MR. HUXLEY ON THE ANATOxMY OF SALPA AND PYROSOMA. 
587 
separate structure, and the cavity in which the gemmiferous tube lies in all the Salpoe, 
and through which it makes its way to the exterior, seems to represent the normal 
separation of the test and tunics. 
The homology of the cellulose test of the Ascidian with the calcareous test of the 
Mollusk has already been adverted to ; and it would seem that the separation of the 
former from the tunics, or its confusion with it, is of as little value as a character 
among the Tunicata, as the imbedding of the shell in the mantle in Limax would be 
in separating it from Helix, whose shell is distinct from the mantle. 
62. It would appear, indeed, that in no natural family is it less possible to draw any 
very broad line of demarcation among the various members than in the Tunicata. 
Tracing them one by one, we find that all the organs of the Salpce have their 
homologues among the other Ascidians ; the various genera passing one into the 
other by almost imperceptible gradations. 
Even the connection of the foetus with the parent by a placenta, a feature appa- 
rently so unique in the Salpce, seems to be not without its analogue in the Didem- 
nidae*. 
The actual fact of a placental circulation indeed has not been observed, but it may 
be surmised, as M. Milne-Edwards'I' has observed the ova to be developed within a 
diverticulum of the vascular system of the parent. 
The peculiarly formed heart, the circulation without distinct vessels, and the 
reversal of its direction are common to all Tunicata. 
63. In all the Tunicata, again, it would seem that the first bend of the intestine 
(whatever its subsequent course) is dorsal, i. e. to the side opposite the ganglion, 
and almost always to the right side. DoUolum, however, seems to be a sinistral 
Tunicate. 
What has been described in the present paper as the “ Tubular System ” was 
found by Lister (Philosophical Transactions, 1834) in Perophora, and described by 
him as transparent vessels that may be supposed lacteals.” 
In Chelyosoma there is a mass of otolithes and a fossa, seemingly analogous to the 
ciliated fossa.” 
The “ languet” of the Salpa has its homologues in Pyrosoma, Chelyosoma and 
Clavelina, and is represented by smaller tentacular filaments in Cynthia, Diazona, 
Synoicum and Polyclinum. 
64. All the Tunicata are hermaphrodite ; and from the small size of the only efferent 
* The only remaining important difference of Salpa from its congeners consists in the Salpa larvae being 
tailless, while, as the beautiful researches of M. Milne-Edwards have shown, the other Ascidian larvae have 
tails. This exception, however, is singularly paralleled among the Amphibia. The larvae of the ordinary 
Amphibia have, as is well known, deciduous tails like the ordinary Ascidians. In the genus Pipa, however, 
which carries its young in cells upon its hack, the larva is tailless. (Leuceart, Ueher Metamorphose, &c. 
SiEBOLD and Kolliker’s Zeitschrift fiir Wissenschaftliche Zoologie, 1851.) Such a veiy striking analogy 
needs no comment. 
t Ohs. sur les Ascidies composees, p. 23. 
