ANATOMY AND PHYSIOLOGY OF SALPA AND PYROSOMA 6l 



Clavelinn and PeropJiora are acknowledged to be closel}- allied 

 genera, and yet in the former the test and outer tunic are separated 

 to their utmost extent ; in the latter ^ they are as closely united as in 

 any Salpa. 



In the Cynthise the test and tunics are generally very distinct, but 

 in Cynthia auipulloides, judging by the descriptions of Van Beneden, 

 they are confounded together.^ 



Again, in the Salpa vaginata of Chamisso, the test makes its ap- 

 pearance as a separate structure, and the cavity in which the gemmi- 

 ferous tube lies in all the Salpce, and through which it makes its wa/ 

 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 cal- 

 careous test of the Mollusk has already been adverted to ; and it would 

 seem that the separation of the former from the tunic, or its confusion 

 with it, is of as little value as a character among the Tnnicata, 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 apparently so unique in the Salpa, seems to be not without its 

 analogue in the Didemnidae.^ 



The actual fact of a placental circulation indeed has not been 

 observed, but it may be surmised, as M. Milne-Edwards * has ob- 

 served 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. 



' See the very beautiful figures and descriptions of Lister in tlie Philosophical Transactions- 

 for 1834. 



- Mem. de I'Acad. Roy. de Bruxelles, tome xx. 



' The only remaining important difference of Salpa from its congeners consists in the 

 Salpa larvce being tailless, while, as the beautiful researches of M. Milne-Edwards have 

 shown, the other Ascidian larva have tails. This exception, however, is singularly paralleled 

 among the Amphibia. The larvse of the ordinarj' 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 back, the larva is tailless. (Leuckart, Ueber Metamorphose, &c. , Siebold and 

 KoUiker's Zeitschrift fiir Wissenschaftliche Zoologie, 185 1.) Such a very striking analogy 

 needs no comment. '' Obs. sur les Ascidies composees, p 23. 



