of the Tunicata and the Polyzoa. 287 



presenting the pedal ganglia of those LainelUbranchiafa, in which the foot is not 

 suppressed, — yet the pedal ganglion presents us under such circumstances with its 

 lowest condition of development, and analogy will not permit us to suppose that in 

 the absolutely footless Tunicate or Polyzoon this ganglion acquires its maximum, 

 and even becomes here the only nervous centre present. It would, indeed, seem 

 as if the solitary nervous centre of the Tunicata and Polyzoa combined the func- 

 tions of the several separate centres oi the LameUibranchiata, while the superior 

 importance of the cephalic element determines its supra-cesophageal position. 



If we now carefully consider the difference of position between the two 

 ganglia, we shall find that this is, after all, unimportant ; in the Tunicata, while 

 the ganglion is always placed between the two external orifices, it is at the 

 same time situated in the interval between the internal and middle tunic, and 

 is consequently in the midst of the sinus ; in the Polyzoa, the two orifices co- 

 alescing, the ganglion can no longer occupy the position it held in the Tunicata ; 

 it is, therefore, carried backwards, and, still bathed in the fluid of the sinus, now 

 becomes situated on the oesophagus, a difference of position which, it will easily 

 be seen, involves no important change of relations, and which is necessarily 

 connected with the difference in the arrangement of the other organs in the 

 respective groups. In the Polyzoa, from their constant motions of retraction 

 and exsertion, the ganglion could not occupy the fixed position which it does in 

 the Tunicata, and, therefore, comes to be situated upon the polypide itself, all 

 whose motions it then necessarily follows. 



7. Generative System. — The construction of the generative'' system in the 

 Tunicata and Polyzoa is also in conformity with the views of the present memoir. 

 Both are hermaphrodite ; in both we have, besides true sexual generation, 

 generation by gemmation, the gemma in the Polyzoa being formed exactly 

 as in the Tunicata from a diverticulum of the sinus system. 



Though our knowledge of the developmental phenomena is in many re- 

 spects so deficient as to afford much less assistance in the present inquiry than 

 could be desired, yet if we compare the erabryological development of an 

 Ascidian as given by Milne-Edwards or Van Beneden, with that of a Polyzoon, 

 we shall still find the results in accordance with the views of the present paper. 

 In the embryo-Ascidian, after the internal organs have begun to assume the 

 definite form which is subsequently to characterize them, we find that the in- 



