Tunicata

A

general impression of the structure of a fairly typical Ascidian can be gathered from the figure of Clavelina (P1. fig. 7). In this form, as in all Haplobranchs, the intestinal loop lies completely behind the pharynx, and the latter is free from the complexities caused by the presence of internal longitudinal bars or vessels; but these are the only conspicuous points—apart from the ar rangements for budding—in which the general structure differs from that of such a typical "simple" Ascidian as Ciona. In both the body is erect, fixed by a multitude of "rootlets," and carrying mouth and atriopore at its upper extremity, the mouth terminal, the atriopore a little to one side. Between the two siphons lies the small solid brain, giving off nerves in front and behind, and defining the dorsal side of the body by its presence. On the opposite (ventral) side the endostyle runs along the whole length of the pharynx, which contracts immediately behind it, and is continued by the dorsal oesophagus into the digestive portion of the alimentary canal. This consists of a descending limb, including the stomach, and an ascending limb (mainly rectum) which crosses the other on its left side, the two being connected at the bend by a mid-gut or intestine. From the lower end of the stomach issues a slender diverticulum which forks repeatedly on the wall of the rectum opposite (the pyloric gland). The rectum opens into the cloaca by a dorsal anus.

The cloacal cavity extends along the dorsal side of the pharynx, the cloacal siphon or atriopore being situated in this case and in Ciona at its anterior extremity. Right and left this cavity is continued over the sides of the pharynx as the lateral atria, or peribranchial cavities, which terminate ventrally on either side of the endostyle. Beneath the endostyle runs a large ventral blood-channel, which is continued behind into the abdomen. A corresponding dorsal blood-channel runs longitudinally between the pharyngeal roof and the cloacal floor, and is also continued into the abdomen. The two are connected by transverse chan nels between the rows of stigmata, and these are connected by smaller channels running longitudinally between adjacent stigmata, the whole constituting a vascular network moulded to the form of the pharynx. The heart is always situated in the course of the ventral blood-sinus, but at different points according to the position of the intestinal loop. In "simple" Ascidians, in which the loop lies against the left side of the pharynx, the heart lies close behind the endostyle, as it does also in the young Clavelina; but during the later growth of Clavelina and its allies the intestinal loop stretches considerably downwards, and the heart is carried with it, in Polyclinids even beneath it into the "postabdomen."

The heart itself in all Tunicates is part of a larger structure, the cardio-pericardial vesicle. This is at first a closed oval sac, arising ventrally to the gut. Its dorsal wall becomes invaginated, and the invaginated epithelium becomes contractile. The inner vesicle constitutes the heart, the outer the pericardium, the two being separated by the pericardial cavity. The lips of the cardiac invagination tend to meet, thereby closing its cavity, except at the two ends, where they remain open to the blood in the ventral sinus. The heart undergoes waves of contraction either from behind forwards, or from before backwards, the whole course of the circulation being subject to periodic reversal.

In Clavelina the roof of the cardiac groove is partly provided by a third structure, the epicardiurn, an epithelial septum which arises as a pair of outgrowths from the hinder wall of the pharynx on either side of the endostyle. These epicardial diverticula fuse behind the pharynx into a single tube, which becomes flattened dorso-ventrally and extends down the abdominal cavity as a thin broad septum between the main dorsal and ventral blood channels. It continues its course into the small branching root lets by which the Clavelina is attached (the creeping stolon). Its relations with the circulatory system appear to be secondary; its real raison d'etre is to be found in the budding process.

In

Ciona, which never buds, the two epicardial outgrowths are present in a modified form; they remain distinct, and the cavity of each expands to form a perivisceral space behind the pharynx, The left sends prolongations into the numerous rootlets of attach ment, and thus also functions as a septum dividing afferent and efferent blood-streams both in the free rootlets and in other tubular outgrowths (the test-vessels) which penetrate the sub stance of the test covering the body itself. In other less primitive Phlebobranchs, in which the test becomes greatly thickened, these adventitious outgrowths become greatly multiplied, but in the majority of these cases the epicardium is vestigial or altogether absent, and the septation of the test-vessels is accomplished by longitudinal folds of their own walls. This secondary process of septation may proceed so far as to divide each vessel into two complete and parallel halves, the so-called double-vessels. In Molgulids, which possess such vessels, the vestigial epicardia are converted into a pair of renal excretory organs ; but both in Molgulids and Ascidia, the test-vessels are mainly or entirely derived from a basal vessel opposite the end of the endostyle.