168 



GASTKOPODA. 



The shell originates from the shell-gland which has shifted towards 

 the end of the body. According to Fol, a ping of strongly refractive 

 substance is very often to be found in the shell-gland ; in some cases, 

 this plug is perhaps formed abnormally, but in Cymbulia it no doubt 

 represents the normal condition. The substance is then said to 

 spread out under the shell which is secreted as a cuticular integu- 

 ment, after the shell-gland has gradually flattened out. It is at first 

 shaped like a watch-glass, then deepens and becomes cup-shaped 

 (Carol in in, Oleodora, etc.), or else it becomes rounded and almost 

 oviform like the embryonic chamber of the Cephalopoda. This is 



V.r 



Fig. 7:"'. — Larvae of Tiedemanniu (.1). Cymbulia Peronii(B) ; and Oreseis acicula (C) 

 (after Kkohn and (tEGEnbaur). d, operculum ; ,/'. foot : .//. fins : s, shell ; v, velum. 



the case in Creseia, Cymbulia, and the Gymnosomata. The shell, 

 which now becomes calcified, grows by the addition of new layers to 

 the margin of the embryonic shell, their boundaries being recognisable 

 as /ones of growth. In this way, the large larval shell which, in the 

 Cavollniidae and Gymnosomata is long and in the Cymbuliidan coiled 

 is formed (Figs. 74, 7, 75 A-C, 76 A, «). 



In the Cacoliniidae, the shell of the adult forms very simply from 

 the larval shell, by the addition of further layers to its anterior 



