170 BULLETIN OF THE 



ings of the ectoderm, and he has made sections of Purpura lapillr.s and 

 Murex erinaceus which show, as he maintains, that simihir rudiments 

 are also formed in them by proHferation from thickenings of the 

 ectoderm. 



Kowalesky ('83, pp. 23-26) shows for Chiton Pohi that the lateral and 

 pedal nerve trunks are formed simply as thickenings of the ectoderm. 



Rabl ('75, pp. 206-208) maintained that the supra-oesophageal gan- 

 glia and the sense organs in Lymnieus, Physa, Ancylus, and Planorbis 

 were formed by an invagination of the ectoderm, and that the pedal 

 ganglia were produced by delamination fi'om the same germinal layer. 

 He has more recently ('83, pp. 57, 58) expressed doubt as to the man- 

 ner in which the pedal ganglia arise in Bythinia tentaculata, because he 

 has seen them so connected to the ectoderm of the dorsal wall of the 

 foot by means of cells as to indicate that they arise by proliferation 

 from that region. 



Sarasin ('82, pp. 45-48), who has also recently studied Bythinia ten- 

 taculata, and who is the only author that has hitherto followed the de- 

 velopment of the entire nervous system in a Gastropod, contends that 

 the whole of it arises from ectodermic thickenings, without any invagina- 

 tion even for the supra-oesophageal ganglia. He also believes that the 

 pedal ganglia arise from the dorsal wall of the foot. 



Fol ('80, pp. 165-169) admits no invagination for the central nervous 

 system in aquatic pulmonates, and he even inclines to the opinion that 

 it may be derived from the mesoderm, which, however, has itself origi- 

 nated from the ectoderm. He considers it an unimportant question, and 

 therefore one which it is useless to discuss, whether the nervous system 

 arises from ectoderm or mesoderm. If the mesoderm were derived from 

 the entoderm, then it would bean important question. He believes that 

 the supra-oesophageal ganglia of land pulmonates (pp. 192-195) originate 

 by invaginations of the ectoderm, while the pedal ganglia arise from the 

 mesoderm of the foot. 



The latest investigations are those of Salensky ('86, pp. 655-759) on 

 the development of Vermetus, one of the Prosobranchs. He concludes 

 that the cerebral ganglia are formed by two invaginations of the ecto- 

 derm, while the pedal ganglia arise by proliferation from the ventral and 

 lateral walls of the foot on each side of the median depression which runs 

 along its ventral face. These ganglia arise separately, and later become 

 connected with each other by a commissure, and with the cerebral gan- 

 glia by connectives, both of which are outgrowths from the ganglia 

 (pp. 694, 695). 



