734 



ZOOLOGY 



SECT. 



cilia which are borne on two special large cells, the anal cells (an. c). 

 The embryonic shell becomes saucer-shaped. A slight ridge in the 

 neighbourhood of the shell represents the border of the mantle. 

 The mid-gut (mesent) has become considerably widened : a 

 diverticulum from it is recognisable, and this afterwards opens 

 on the exterior to form the anus. A diverticulum of the fore- 

 gut (rad) at the same time forms the rudiment of the radular 

 sac. The statolith-sacs appear as depressions of the ectoderm at 

 the sides of the mouth : these grow inwards and become sac-like, 

 subsequently lying at the sides of the foot, which has meantime 

 attained a considerable size. 



The trochophore-stage, which is so well marked in the case 

 of Patella, occurs in other Gastropods, though, as a rule, presenting 



tent-4 



vel 



tent 



sh 



FIG. 657. Veliger stage of Vermetus. cer. g. cerebral ganglia ; eye. eye ; /. foot ; mo. mouth : 

 ot. statocyst ; sh. shell ; tent, tentacle ; vel. velum. (After Lacaze-Duthiers.) 



modifications perhaps traceable to the enclosure of the embryo in 

 an egg-shell and to the presence of much "food-yolk. The history 

 of the blastopore is not the same in all cases ; in Paludina it becomes 

 converted into the anus ; in some the mouth is developed from 

 its anterior portion ; in others the stomodseal invagination arises 

 after its complete closure, or may, with the mantle-cavity, only 

 become developed after the symmetry has been disturbed by torsion. 

 In most of the Gastropoda the pre-oral circlet or velum (Fig. 657, 

 vel.) undergoes a development not observable in the Pelecypod 

 embryo, and becomes greatly extended as a bilobed flap, the strong 

 cilia with which it is bordered rendering it a very efficient organ 

 of locomotion for the larva. With the full development of the 

 velum the larva passes into the veliger stage (Fig. 657). In this 



