84 ADAM SEDGWICK. 



ment is of great importance ; but before it can be of any value 

 to us, we must be able to decide whether the neural surfaces of 

 the Coelenterata, Annelida, &c., are homologous. It will be 

 generally admitted that the nervous systems of the Annelida, 

 Arthropoda and MoUusca are built upon the same type ; and 

 that the ventral surface of the body is homologous in each of 

 these three groups. The late Prof. Balfour put forward the 

 hypothesis that the nervous system of these types was homo- 

 logous with that of Coeleuterata. He says : 



" In the first place it is to be noted that the above specula- 

 tions render it probable that the type of nervous system from 

 which that found in the adults of the Echinodermata, Platyel- 

 minthes, Chsetopoda, MoUusca, &c., is derived, was acircumoral 

 ring, like that of Medusae, with which radially-arranged sense- 

 organs may have been connected j . . . , Its anterior 

 part may have given rise to supra-oesophageal ganglia and organs 

 of vision; these being developed on the assumption of a 

 bilaterally symmetrical form, and the consequent necessity 

 arising for the sense-organs to be situated at the anterior end 

 of the body. If this view is correct, the question presents 

 itself as to how far the posterior part of the nervous system of 

 the Bilateralia can be regarded as derived from the primitive 

 radiate ring. 



" A circumoral nerve-ring, if longitudinally extended, might 

 give rise to a pair of nerve-cords united in front and behind, 

 — exactly such a nervous system, in fact, as is present in many 

 Nemertines (theEnoplaand Pelagonemertes), in Peripatus and 

 in primitive moUuscan types (Chiton, Fissurella, &c.). From 

 the lateral parts of this ring it would be easy to derive the 

 ventral cord of the Chsetopoda and Arthropoda. It is especially 

 deserving of notice, in connexion with the nervous system of 

 the above-mentioned Nemertines and Peripatus, that the com- 

 missure connecting the two nerve-cords behind is placed on the 

 dorsal side of the intestines. As is at once obvious, by refer- 

 ring to the diagram (fig. 231 b), this is the position this com- 

 missure ought, undoubtedly, to occupy if derived from part of 

 a nerve-ring which originally followed more or less closely the 



