(1264). The heart, above described, is extremely thin and transparent, and is lodged in a distinct pericardium, which separates it from the other viscera.

(1265). Notwithstanding this apparently simple arrangement of the vascular system in the Ascidians, the nature of the circulation of the blood, throughout the class, is extremely curious, the action of the heart being completely reversed at brief intervals, and the course of the blood entirely changed - a phenomenon which is easily witnessed in any of the smaller and more transparent species, when placed under the microscope. The contractions of the heart succeed each other with regularity; but they are sluggish, not extending at once through the whole organ. The systole commences at one extremity, and is propagated by an undulatory movement towards the opposite end by a sort of peristaltic action. For some time the contractions succeed each other with rapidity, passing on in the same course, when they suddenly cease, and, after a pause, recommence from the other end of the viscus. The blood, thus impelled alternately from behind forwards, and then in the contrary direction, ascends towards the branchial apparatus; nevertheless it docs not appear to be conducted there by closed vessels, but seems to be diffused between the inner tunic of the abdomen and the viscera, where it flows in currents that vary in their direction as the movements of the animal, or any other mechanical causes, affect their passage.

The chief portion of the blood, however, ascends by the dorsal or the ventral surface of the abdomen, and, after having bathed the surface of the viscera, gains the base of the branchial sac. When the contractions of the heart are directed forwards, the ascending current of blood passes along the anterior wall of the abdominal cavity and enters a capacious sinus, situated in front of the respiratory chamber, which gives origin on each side to a series of large transverse vessels; and these intercommunicating with each other by means of innumerable branches disposed vertically, a rich vascular network is formed, that, after spreading all over the walls of the branchial cavity, pours its blood into another vertical sinus situated at the opposite side of the thoracic cavity, into which is likewise poured the vitiated blood derived immediately from the system. Lastly, the circulating fluid, again diffusing itself between the viscera, descends along the dorsal region of the abdomen and again reaches the heart. Were the circulation constant in the above direction, as Milne-Edwards observes, it would somewhat resemble that of other Acephalous Mollusca. The heart might then be compared to an aortic ventricle, and the anterior thoracic sinus to a branchial vein.

But, owing to the contrary directions of the currents of blood, due to the changing action of the heart, the vessels that during one minute perform the functions of veins, are in the next converted into arteries.

(1266). When we consider the fixed and immoveable condition of an Ascidian, and its absolute deprivation of all prehensile instruments adapted to seize prey, it is by no means evident, at first sight, how it is able to subsist, or secure a supply of nourishment adequate to its support; neither is the structure of the mouth itself, or the strange position which it occupies, at all calculated to lessen the surprise of the naturalist who enters upon the consideration of this part of its economy. The mouth, in fact, is a simple orifice, quite destitute of lips or other extensible parts, and situated, not at the exterior of the body, but at the very bottom of the respiratory sac (fig. 243, and fig. 244, g.) It is obvious, then, that, whatever materials are used as aliment, they must be brought into the body with the water required for respiration; but even when thus introduced into the branchial cavity, the process by which they are conveyed to the mouth and swallowed still requires explanation. We have before noticed that the interior of the branchial chamber is covered with multitudes of vibratile and closely-set cilia, well described by Mr. Lister*, which, by their motion, cause currents in the water. When these are in full activity, observes that gentleman in the paper referred to, the effect upon the eye is that of delicately-toothed oval wheels, revolving continually in a direction ascending on the right, and descending on the left of each oval, as viewed from without; but the cilia themselves are very much closer than the apparent teeth; and the illusion seems to be caused by a fanning motion given to them in regular and quick succession, which produces the appearance of waves; and each wave answers here to a tooth.

* Phil. Trans, for 1834, p. 378.

(1267). Whatever little substances, alive or inanimate, the current of water brings into the branchial sac, if not rejected as unsuitable, lodge somewhere on the respiratory surface, along which each particle travels horizontally, with a steady, slow course to the front of the cavity, where it reaches a downward stream of similar materials; and they proceed together, receiving accessions from both sides, and enter at last the oesophagus, placed at the bottom (fig. 244, g), which carries them, without any effort of swallowing, towards the stomach.

(1268). The oesophagus (fig. 244, h) is short, and internally gathered into longitudinal folds. The stomach (i) is simple, moderately dilated, and has its walls perforated by several orifices, through which the biliary secretion enters its cavity. The liver is a glandular mass intimately adherent to the exterior of the stomach and the intestinal canal (fig. 243, e e), of variable length, and, more or less convoluted in different species, after one or two folds terminates in the rectum, which, emerging from the peritoneal investment covering the intestine, has its extremity loosely floating in the cavity communicating with the second orifice (f): into the latter a bristle is introduced in the figure, having its extremity inserted into the anal extremity of the digestive tube. Excrementitious matter, therefore, when discharged from the rectum, escapes from the body through the common excretory aperture, generally situated upon the least elevated protuberance of the outer covering*. It would seem that the food of Ascidians consists of very minute particles of organized matter; for although small Crustacea and other animal remains have been occasionally met with in the branchial chamber, nothing of this nature has been observed in the stomach itself; and, as must be obvious to the reader, the oral aperture seems but little adapted to the deglutition of bulky substances.