MR. HUXLEY ON THE ANATOMY OF SALPA AND PYROSOMA. 
585 
much increased in size, and eventually terminates at a short distance from the gene- 
rative glands, forming on each side the band of which Savigny speaks as passing 
towards the liver. 
Midway between the ganglion and the point of division, the diverging bands give 
off each a thin band, which runs to the lateral oval cellular masses. 
Section III. — The Homology of Structure o/’Salpa and Pyrosorna, and of these with 
the ordinary Ascidians. 
59. It seems to have been pretty generally admitted by naturalists, that the Tuni- 
cata are susceptible of division into two great classes, the Monochitonida and Dichi- 
tonida, characterized by certain differences in the structure of the branchiae and in 
the degree of adhesion of the inner and outer tunics. 
Of the two species whose structure has been described, Salpa and Pyrosorna, the 
former was placed among the Monochitonida (or “ those having the inner sac ad- 
herent throughout to the outer tunic”), while the latter was reckoned among the 
Dichitonida (or those “ whose inner sac is adherent to the outer tunic, at its two 
orifices, only”). 
Now there is an ambiguity which must be noticed here at starting, as it is one 
which has caused much confusion, and must, unless cleared up, cause our conception 
of the real structure of the Ascidians to be very indistinct. Authors speak of the 
greater or less adherence of the outer and inner sacs, and consider the “ outer sac ” 
of the ordinary Ascidian to be homologous with the outer tunic of the Salpa. The 
“inner sac,” again, is with them homologous with the inner tunic of the Salpa. But 
it is not so ; every Ascidian, as M. Milne-Edwards has clearly shown in Clavelina, 
consists of three tunics : an outer, the test ; a middle, which is here called outer 
tunic ; and an inner, the inner tunic. The inner tunic of the Salpa answers to the 
inner tunic of Clavelina, but its outer tunic answers to the test and the outer tunic 
together (90.)*. 
However, with regard to the two genera in question, whatever be the nature of the 
two membranes of which they are composed, there is absolutely no distinction what- 
ever to be drawn between them. The inner membrane is just as much or as little 
adherent to the outer in Pyrosorna as in Salpa. In each case the wide sinuses between 
the two membranes form the sole vascular system. 
60. It may be said that there is an essential difference between Salpa and Pyro- 
* This essential difference between the test and the two tunics of the Ascidians has its origin in the embryo. 
The tunics are formed by the ordinary process of development, while the test having a totally different chemical 
composition, is in a manner secreted round, and envelopes the whole embryo. 
There seems to be a certain independence in the mode of growth of the embryo and that of the test, the 
former lying at first quite free in the latter ; and it appears to depend entirely upon the relative rates of growth 
of the two whether the resulting Ascidian shall be Monochitonidous or Dichitonidous. 
The test of the Ascidian composed of cellulose is every way homologous with the test of the Mollusk com- 
posed of carbonate of lime. 
