1208 



TUNICATA. 



the mantle. Besides these, Meckel* found 

 in the Ascidia gclatinosa 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 branchiae 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 branchiae. 



The great nerve-ganglion in Chelyosoma, $'" 

 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. 111. 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.lll.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- 

 lyosoma, considers probably to appertain to the 

 function of hearing (fig. 111. g, h). One of 

 these bodies has the appearance of a minute 

 bladder, filled w r ith a whitish substance. It 

 measures \'" long, and ^"' 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 

 f '" 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 ^'" 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 AscidicB 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 

 oculiform 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. vilrca, A. virginea, A. pru- 

 num, 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 Amaurou- 

 cium 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 

 larvae 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 

 larvae 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 



* Schalk, De Ascidiarum Structuro, Halle, 1814. 



Van Beneden, Mem. Brux. Acad. vol. xx. p. 40. 



