44 



PROCEEDINGS OF THK M ALACOLOGICAL SOCIKTY. 



system I need not dwell ; it agrees in essentials with previous 

 descriptions (Fig. I), but there are several points to which I 

 must more particularly call attention. (1) There is no sign of 

 any separation of the cerebral ganglion into cerebral and pleural 

 enlargements. This point was shown in the first place in a model 

 reconstructed from sections taken transversely to the long axis of the 

 animal, and was confirmed by horizontal sections (^Fig. II). (2) The 

 cerebral ganglion has no definite posterior limit, but gradually tapers 

 away (as described by Drew) into the cerebro-visceral connective, 

 while the whole of this connective, or, in fact, the entire ring formed 

 by the cerebral and visceral ganglia with their commissures and 

 connectives, is to some extent ganglionic throughout. (3) The two 

 roots of each cerebro-pedal connective rise from the inner side of the 



C.COtTV. 



C.G 



Fig. II. — Horizontal section of right cerebral ganglion of Nncula nucleus, a.kt. 

 anterior root of cerebro-visceral connective ; p.rt. its posterior root ; 

 c.coM. cerebral commissure ; c.g. cerebral ganglion ; ces. oesophagus. 



cerebral ganglion, close together and both at a point posterior to the 

 origin of the anterior pallial nerve, which according to Pelseneer 

 should spring from the pleural enlargement. (4) The distal free part 

 of the otocystic nerve is of practically the same diameter as the hinder 

 root of the cerebro-pedal connective (as stated by Drew). (5) The 

 pedal ganglia are united by two serially arranged commissures 

 (Fig. Ill) : a multiple connection between these ganglia, suggestive of 

 the ladder-like condition in Amphineura and low Gastropods, has been 

 noted by Rawitz in TJnio,^ and by iStempell' in certain Protobranchs, 

 but has not hitherto been recorded for Nncula. 



In conclusion, a few words with regard to Drew's comparison between 

 the nervous systems of Nucula and an Amphineuran. In the Chitons 

 (the most generalised Amphineura so far as concerns the nervous 



Rawitz, "Das zentrale Nervensystem der Acephalen"; 



(1887), p. 438. 

 Stempell: Zool. Jahrb. (Fauna Chilensis), i, p. 405. 



Jena Zeits., Bd. xx 



