SHELL-GLAND OF CYCLAS AND PLANULA OF LIMN^US. 321 



The facts which I observed in Liranseus in every way con- 

 firmed and strengthened the views which I had already 

 expressed in the paper communicated to the Royal Society 

 before leaving England. 



Some (and but a few) of the points in the earlier of these 

 two papers, additional to those noted in the paper of 1873, 

 are — 1st. The correspondence (as first indicated by Selenka 

 in Purpura, and by Kovvalewsky in Oligochaeta) of the in- 

 vaginate and circumcrete forms of Gastrula. 2nd. The con- 

 tinued proliferous activity of the large endodermal cleavage- 

 spheres after enclosure by the smaller circumcrescent spheres 

 in the development of Aplysia, and of the corresponding 

 cells in Pisidium. 3rd. The origin of the cephalic nerve- 

 ganglia from ectodermal cells within the area of the velum. 

 4th. The mode of formation and evanescence of the shell- 

 gland, 5th. The rectal peduncle or pedicle of invagination 

 in Pisidium. 



In the paper on Limnseus I was able to adduce facts which 

 strengthened my views as to the activity of the endodermal 

 cells subsequent to their inclusion within the ectoderm; to 

 confirm my account of the rectal pedicle of Pisidium by the 

 discovery of an identical structure in Limn(BUS ; to show that 

 what Lereboullet had taken for the mouth of Limnoius was 

 its blastopore (the name which I have given to the orifice of 

 invagination, by means of which the endodermal cells become 

 enclosed in the ectodermal in so many animals) which closes, 

 in most cases, at an early period of development, and that 

 what he had taken for an " anal cone" is my shell-gland (as 

 I had previously pointed out, * Proc. Roy. Soc.,' 1874). I 

 also showed that the Pulmonata are not, as had been sup- 

 posed, devoid of a ciliated velum in the larval state, and that 

 the velum is not even reduced to a rudimentary condition, as 

 had been supposed, but is well developed and prominent at 

 an early stage of development. Further, I pointed out the 

 existence of the "shell-gland" in various MoUusca, namely, 

 in Pisidium, Aplysia, Neritina, Paludina, Limnseus, Tere- 

 bratula, and Loxosoma, and discussed its relationship to the 

 open pit in the mantle which I discovered in the embryo 

 Cephalopoda. 



These and some other points in my publications relative 

 to Molluscan development have been variously received by 

 subsequent writers. A Genevese, by name Fol, with whom 

 it was my misfortune to spend two hours at Messina, in 

 May, 1874, has made them the ground of a gross personal 

 attack in M. Lacaze Duthier's ' Journal of Zoology.' He 

 has, however, been compelled, in the face of evidence sub- 



VOL. XVI. NEW SE». X 



