332 BULLETIN OF THE 



and atrial apertures are formed after the embryo is considerably ad- 

 vanced (Figs. 31 and 32), precisely as in the solitary form. Accord- 

 ing to Vogt,* this tunic splits into two layers, the outer becoming the 

 cellulose test, and the inner the outer tunic ; nothing of this kind 

 was observed, and appearances seemed to indicate that the test is an 

 excretion from the surface of the body, as stated by Huxley (p. 585), 

 and Leuckart ; but this must still be regarded as one of the unsettled 

 points in the history of the genus. 



The Body Cavity. — The constrictions (Figs. 27, 28, 29, and 30, 4) 

 which mark out the zooids upon the surface of the tube gradually 

 extend inward, and enclose part of the sinus cavity, which thus 

 becomes converted into a body cavity, into and out of which it is 

 quite easy to watch the blood corpuscles of the solitary form make 

 their way. This direct connection of the sinus systems of the young 

 zooids with that of the solitary Salpa persists long after all the 

 principal organs of the former have attained their essential character- 

 istics ; and Huxley states (p. 574) that he has seen "one of the large 

 blood corpuscles of the parent entangled in the heart (which was then 

 not more than one five-hundreth of an inch long) of a very young 

 foetus." This connection of the two circulations may be seen with 

 such perfect distinctness in our species that the only way in which 

 we can reconcile Vogt's observations upon the circulation of the chain- 

 zooid of S. pinnata with those here detailed is by assuming that 

 there is a very decided and remarkable difference between the 

 species in this respect.t 



Digestive and Nervous Organs, and the Egg. — In Fig. 28 part of 

 the proximal portion of a stolon a little more advanced than the one 



* Sur les Tuniciers nageants de la Mer de Nice (p. 42). 



t According to Vogt (p. 47), the chain-salpa of S. pinnata is provided with an organ, 

 the " stolohlast," homologous in structure and function with the placenta of the soli- 

 tary emhryo. He says that this is situated near the nucleus and heart, and is com- 

 posed of two chandlers, into one of which the blood of the emhryo makes its way, and 

 thus comes into close contact with the blood circulating in the sinus system of the 

 parent, which has access to the second chamber. He says that there is no communi- 

 cation between the two, and that none of the blood of the parent gains access to the body 

 cavity of the zooid. Unfortunately his figures, and the letters of reference especially, 

 are so indefinite that very little can be made out by the study of them ; but the expla- 

 nation has suggested itself that the " stolohlast " may be nothing more than the rudi- 

 mentary testicle, as Vogt says that it begins to disappear at the same time that the 

 development of the latter begins, and disappears entirely after this is formed. 



