ANATOMY AND PHYSIOLOGY OF SALPA AND PYROSOMA 59 



Midway between the ganglion and the point of division, the 

 ■erging bands give off e 

 lateral oval cellular masses. 



diverging bands give off each a thin band, which runs to the 



Section III. — The Homology of Structure of Salpa and Pyrosoma, 

 and of these with the oj-dinary Ascidians. 



59. It seems to have been pretty generally admitted by naturalists, 

 that the Tunicata are susceptible of division into two great classes, the 

 Monochitonida and Dichitonida, 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 

 Py7-osonia, the former was placed among the Monochitonida (or 

 " those having the inner sac adherent 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,hvit 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 whatever to be dra\\'n between them. 



' This essential difterence 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 Mono- 

 chitonidous or Dichitonidous. 



The test of the Ascidian composed of cellulose is every waj- homologous with the test of 

 the MoUusk composed of carbonate of lime. 



