Van Beneden, to whom we are indebted for so much information with regard to simple Ascidians in general and Cynthia ampulla in particular, has observed in living specimens of this species, that the blood is distributed to each branchial trunk at the same time, and with the same direction, ascending and de scending alternately. According to this, the heart, contracting in one direction, sends blood towards the branchim, filling all the vessels at one time, and recalls it by contract ing in an opposite direction. That a series of contractions in one direction, during a certain time, may take place without engorgement, there must be a direct communication between the branchi and the peri-intestinal cavity; and this is afforded, according to Van Beneden, by the respiratory tentacles, see p. 1202.
Seen under the microscope, the contractile tissue of the heart affords no trace of mus cular fibre. The contractility remains some time after the removal of this organ from the body ; and that without being irritated.
Nervous system. — A single ganglion, oval, soft, consisting of a saclike neurilem ma, enclos ing nerve-cells or neurine, placed in the sub stance of the mantle, and between its two tubu lar orifices, gives off four branches, two of the branches forming a loop around either tube, and other lesser filaments distributed about the mantle. Besides these, found in the Ascidia gelatinosa one larger and two smaller ganglia between the stomach and the branchial sac. Cuvier considered the large ganglion to be the analogue of the " inferior " ganglion of Molluscs, found in the Bivalves between the branchim and towards the origin of the tube that admits the water ; and ob served that he had not seen any nerves ter minating at a brain, nor the brain itself, which must be situated by the mouth at the base of the branchim.
The great nerve-ganglion in Chelyosoma, i"' long and *'" broad, cylindrical, and yellow coloured, lies near the middle of the inferior surface of the plated test, and a little to the left (fig.777. f). From its anterior angle it sends four branches, two of which form a half circle around the star-shaped muscular ap paratus of the branchial orifice, the other two losing themselves in the muscles bordering the nearest plates. From the posterior angle arise eight nerves, four going to the anal orifice, and four to the lateral muscles of the plates ( fig. 777. d, g).
Special senses. — In the last-named genus there are two remarkable bodies found in connection with the nerve-ganglion, which Professor Eschricht, to whom we are indebted for a careful anatomical description of Che Iyosoma, considers probably to appertain to the function of hearing (fig.777. g, h). One of these bodies has the appearance of a minute bladder, filled with a whitish substance. It measures I"' long, and I"' broad, and lies to the left of, and quite close to, the ganglion, being at its posterior extremity strongly ad herent to it, or to the base of one of the nerves proceeding from it, by means of a stalk-like attachment. Through its diaphanous walls a row of arched transverse striae are discernible, which are either folds of the parietes, or a par tition. The other body is pear-shaped, about im long, lying anterior to the ganglion and the little bladder-like body, and, like them, be tween the serous membrane and the respira tory sac ; its stem is placed between these two bodies, and its head advances up to the hindmost muscle of the branchial orifice. It seems to be of tolerably firm consistence, but it is not at all bony or horny. Its thick, an terior portion is barely i'" broad, and has anteriorly a deep hole, which seems to lead into a large cavity ; the border on all sides of this pit is prominent, gently declining to a blackish little body within.
The Ascidice have frequently around the ex tremity of each process of the mantle, i.e. the branchial and anal tubes, a row of coloured points or ocelli, similar to the imperfect organs of sight present in the majority of the bivalve Acephalans, where they are arranged along the margin of the mantle, or dotting the edges of the siphonal orifices. The number of these oculiforrn points correspond with the number of processes or folds that the margins of the tubes respectively bear ; about eight in the branchial, and six in the anal tube. They are usually red, as in A. vitrea, A. virginea, A. pru awn, and others; in A. mentula they are yellow, with a central red spot.
M. Milne-Edwards has observed similar eye like points around the oral tubes of Ainazirou than and Parascidia, belonging to the botryl lian group of Tunicates.
Some of the Salpce also have ocular spots.
Besides these oculiform points in the adult animals, Van Beneden has observed in the larvce of Cynthia ampulla, on the side of the gibbous or anterior portion representing the head, some black points that he regards as true eyes. Speaking of the development of the young Ascidian in the egg, Van Beneden describes the separation of the contents of the incubated ovum into an external layer, to form the skin and the tail of the young animal, and a second layer, of which subsequently the walls of the alimentary canal are formed ; and, thirdly, an internal vitelline mass. " In the thickness of the external layer," says Van Beneden, " there appears towards the middle of the body, and rather inferiorly, a cell filled with black pigment, which must be regarded as the organ of vision. It persists during the whole term of the animal's locomotive exist ence, and disappears after it becomes fixed. These organs," he adds, " which we may well call eyes, although so simple, are constant ; sometimes two are discernible on one side." M. Milne-Edwards has also seen in the larvme of the compound Ascidians one or two blackish points, but towards the posterior extremity of the body ; he merely notices them, without assigning for them any function. " We consider ourselves," says Van Beneden, " sufficiently authorised, by all that we have seen in the embryos of the free inferior ani mals, to elevate these pigment-cells to the dignity of an organ of special sense. To say that the presence of an eye implies that of an optic nerve and of a brain does not appear to us to be more just than to say that there are muscles when there is movement. We have the example of the Hydrce, that, without mus cles, without nerves, without brain, and with out a special organ of feeling, are sensible to light, and avoid or approach at will bodies whereby they are affected. And if the Hydra, without special apparatus, is sensible to light, we do not see why a cell of pigment may not be the first rudiment of the organ of sight. The appearance of the eye and the ear, in the animal series, takes place in similarly simple rudimentary forms."* Generation. —The Ascidiadce are all herma phrodites. Between the fold of the intestine, and close to the liver, when the latter exists, there is observed a whitish glandular organ; this is the ovary. An undulatory duct passes from it,. which, following the rectum, opens near its extremity into the cloaca. Opposite to the opening of the oviduct, Carus noticed the orifice of another glandular organ, which he thought to be either a male organ, or serving to furnish the gelatinous covering of the ova.