Miscellaneous. 411 



like all the rest of the wall, composed of easily recognizable cubical 

 cells. The rounded cells, now interposed on the outer side, com- 

 mence an active proliferation in all directions, and quickly form an 

 oval aggregation, which begins to push inwards, towards the anterior 

 wall, the posterior wall of the vesicle, which thus becomes com- 

 pressed. 



The oval aggregation of cells is nothing but the ganglion properly 

 80 called, which has only to become enlarged and to extend beyond 

 the sides of the vesicle in order to re; Jize the adult state. As to the 

 vesicle, which always retains its walls in their integrity, and with 

 their histological structure so different from that of the ganglion, it 

 has only to open at the bottom of a slight depression of the branchial 

 sac, which advances towards its superior extremity, in order to con- 

 stitute the ciliated sac of Huxley. 



From this description we see that the primitive nervous canal, the 

 Nervenrohr, which was supposed to form the ganglion directly by 

 its obliteration, is nothing but the ciliated sac, the canal of the sub- 

 nervian gland which gives origin to no nerve ; and the ganglion 

 properly so called only proceeds from it indirectly, and only appears 

 at a very late period. 



Such are the positive facts that I have been able to ascertain in 

 Pyrosoma. What remains for me to say is only an induction which 

 needs to be verified, but which appears to me to be founded upon 

 sound arguments. Pyrosoma, notwithstanding its relations with the 

 Thaliaceaj, is, by its general organization, a true compound Ascidian ; 

 it is therefore allowable to think that the neural canal observed in 

 the larvfe of Ascidia, and the ccnhral vesicle, which is only a part of 

 it, may, as in Pi/}'osoma, be merely the rudiment of the canal of the 

 subnervian gland. 



This opinion is the more probable because the anterior portion of 

 this canal opens in the Ascidian larva also into the branchial sac 

 and the ganglion properly so called is formed at its posterior part, 

 although its mode of origin has not been exactly ascertained. 



Although we know the origin of the subnervian canal, at least so 

 far as Pyrosoma is concerned, we have said nothing of its functions. 

 Without having yet any positive evidence, I believe, with the 

 majority of authors, that it really acts as a sensory organ, and pro- 

 bably an olfactory organ. It may be objected that there is no 

 nerve ; but the posterior wall of the canal is applied so directly 

 against the branchial surface of the ganglion, that it is very difficult 

 ■ to assert that some nervous fibriUa?, not longer than the thickness of 

 a cell, do not directly traverse this wall ; it is not towards the vesti- 

 bule that we must seek for these nerves, but at the bottom of the 

 canal, or in the gland, which is perhaps only an organ destined to 

 amplify the sensations. 



At any rate, if we have a gland here, its canal is not an excretory 

 duct; for, besides that it is easy to see (as has been done by all authors) 

 that in the living animal the movement of the cilia is directed to- 

 wards the canal, and not outwards, one can easily ascertain, as I 



