with the Brachiopoda and Tunicata, and even some with the Mollusca proper; and it has not, therefore, appeared advisable to remove them to the division of the Anarthropoda.

DISTRIBUTION of Polyzoa in SPACE.-The Polyzoa, like all the Molluscoida, are exclusively aquatic in their habits, but, unlike the remaining two classes, they are not exclusively confined to the sea. The marine Polyzoa are of almost universal occurrence in all seas. The fresh-water Polyzea, however, not only differ materially from their marine brethren in structure, but appear to have a much more limited range, being, as far as is yet known, principally characteristic of the north temperate zone. Britain can claim the great majority of the described species of fresh-water Polyzoa, but this is probably due to the more careful scrutiny to which this country has been subjected. Fresh-water Polyzoa have also been found in the southern hemisphere, in Australia and India.

DISTRIBUTION OF POLYZOA IN TIME.-The Polyzoa have left abundant traces of their past existence in the stratified series, commencing in the Lower Silurian Rocks and extending up to the present day. The Oldhamia of the Cambrian Rocks of Ireland, and the Graptolites, have been supposed to belong to the Polyzoa; but the former is very possibly a plant, and the latter should be referred to the Hydrozoa. Of undoubted Polyzoa, the marine orders of the Cheilostomata and Cyclostomata are alone known with certainty to be represented. Several Paleozoic genera-such as Fenestella (the Lace-coral), Ptilodictya, Ptilopora, &c.—are exclusively confined to this epoch, and do not extend into the Secondary Rocks. Amongst the Mesozoic formations, the Chalk is especially rich in Polyzoa, over two hundred species having been already described from this horizon alone. In the Tertiary period, the Coralline Crag (Pleiocene) is equally conspicuous for the great number of the members of this class.

CHAPTER XLIII.

TUNICATA.

CLASS II. TUNICATA (Ascidioida).-The members of this class of the Molluscoida are defined as follows:-Alimentary canal suspended in a double-walled sac, but not capable of protrusion and retraction; mouth opening into the bottom of a respiratory sac, whose walls are more or less completely lined by a network of blood-vessels" (Allman). Animal simple or composite. An imperfect heart in the form of a simple tube open at both ends. Sexes mostly united; a metamorphosis in development.

The Tunicaries are all marine, and are protected by a leathery, elastic integument, which takes the place of a shell. In appearance a solitary Ascidian (fig. 183, C) may be compared

Fig 183.-Morphology of Tunicata. A, Diagram of the structure of a simple Tunicate: Test; Second muscular tunic; s Branchial sac; 6 Branchial aperture; a Atrial aperture; c Atrium; o Opening of the gullet;g Stomach, leading into the intestine; an Anal aperture; n Nerve-ganglion. B, Botryllus smaragdus-a small portion of a colony of the natural size, and a single system of the same enlarged; co Common atrial aperture; br Branchial aperture of one of the zooids C, Molgula Manhattensis, a simple Ascidian. The arrows in A and C show the direction of the water-currents.

to a double-necked jar with two prominent apertures situated close to one another at the free extremity of the animal, one

of these being an ingestive and branchial aperture, whilst the other serves as an excretory aperture. The covering of an Ascidian is composed of two layers. Of these the outer is called the "external tunic," or "test," and is distinguished by its generally coriaceous or cartilaginous consistence. It is also remarkable for containing a substance which gives the same chemical reactions as cellulose, and is probably identical with this characteristic vegetable product. Sometimes it contains spicules or plates of calcareous matter. The test is lined by a second coat, which is termed the "second tunic," or "mantle," and which is mainly composed of longitudinal and circular muscular fibres. By means of these the animal is endowed with great contractility, and has the power of ejecting water from its branchial aperture with considerable force. The mantle lines the test, but is only slightly and loosely attached to it, especially near the apertures. The ingestive or branchial aperture (fig. 183, A, b) is generally surrounded by a circlet of small, non-ciliated, non-retractile tentacles, and opens into a large chamber (fig. 183, A, s), which usually occupies the greater part of the cavity of the mantle, and has its walls perforated by numerous apertures. This is known variously as the "pharynx," the "respiratory sac," or the "branchial sac." The last of these names is the best, as it is not certain that the perforated respiratory sac is really the homologue of the pharynx. If this should be its real nature, then the branchial opening in the test is truly the mouth; but good authorities regard the branchial sac as wholly unconnected with the alimentary canal. Inferiorly the respiratory sac leads, by a second aperture (fig. 183, A, 0), into an oesophagus, which conducts into a capacious stomach (g). If the branchial sac be regarded as not representing a dilated pharynx, then its lower aperture is the true mouth.

From the stomach an intestine is continued, generally with few flexures, to the anal aperture (an), which does not communicate directly with the exterior, but opens into the bottom of a second chamber, which is called the "cloaca," or "atrium" (c). Superiorly the cloaca communicates with the external medium, by means of the second aperture in the test (a). The first bend of the intestine is such that, if continued, it would bring the anus on the opposite side of the mouth to that on which the nervous ganglion is situated. The intestine, therefore, is said to have a "hæmal flexure;" whereas the flexure in the case of the Polyzoa is "neural." The intestine, however, in the Tunicata does not preserve this primary hæmal flexure, but is again bent to the neural side of the