THE ANATOMY OP PLEUKOTOMARIA BEYRICHII. 241 



definite accumulatiou of iierve-cells met with in other Proso- 

 branchs. 



Fischer and Bouvier seem to have been misled, either by 

 this coloration or by the slight swelling of the cords in 

 certain of these regions, iuto the belief of the existence of 

 definite ganglionic areas, and they indicate such regions by 

 means of dots in their figures ; they appear to have over- 

 looked the presence of nerve-cells along the connectives, and 

 the still more important, though slight, accumulation and 

 coloration at the point of origin of the visceral nerve. 



The Cerebral Ganglia. — The circum-oesophageal nerve- 

 ring is much enlarged on either side of the anterior part of 

 the buccal cavity, and since the tentacular and certain other 

 nerves which are characteristic of the cerebral ganglion of 

 other Prosobranchs arise from this region, we may regard it 

 as representing that ganglion. The cerebral ganglia are, 

 then, a pair of elongate band-like structures widening out 

 below ; they are connected together above the buccal mass by 

 a slightly narrow region {cb. c), which represents the 

 cerebral commissure of more specialised forms, but which 

 here is indistinguishable from the ganglia themselves, since 

 both in its size and in the number of its ganglionic cells it 

 passes imperceptibly into the laterally placed ganglionic 

 areas. The cerebral ganglia give origin to five pairs of 

 nerves supplying the lips (tigs. 21, 22, and 29), and to a pair 

 of laterally placed tentacular nerves {t. n.), from which in 

 turn the optic nerve arises. Arising with the most ventral of 

 these labial nerves is a broad nerve which runs downwards 

 and below the buccal mass (figs. 21 and 29) ; this nerve 

 gives off a sixth lip-nerve, and is then continued ven- 

 trally to the mouth and close to the lips, to meet and fuse 

 with a similar nerve from the opposite side of the body, thus 

 constituting the labial commissure (Z. c.) so characteristic of 

 the Diotocardia and archi-Taenioglossa. 



Yet another nerve arises from the ventral continuation of 

 the cerebral ganglion, but in order to see this properly the 

 mesial aspect of the ganglion must be examined. Such a 



VOL. 44, PAKT 2. NEW SEKItS. Q 



