286 Dr. G. J. Allman on the Homology of the Organs 



racteristic contractility. Now these muscles are exactly represented by 

 equivalent fibres which are developed in the homologous tunic or endocyst of 

 the Polyzoa, and constitute the " parietal muscles" of these animals. The cir- 

 cular bands of Salpa and Doliolum appear to be developed in the internal tunic, 

 and have their representatives in the sphincters occiu-ring in the inverted 

 tunic of the Polyzoa. Striated muscular fiibre exists in many, if not in all the 

 Polyzoa, and a similar condition of this tissue has been detected by Eschricht 

 and Huxley in Salpa. 



6. Nervous System. — Between the great nervous ganglion in the Tunicata 

 and the Polyzoa there is apparently a marked difference in position, this gan- 

 glion in the Tunicata being placed between the respiratory and cloacal orifice, 

 while in the Polyzoa it is situated on the oesophagus near its oral extremity, and 

 this difference might at first lead to the belief, that the homological identity 

 which we have witnessed between the other organs of the two groups fails to show 

 itself in the nervous system ; still, however, it can be rendered evident, that no 

 exception is here offered to the unity of plan already demonstrated, and that 

 the two ganglia are strictly homologous. The ganglion is manifestly iden- 

 tical in function in the two groups, for in each we have nerves passing off from 

 it both to the respiratory apparatus and to the oesophagus and region of the 

 mouth, a distribution in which it corresponds with that of both the branchial 

 and cephalic ganglia of the higher MoUusca, whose offices it thus seems to 

 combine. 



In several of the Tunicata, a well-defined otolithic capsule has been dis- 

 covered in connexion with the ganglion ; and Mr. Huxley has suggested to 

 me that this gano-lion ought therefore to be considered as homologous with the 

 pedal ganglion of the Lamellibranchiate MoUusca, since in these the otolithic cap- 

 sule is always found in connexion with the pedal ganglion. To this view, how- 

 ever, several objections appear to me to present themselves ; the ganglion of the 

 Tunicata and of the Polyzoa has functions devolving on it which we never see 

 performed b}' the pedal ganglion o^ the Lamellibranchiata ; the development 

 of the pedal ganglion, moreover, bears a constant relation to that of the foot, 

 and though the obliteration of the foot does not necessarily bring with it the 

 absence of the ganglion — as in Teredo, for example, where the researches of 

 QuATKEFAGES have shown the existence of a pair of minute ganglia, manifestly re- 



