Report) and Buccinum, or the contractile vesicle found by Semper 

 in Ampullaria, Cypraea, Murex and Ovulum, or the dorsal vesicle 

 F . l38 of the Pulmonates (snails). There is, 



necklace consisting of four yellowish cells, situated next to the 



Meanwhile the more the posterior part of the body grows, the 

 larger and more spiral becomes the shell, until the helmet shape 

 of the adult is approached. At this stage also the gill-cavity 



In a succeeding stage the foot has increased in length, the 

 spire of the shell has begun to topple over as it were and fall 



« ardiuin l.< in- f. • u ,1 \\r>\ i. and permanent kidney and gills grow 

 out. The gills originate from the ectoderm. It is not until this 

 period that the end of the intestine and anal outlet is formed. 

 The provisional larval organs now begin to disappear, the ceph- 



kidm-ys disappear. Of the larval visceral organs only S^&fa. 

 the heart remains which, though smaller, still pulsates, fjyfc^fc 

 It now rests under the mantle in the branchial cavity. \ " J 

 There are now two gill-leaves, and finally the perma- 

 nent heart is formed. The further changes consist in 



the perfection of all these organs and the development 



of the. shell into the helmet shape of the adult. Fig. Caiyptnea 

 139 (after Morse) represents the common Calyptrcea Stnata ' 

 striata of our own coast. We have seen that the usual five stages 

 have been undergone, i.e. the egg, morula, gastrula (not so well 

 marked as in the pond snail, Fig. 141), veliger and adult. 



The metamorphoses of Trochus represent another type of de- 

 velopment in the Gastropods, which illustrate points less clearly 

 wrought out in the Calyptraea. 



The eggs of Trochus varius are very small, spherical, and laid 



