ON THE DEVELOPMENT OF THE POND-SNAIL. 377 



lOj 11, 12, we have various views of the gastrula oi Lymn(Bus. 

 In assuming this form the embryo gets rid of a very delicate 

 envelope, which appears to be of a slightly viscid nature, 

 and which, together with the E-ichtungsblaschen, is now lost. 

 It is seen in a loose detached condition in the stage repre- 

 sented in fig. 7. 



The gastrula of Lymnaus has been figured and described 

 by LerebouUet, who takes the fossa and orifice of invagina- 

 tion for the rudiment of the adult's mouth. I believe, how- 

 ever, that this is a mistake, and that the orifice of invagina- 

 tion in Lymnaus closes up, as I have observed, in the 

 gastrula of the Lamellibranch Pisidium, and in that of 

 Limax and of Polycera, Tergipes, and Doris. 



The Lymna^us-gastrula has the same curious cushion-like 

 form as observed in the Nudibranchs. The orifice of invagi- 

 nation, in its most strongly marked period of development, 

 is a long, trough-like depression, running from one side of 

 the cushion towards the middle, and there sinking deeply 

 into the substance of the mass. Accordingly, as it is turned 

 this way or that, the extent and direction of the orifice 

 presents apparent difierences, which are, however, merely 

 apparent. 



The figures will give a more correct notion of these appear- 

 ances than any description. 



Besides, by LerebouUet, who did not appreciate its true 

 character, the gastrula of Lymnceus has been figured dia- 

 gramatically by Professor Haeckel in his Gastrsea-theory. 

 (See PI. VII.) 



I have elsewhere distinguished two classes of gastrula 

 forms, according to the mode of their development, namely, 

 "invaginate gastrulse" and " delaminate gastrulae," the 

 latter forming by an internal movement of stratification in a 

 mass of embryonic cells, and not by a process of involution. 

 The Lymneeus-gastrula is clearly an " invaginate" one ; but 

 amongst invaginate gastrulae we may distinguish those which 

 are formed by emboly (the growth inwards of a number of 

 small cells), and those formed by epiboly, in which large 

 cells remain, as it were, stationary, and are grown over by 

 smaller cells. These terms are adopted from Selenka, who 

 has given a very valuable account of the development of 

 Purpura lapillus in the ' Niederlandisches Archiv fiir 

 Zoologie,' Bd. I, July, 1872. 



I am obliged to leave for further inquiry the interesting 



this there is certainly not much if any difference in the size of the com- 

 ponent cells, those in the apex of the pyramid being only apparently larger 

 on account of their prominence. 



