168 



GASTROPODA. 



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

 the end of the body. According to FOL, a plug 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 Cymlwlia 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 

 (Cavolinia, Cleodom, etc.), or else it becomes rounded and almost 

 oviform like the embryonic chamber of the Cephalopoda. This is 



FIG. 75. Larvae of Tiedemannia (A), Gymbulia Peronii (R] and Creseis acicula (C) 

 (after KROHN and GEGENBAUU). d, operculum ; /, foot ; ft, fins ; s, shell ; v, velum. 



the case in Crewi*, Gymbulia, 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 zones of growth. In this way, the large larval shell which, in the 

 Cavoliniidae and Gymnosomata is long and in the Cymlniliida^ coiled 

 is formed (Figs. 74, y, 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 



