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. 200, C) may be compared 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. 200, A, b) is generally surrounded by a circlet of small, non-ciliated, non-retractile tentacles, and opens into a large chamber (fig. 200, 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. 200, 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.
Fig. 200. - Morphology of Tunicata. A, Diagram of the structure of a simple Tunicate: t Test; t' Second muscular tunic; s Branchial sac; b 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.
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 "haemal flexure;" whereas the flexure in the case of the Polyzoa is " neural." The intestine, however, in the Tunicata does not preserve this primary haemal flexure, but is again bent to the neural side of the body, the nervous ganglion coming finally to be situated between the mouth and the rectum. As just stated, the anus is not in direct communication wtth the exterior, but opens into a large cavity, called the " cloaca," or " atrial chamber," which, in turn, opens externally by the second aperture of the animal. This cloaca is a large sac lined by a membrane which " is reflected like a serous sac on the viscera, and constitutes the 'third tunic,' or 'peritoneum.'" From the cloaca " it is reflected over both sides of the pharynx" (respiratory sac), "extending towards its dorsal part very nearly as far as that structure which has been termed the 'endostyle.' It then passes from the sides of the pharynx to the body-walls, on which the right and left lamellae become continuous, so as to form the lining of the chamber into which the second aperture leads, or the 'atrial chamber.' Posteriorly, or at the opposite end of the atrial chamber to its aperture, its lining membrane (the 'atrial tunic') is reflected to a greater or less extent over the intestine and circulatory organs. . . . Where the 'atrial tunic' is reflected over the sides of the pharynx, the two enter into a more or less complete union, and the surfaces of contact become perforated by larger or smaller, more or less numerous, apertures. Thus the cavity of the pharynx acquires a free communication with that of the atrium; and as the margins of the pharyngo-atrial apertures are fringed with cilia working towards the interior of the body, a current is produced, which sets in at the oral aperture and out by the atrial opening, and may be readily observed in a living Ascidian" (Huxley).