.810 PROF. G. C. BOURNE ON THE [NoV. 17, 



The following papers were read : — 



1 Contributions to the Morphology of the Group Neritacea 

 of Aspidobranch Gastropods. — Part I. The Neritidw. 

 By Prof. Gilbert C. Bourne, D.Sc., F.Z.S. 



[Received October 27, 1908.] 



(Plates XLYI.-LXVI.* and Text-figure 172.) 



While the Haliotida?, Fissurellidse, Pleurotomariida?, Trochidae, 

 Patellida?, and other members of the Aspidobranch Gastropoda 

 have received a large amount of attention from morphologists, 

 the Neritacea have, until recent years, attracted little interest. 

 The anatomy of so familiar a species as the European Neritina 

 fluviatilis was imperfectly known till the appearance of Lenssen's 

 memoirs in 1899 (25) and 1903 (26). Thiele's (39) short but 

 accurate descriptions of various oi-gans of ti-opical species of 

 Neritidje have added largely to our knowledge of the group, but 

 even when these are taken into considei'ation it can hardly be 

 said that a sufficiently comprehensive comparative account of the 

 Neritidaj exists in a form available for students of molluscan 

 anatomy. 



It has been too readily assumed that the Neritacea, forming, as 

 they do, an extremely specialized section of the Rhipidoglossa, 

 are unlikely to retain an}^ considerable traces of primitive 

 organization, or to yield evidence bearing on the ancestiy of the 

 Gastropoda. Thus Pelseneer (30) writes : " D'autre part les 

 Neritaces sont plus specialises que tons les autres Rhipidogiosses 

 (Haliotis etc.) ou la commissure viscerale est deja croisee, par : 

 1". L'existence d'une seule branchie et d'un seul osphradium. 

 1°. L'existence d'un seul rein. 3°. L'existence d'un orifice 

 genital propre. 4°. L'existence d'yeux a cavite fermee. 5". La 

 separation plus complete des ganglions pleuraux et pedieux." 

 There is no doubt that the Neritacea ai-e specialized in these 

 respects, but this is no reason for regarding them as proba])ly 

 uninteresting subjects for anatomical study, for animals highly 

 specialized in some respects may, nevertheless, retain many 

 primitive features, and there are so many points in which the 

 Neritacea seem to approximate to the Pectinibranchs, e. g. the 

 existence of a single kidney with a slit-shaped opening into the 

 mantle-cavity, the complex genital ducts with accessory glands, 

 &c., that it has long been a matter of interest to determine 

 whether they are intermediate between the less specialized 

 Aspidobranchia and the Pectinibranchia, or whether their 

 apparent resemblances to the latter group are due to convergence. 

 Finally, the Helicinidfe, interesting because they are terrestrial 

 and pulmonate, have not been the subject of any comprehensive 



* For explanation of the Plates see p. 884. 



