NOTES ON THK DEVELOPMENT OF MOLLUSCA. 369 



cells being much smaller in size, and are formed of clear proto- 

 plasm, i am inclined to think that the next stage is formed by 

 other four cells being segmented oft' from the four large yolk 

 cells (fig. 8) ; be this as it may an epiblastic cap is soon formed, 

 which spreads over the yolk cells. Fig. 10 shows that at the 

 usual position, at the tip of the blastopore, mesoderm cells (m) 

 are segmented off from the yolk cells. 



The further development of Janthina presents us with 

 nothing which is not common to most other Prosobranchs. I 

 would, however, draw attention to the-large violet mass which is 

 seen on the right side of the embryo close to and dorsal to the 

 anus. A similar pigmented mass is met with in many Opitho- 

 branchs, large and red on the right side in Philine aperta, a 

 small violet spot on the left side of Elysia virides ; in Plenro- 

 branckidium, as first noticed by Lankester (loc. cit.), there is 

 one on each side coloured pink. It would be interesting to dis- 

 cover the meaning of this pigment. 



It is well known that the nervous system arises from the epi- 

 blast throughout the animal kingdom, with the exception of 

 certain nerve elements in the Ccelenterata, which have a hypo- 

 blastic origin, and of the central nervous system in Prosobranch/ 

 Gasteropoda, and in Cephalopoda, which is stated to arise from 

 the mesoblast. It is quite comprehensible how the former ex- 

 ception to the general rule should arise, but the two latter are, 

 to say the least of it, very anomalous ; and it was for the purpose 

 of attempting to clear up the difficulty that the following re- 

 searches were made. 



In all the groups of the Gasteropoda, with the exception 

 above noted, the origin of the nervous system from the epiblast 

 has been observed with more or less accuracy. I shall now 

 describe the manner in which the central nervous system develops 

 in two genera of the Prosobranchiata. 



In Purpura lapilhis, at the stage represented in fig. 12, two 

 large thickenings of the transparent skin are seen with the 

 velum in close connection with the developing eyes and incipient 

 tentacles, and two similar masses occur in the foot, having an 

 intimate relationship with the otocysts. A transverse section 

 through this region shows us that the nervous system is in pro- 

 cess of development by proliferation from these paired thick- 

 enings of the epiblast. Pig. 14 gives a more highly magni- 

 fied view of one of the supra-oesophageal ganglia thus being 

 formed. It is worthy, while to compare this figure with that 

 given by E,abl of Planorbis.^ Prom observations made by 

 transverse sections I have no hesitation in expressing my con- 

 viction that a precisely similar origin of the nervous system occurs 

 ' C. Rabl, 'Morpli. Jalirb.,' v, 1879 (Taf. xxxv, fig. 20). 



