ACTINOZOA. 167 



The nervous system of the Ctenophora consists 

 either of a single ganglion or of a pair of ganglia 

 closely approximated, giving origin to a number 

 of delicate nerve-like cords. The ganglion lies 

 deeply seated within the pyramidal mass of ecto- 

 derm included between the apical canals, towards 

 the narrow extremity of which its apex is directed, 

 while its base rests upon the surface of the cteno- 

 cyst. In form it is sub-pyriform or bluntly conical : 

 anatomically, it seems resolvable into a thin tran- 

 sparent w^all, enclosing granular contents ; in 

 colour, it is most frequently pale 3^ellow. From 

 this central mass issue two principal series of 

 nervous cords, one of which arches inwards towards 

 the walls of the digestive cavity, and, in some 

 cases, separates into four sets or bundles to supply 

 the principal regions of the body. The nerves of 

 the second series, usually eight in number, are 

 distributed along the rows of swimming combs so 

 as to lie between the latter and their corresponding 

 canals. These cords appear dilated at intervals 

 into numerous minute ganglionic enlargements, 

 giving off secondary filaments, one for each of the 

 ciliated plates ; an arrangement, which, if corrobo- 

 rated by subsequent investigations, would go far 

 to throw some light upon the singular and quasi- 

 independent movements which these combs per- 

 form in the living animal. There is still, however, 

 much diversity of opinion as to the true interpre- 

 tation of the parts just described. Kolliker, while 

 recognising in Chiajea the presence of delicate 

 cords extending from comb to comb, expresses 

 himself, nevertheless, as very doubtful of the ex- 

 istence of a nervous system in any of the Cteno- 

 phora which he had himself investigated. Agassiz 



M 4 



