200 SUMMARY OF CURRENT RESEARCHES RELATING TO 



probably, all Opisthobranchs — do not take in water; tborc remain, there- 

 fore, only tbe Prosobranchs, and among tbem tbe sand-dwellers appear 

 to bo especially endowed with this property. 



a- Cephalopoda. 



Growth of Cephalopod Shells.* — Mr. F. A. Bather discusses the 

 two views as to the mode of formation of the shells of Cephalopods. 

 The investigation of Nautilus led to the secretion-hypothesis, according 

 to which the anterior portion of tho mantle secretes calcareous matter, 

 which it deposits in successive layers on tho margin of the aperture. 

 Dr. E. Eiefstabl has been led to propose what may be called the intus- 

 susception-hypothesis. Microscopical investigation of the shell of Sepia 

 seems to show that each septum is absolutely developed from the pre- 

 ceding, and is removed therefrom by growth of the intervening zone of 

 the outer shell-wall ; the growth being effected not by apposition, but 

 by intussusception. Mr. Bather summarizes the evidence of Biefstahl, 

 and urges certain objections ; he thinks that facts do not confirm the 

 intussusception-hypothesis, and that some of them do favour the old 

 view of formation of the shell by secretion, 



y. Gastropoda. 



Form and Development of Spermatozoa in Murex.j — M. E. 

 Koehler has investigated the development of the spermatozoa in Murex 

 brandaris and Jf. biunculus, where, as in various other Prosobranch 

 Mollusca, these bodies are of two forms. A layer of parietal proto- 

 plasm contains numerous nuclei, whence originate all the seminal 

 elements ; but these nuclei, when they leave the protoplasmic layer, and 

 become organized into cells, are of two very distinct kinds ; some are 

 large, and contain a large nucleus, have granular protoplasm and an 

 enveloping membrane ; these are the mother-cells of the vermiform 

 spermatozoa. Others are much smaller cells, without any membrane, 

 and connected by protoplasmic processes with neighbouring cells ; these 

 are the mother-cells of the filiform spermatozoa. 



The large cells present a series of modifications, which first affect the 

 nucleus ; it becomes homogeneous, contracts a little, and loses somewhat 

 the regularity of its contour ; these changes are accompanied by a frag- 

 mentation of the nucleus, and an increase in the size of the cells ; one 

 of the nuclei produces a bundle of fibrils, one of the extremities of which 

 will become the tuft of characteristic cilia, while the other will develope 

 into a central filament. As the latter elongates, it meets the wall of the 

 cell, and then enlarges to form the head of the vermiform spermatozoa. 

 The author promises further details and a full justification of his belief 

 that these spermatozoa are formed in the place of ova. He has 

 observed that the mother-cells arise directly from the primordial sexual 

 cells, like the ova in an ovary. If we admit that Prosobranchs are 

 more ancient than Pulmonates, it seems justifiable to believe that this 

 existence of two kinds of spermatozoa indicates a tendency towards 

 hermaphroditism ; the abnormally formed spermatozoa are very variable 

 in form, and have no function ; they get that of ova only when herma- 

 phroditism is complete. 



* Geol. Mag., iv. (18S7) pp. 446-9. 



t Comptes Rendus, cvi. (1888) pp. 299-301. 



