824 PROF. G. C. BOURNE ON THE [N"oV. 17, 



of these two papers Lenssen deals with the digestive and genital 

 systems, giving a detailed account of the bucco-pharyngeal 

 cavity, the odontophore, the oesophagus with its glandular 

 appendages, and the stomach. He and Gilson (18) are the only 

 authors besides Thiele who have published an accurate account 

 of this system of organs in the Neritacea. Gilson and Lenssen 

 discovered the remarkable fact that in Xeritina, a dioecious 

 •Gastropod, the female ducts are diaulic, whereas the male ducts 

 are monaulic ; and the latter author gives a thorough and accurate 

 •description of the very complex arrangements of both male and 

 female organs. I have only to say that I have carefully verified 

 Lenssen's statements and find nothing to correct and very little to 

 add to them as regards the species examined, Xeritinafiuvicitilis, 

 but I find considerable and important differences in some of the 

 tropical Neritin?e. 



In his second paper Lenssen deals with the nervous, circulatory, 

 respiratory, and excretoiy systems of Xeritina fluviatilis. Here 

 he has not been in some respects as accurate as in his first paper. 

 For instance, in the description of the nervous system (p. 297) 

 he confuses the labial with the buccal commissui'e. It is clear 

 both from his text and figure that the commissure that he dis- 

 covered is the buccal commissure, but he calls it the labial. 



It is practically impossible to dissect out the true labial 

 commissure in so small an animal as X. fluvudilis, and it is 

 exceedingly difiicult to trace it in sections; but I have satisfied 

 myself that it exists. In other respects Lenssen's account and 

 figure of the nerve-centres appear to be correct, and I can confirm 

 his statement that there are no transverse commissures behind 

 the single large commissure uniting the anterior ends of the pedal 

 ■cords. 



As regards the visceral and pallial nerves Lenssen makes a 

 considerable advance on his predecessors and he accepts Bouvier's 

 identification of the subintestinal ganglion. He discovered, 

 apparently without being aware that Boutan and Bela Haller had 

 anticipated him in this matter, the ganglion on the subintestinal 

 nerve ,at the point where the latter turns rather sharpl}^ from 

 Tight to left to course close below the surface on the dorsal side 

 of the pedicle attaching the anterior" part of the body to the 

 visceral mass. His account of the relations of this ganglion and 

 of the nerves given oflf from it is for the most j)art very exact, but 

 he does not appear to have observed that the genital nerve {loc. 

 cit. pi. i. fig. 1, ns.) almost immediately enlarges to form a 

 ganglion of considerable size, closely attached to the oviduct (or 

 sperm-duct). He further describes a strvicture which he hesitates 

 to identify as the rudiment of the right ctenidium. " A cet 

 endroit," he says, " il existe un organe creux, I'homologue, peut- 

 ■etre des mamelons decouverts chez les patelles et d'autres pi-oso- 

 hranches. Cet organe renferme un grand nombre de globules 

 sanguins et semble, par consequent, dependre soit de i'appareil 

 <;irculatoire, soit de I'appareil respiratoire. II fait saillie dans la 



