208 PROFESSOR HUXLEY ON THE ANATOMY 



The vascular system of Fyrosoma is exceedingly simple ; nor could we anywhere find a 

 more convincing example of the validity (in some cases, at any rate) of Milne-Edwards's 

 views of the circulation in the Mollusca than is offered by this animal. The heart lies 

 close to, and apparently connected with, the right side of the posterior and haemal wall of 

 the pharynx, between the endostyle and the bend of the intestine ; and it appears to have 

 exactly the same structure as in P. atlanticum. There are no vessels, the whole inter- 

 space between the inner tunic and the outer, or between these and the atrial tiinic, being 

 one vast blood-sinus, with which the canals in the branchial bars communicate at each 

 end. I have spoken of the haemal and of the hypopharyngeal sinuses merely as a matter 

 of convenience ; in point of fact, the general blood-cavity is not naturally divided into 

 distinct sinuses*. 



The nervous system consists, as in Fyrosoma atlanticum, of a single ganglion, of an oval 

 shape when viewed sideways, but somewhat heart-shaped when seen from above, its nar- 

 rower end being turned backwards. It is about xioth of an inch long, and is composed of 

 a dark granular mass invested by a delicate structureless membrane. It lies between the 

 inner and outer tunics, the former being raised, so as to form a slight protuberance over it. 

 On the posterior half of this protuberance lies the broad lower median portion of the peri- 

 pharyno-eal ridge. In its anterior half, the opening of the ciliated sac appears. The prin- 

 cipal nerves given off from the ganglion are the following. Two, a smaller internal and a 

 laro-er external, pass from the antero-lateral parts of the ganglion, forwards towards the 

 oral aperture, branching as they go. I suspect that a nerve runs up on each side, be- 

 neath the peripharyngeal ridge ; but I cannot make sure of the fact. A considerable 

 nervous trunk is given off to the poster o-lateral walls of the body ; and, finally, two deli- 

 cate trunks arise posteriorly, one on each side of the middle line, which run back, so as to 

 have the languets between them, and passing up at the sides of the cesophageal aperture, 

 are lost under the divisions of the posterior epipharyngeal ridge. 



In Fyrosoina atlanticum I observed a mass of deep-red otoliths in contact with the 

 posterior end of the ganglion {I. c. p. 583), but no such structures are discernible in the pre- 

 sent species. The ciliated sac (' tubercule anterieur ' of Savigny)— an organ of univerl^l 

 occurrence among Ascidians — is mPyrosoma gigantemn an elongated, laterally compressed, 



* I have described the circulatory system of Salpa in similar terms to these, in my memoir on Salpa and Pyrosoma, 

 and notwithstanding the criticism my statements have received both from M. Vogt and Prof. Leuckart, I must main- 

 tain their correctness. M. Vogt affirms that I have committed ' a grave error ' in declaring the blood-canals of 

 Salpa to be lacunse between the two layers of the mantle,— apparently supposing that I mean thereby the test and the 

 external tunic, and forgetting my careful discrimination of test, outer tunic and inner tunic, at p. 585 of the memoir 

 cited. In fact, nothing can be easier than to observe the entire distinctness of the inner and outer tunics in a bud or 

 embryo of Salpa or Pijrosoma— to see that the viscera and blood-canals do really lie between these tunics, and that 

 they are by no means, as M. Vogt states, lodged in cavities excavated in the ' inner mantle.' Prof. Leuckart has 

 equally mistaken my meaning when {I. c. p. 14) he ascribes to me a participation in Eschricht's opinion as to the 

 existence of a serous' sac surrounding the body of Salpa. My words in the passage cited by Prof. Leuckart are, " In 

 very young SalpcB, this space [the interval between the inner and outer tunics] is like the cavity of a serous sac." Still 

 less can I find in my memoir any such opinions as those ascribed to me in the note to p. 43 of Prof. Leuckart's 

 valuable memoir. While on the subject of errors, however, I am glad to take the opportunity of painting out that 

 several statements made at second-hand in my memoir, regarding Ascidians other than those specially described, 

 are incorrect. The diagram of Pdonaia (pi. 19), again, is altogether erroneous— this Ascidian differing, as I have 

 since found, in no essential respect from Cynthia, 



