208 PROFESSOR HUXLEY ON THE ANATOMY 
The vascular system of JPyrosoma is exceedingly simple ; nor conld 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 tunic, 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 Tyrosoma 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 t46^1i 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- 
pharyngeal 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 
* 
larger 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 postero-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 oesophageal aperture, 
are lost under the divisions of the posterior epipharyngeal ridge. 
In Pyrosoma 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 universal 
occurrence among Ascidians — is mPyrosoma gigaateum 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 lacunae 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 Fyrosoma — to see that the viscera and blood-canals do really lie between these tunics, and tha 
Vogt states, lodged in cavities excavated in the ' inner mantle/ Prof. Leuckart has 
M 
(I. c. p. 14) 
* " In 
existence of a serous sac surrounding the body of Salpa. My words in the passage cited by Prof. Leuckart ar 
very young Salpa, this space [the interval between the inner and outer tunics] is like the cavity of a serous sac." Sti 
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 
opportunity of pointing out that 
valuable memoir. While 
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several statements made at second-hand in my memoir, regarding Ascidians other than those specially describe , 
are incorrect. 
since found, in no essential respect from Cynthia. 
again 
Ascidian differing, as I have 
