228 



GASTEROPODA AND PTEROPODA. 



thick, and then splits into a splanchnic and somatic layer, of 

 which the former attaches itself to the hypoblast, and gives rise 

 to the muscular and connective-tissue wall of the alimentary tract, 

 and the latter attaches itself to the epiblast, and forms the mus- 

 cular and connective-tissue wall of the body and other structures. 

 The two layers remain connected by protoplasmic strands, and 

 the space between them forms the body cavity (fig. 107). In 

 most instances there would appear to be at first no such definite 

 splitting of the mesoblast, but the layer has the form of a scattered 

 network of cells between the epiblast and the hypoblast. Finally 

 certain of the cells form a definite layer over the walls of the 

 alimentary canal, and constitute the splanchnic mesoblast, and 

 the remaining cells constitute the somatic mesoblast. 



We must now return to the embryo at the time when the 

 blastopore is becoming narrowed. First of all it will be necessary 

 to define the terms to be applied to the various regions of the 

 body and these will 

 best be understood by 

 taking a fully formed 

 larva such as that re- 

 presented in fig. 101. 

 The ventral surface 1 

 consider to be that 

 comprised between the 

 mouth (m) and the 

 anus, which is very 

 nearly in the position 

 (i) in the figure. As a 

 great protuberance on 

 the ventral surface is 

 placed the foot/! The 

 long axis of the body, 



FIG. 101. DIAGRAM OF AN EMBRYO OF PLEU- 

 ROBRANCHIDIUM. (From Lankester.) 



f. foot ; ot. otocyst ; m. mouth ; v. velum ; 

 ng. nerve ganglion ; ry. residual yolk spheres ; s/is. 

 shell-gland ; i. intestine. 



at this period though not necessarily in the adult, is that passing 



forwards and divided transversely into two parts, an anterior continued from the front 

 mesoblast, and a posterior from the hinder mesoblast. 



If RtibPs account is correct, there is a striking similarity between the origin of the 

 mesoblast in Mollusca and in Chaetopoda. It appears to me very probable that the 

 mesoblastic bands are formed (as in Lumbricus) not only from the products of the 

 division of the mesoblasts, but also from cells budded off from one or both of the 

 primary germinal layers. 



