1908.] ASPIDOBRANCH GASTROPOD MOLLUSCS. 853 



the subject of the cerebral ganglia, the labial commissure, the 

 buccal commissure, and the pedal centres to the excellent figures 

 and descriptions of Bouvier, I. will pass on at once to the 

 •disputable points. 



It has been clearly demonstrated by Bouvier (10), Boutan (7), 

 and Haller (20) that the marine Neritce {JSferita) and Septaria 

 are typically streptoneui'ous, the supra-intestinal branch of the 

 visceral commissure existing as an extremely fine nerve whose 

 relations are normal. Lenssen (26) has shown that with one 

 exception the general character of the nervous system is the same 

 in Neritina fiuviatilis, but this exception is important. He could 

 not find any trace of the supra-intestinal nerve, nor of any 

 connection between the visceral and branchial ganglia. I have 

 taken great pains to discover this nerve in the same species, but 

 have net succeeded in discovering it either in sections or by 

 dissection. 



Though it would seem on a priori grounds improbable that the 

 nervous system of Neritina jiuviaiilis, so similar to that of 

 Nerita in all other details, should difler from it in the 

 deficiency of so characteristic a nerve, I can only add my 

 testimony to that of Lenssen, and state that I believe that it 

 ■does not exist. If we are right in this conclusion, it afibrds 

 an additional reason for separating, as I have done, Neritina 

 Huviatilis from the tropical freshwater forms which I have classed 

 together lurder the name Paranerita. In the latter there is no 

 question of the completeness of the crossed visceral connnissure. 

 I have found it in all my specimens, and its course is very 

 ■correctly described by Bouvier. In this, as in other respects, 

 I find that the nervous systems of Nerita and Paranerita are 

 practically identical. 



But in spite of all the labour that has been bestowed upon the 

 subject there is still an uncertainty or deficiency in. the descriptions 

 and figures hitherto given of the ganglia on the visceral com- 

 missure. I have devoted a considerable amount of time to this 

 ■question, working both by means of sections and dissections and 

 hope to elucidate some points that have hitherto been obscure. 



Not being satisfied with existing figures of the pleuropedal 

 centres, I have given in fig. 36 a drawing founded on the 

 combined results of dissections and reconstruction of serial 

 sections. As the drawing is fully lettered, I may spare the 

 reader the trouble of a detailed description of the nerves issuing 

 from the pedal and pleural ganglia. I have made use of the same 

 lettering as Bouvier to designate the different nerves, so that my 

 drawing can be more easily comipared with his. I wdll only call 

 attention here to the branch of the cerebro-pleural connective 

 labelled s, and to the nerves labelled op. The foi-mer diverge 

 gradually from the connective, pass to the outside of the anterioi- 

 extrinsic muscles of the pharyngeal bulb, and are distributed to 

 the walls of the cephalic region behind the tentacles. They are 

 ■quite distinct nei^ves, but do not appear to have been noticed 



