MOLLUSCA. 251 



remainder of the lower layer cells are the mesoblast. The 

 mesenteron gradually extends itself till it meets the stomodaeum 

 (fig. 127). The proctodseum is formed as a shallow pit close to 

 the first formed part of the mesenteron. 



The mesoblast gives rise not only to the organs usually 

 formed in this layer, but also to the nervous centres, etc. 



The mantle and shell. The mantle first arises as a thick- 

 ening of the epiblast on the dorsal surface of the embryo. The 

 thickened integument, with the subjacent mesoblast, soon forms 

 a definite projection, in the centre of which appears a circular 

 pit (figs. 1 14 chs and 1 15 slis}. This pit, which has already been 

 spoken of as the shell-gland, resembles very closely the shell- 

 gland of other Mollusca. The fold around the edge of the shell- 



Fi<;. 115. DIAGRAM OF A VERTICAL SECTION THROUGH THE MANTLE REGION OF 

 AN EMBRYO LOLIGO. (From Lankester.) 



[This figure is turned the reverse way up to fig. 114.] 



ef>. epiblast ; y. food-yolk ; in. mesoblast ; ;'. cellular yolk membrane ; shs. shell- 

 gland. 



gland grows inwards so as gradually to circumscribe its opening, 

 which before long becomes completely obliterated ; and the 

 gland forms a closed sack lined by epiblast which grows in an 

 anterior direction (figs. 1 14 and 127 ccJi). 



The edges of the mantle now begin to project, especially on 

 the posterior side (fig. 127), and within the cavity formed by this 

 projecting lip there are placed the anus (an], gills, etc. The pro- 

 jecting lip of the mantle is formed both of epiblast and meso- 

 blast. The whole of the anterior side of the mantle is filled by 

 the elongated shell-sack (cell), within which the shell or pen soon 

 becomes secreted. 



