42 rUOCEEDINGS OF J'HK MALACOLOGICAL SOCIETY. 



On the other hand, Drew, in dealing with the central nervous system 

 of JVucuIa delphinodonta,^ strongly insists upon its essentially primitive 

 nature, and finds neither in the developmental processes through 

 which it passes nor in the adult condition any sure indication of 

 the Gastropod type of specialisation. Apart from the generalised 

 structure of the nervous system as a whole, his two main points 

 are — (1) that the cerebral ganglia develop from a single rudiment 

 on either side, without signs of a pleural enlargement, and (2) that 

 there is no indication of a pleural enlargement in the adult either 

 of this species of Nacula or in the other forms of Protobranchiata 

 examined by him. Another fact of some importance in connection 

 with his general conclusions is furnished by the mode of development 

 of the cerebro - visceral connectives. He writes:^ "They are first 

 found very close to the surface, almost, if not quite, in contact with 

 the ectoderm. Later they sink deeper into the body. The cerebro- 

 visceral commissures are quite thick, and differ from the cerebro-pedal 

 commissures in having much the same structure as the ganglia 

 themselves." 



It will be remembered that Pelseneer's comparison was suggested 

 not only by the double cerebral ganglia found by him and Stempell ^ 

 in several Protobranchs, but also by the corresponding duplicity of 

 the cerebro-pedal connectives. This duplicity of the connectives has 

 been confirmed by Drew, but he suggests that possibly the posterior 

 of the two roots — the pleuro-pedal connective according to Pelseneer 

 and Stempell — is the proximal end of the otocystic nerve, and not 

 really a connective at all. In Pelecypods the otocystic nerve arises 

 in the cerebral ganglion, and normally accompanies the cerebro-pedal 

 connective, intimately blended with it, to a point near the pedal 

 ganglion, and then branches off to the otocyst. In Solenomya, 

 laowever,* the nerve has been found to leave the cerebral ganglion 

 independently behind the pedal connective, and to run entirely free 

 to the otocyst. This fact suggested to Drew that possibly in the 

 Protobi'anchs with two roots to the cerebro-pedal connectives, a tran- 

 sitional stage between Solenomya and the normal condition is realised, 

 in which the proximal and distal ends of the otocystic nerves are free, 

 while the intermediate parts are fused with the cerebro-pedal con- 

 nectives. This idea is strengthened by the fact that the distal, free 

 part of the otocystic nerve in Nucula is of the same calibre as the 

 posterior root of the cerebro-pedal connective. The only objection, 

 and a very important one, is that in Solenomya each pedal connective, 

 although emerging single from the cerebral ganglion, arises within 

 the ganglion by two independent roots. This is a point that requires 

 further confirmation, but if confirmed it would be fatal to Drew's 

 suggestion. 



1 Drew, I.e., p. 370. 



2 Drew, I.e., p 372. 



3 Stempell, " Beitrage zur Kenntniss der Nueuliden": Zool. Jahrb. (Fauna 



Chilensis), i (1898), p. 403. 

 * Stempell, "Zur Anatomie von Solemya tognta" : Zool. Jahrb., Bd. xiii (1899), 

 p. 147. 



