REPRODUCTIVE SYSTEM AND SEXUAL SUCCESSION OF L. BULIMOIDES 197 



ditic condition. It is to be noted that in the great majority of mesogastropods — whether bisexual or 

 hermaphrodite — there is a trend towards sperm dimorphism, and in some species the oligopyrenic 

 sperms reach a grotesque size. [Fretter (1953) 1 has demonstrated huge giant sperms in Clathrus 

 transporting vast numbers of attached normal sperms.] Parthenogenesis, too, appears to occur in at 

 least one species, Paludestrina jenkinsi. The sexual condition in the mesogastropods as a whole is 

 therefore too specialized to make them easily acceptable as ancestors of the pulmonates or the opistho- 

 branchs. The two latter groups have probably originated close together at a higher archaeogastropod 

 level and have retained their hermaphroditism. Each group began with a protandrous sexual succes- 

 sion and the succession also involves the pallial genital duct which, like the higher prosobranchs, these 



OPISTHOBRANCHIATA 



HIGHER PROSOBRANCHS 



PULMONATA 



ELLOBIIDAE 

 [MORTON_ I9S3a] 



-~~CALYP TRAEIDA £ 

 [ORTON_ 1909] 



FlSSURELLlDAE 



[BACCI-1947] LOWER PR0S0BRANCH5 



Fig. 17. Diagram to illustrate the theory of the evolution of sex in the Gastropoda discussed in this paper. Black indicates 

 a condition of protandrous hermaphroditism, stipple simultaneous hermaphroditism and white separation of male and female 

 individuals. 



groups have acquired. The higher members of both subclasses have evolved in the direction of 

 simultaneous hermaphroditism, with the sexual stages telescoped together and eggs and sperm pro- 

 duced side by side as in Helix, for example, or in separate acini of the ovotestis (higher nudibranchs). 

 A similar but less well-marked trend seems to have been to cut out localized breeding seasons and to 

 produce eggs and sperm simultaneously all the year round. 



A more detailed discussion of the evolution of hermaphroditism in gastropods is intended in a later 

 account, after the presentation of further work. It is already clear, however, that a study of Limacina 

 is of great value for an understanding of the early condition with which the sexual evolution of many 

 opisthobranchs and pulmonates would seem to have begun. 



SUMMARY 



The pelagic Mollusca collected during the Benguela Current Survey by the ' William Scoresby ', in 

 March 1950, comprise the following species: Ianthina ianthina, I. globosa, Atlanta peroni, Limacina 

 inflata, L. bulimoides, Diacria trispinosa, Euclio pyramidata, Cavolinia inflexa, Cymbulia peroni, 

 Thliptodon diaphanus, Pneumodermopsis paucidens, as well as two species of larval lamellibranchs, one 

 prosobranch larva, and several larval cephalopods. The numerically important members of the plankton 

 were the two lamellibranch larvae at stations close inshore, and the two pteropods, Pneumodermopsis 



1 Also in personal conversation. 



5-2 



