754 



ZOOLOGY 



SFX'T. 



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, present- 

 ing 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 in- 

 vagination 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 Gas- 

 tropoda the pre-oral 

 circlet or velum (Fig. 

 647, ml.) undergoes a 

 development not ob- 

 servable in the Pele- 

 cypod embryo, and 

 becomes greatly ex- 

 tended as a bilobed 

 flap, the strong cilia 

 with which it is bor- 

 dered 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. 647). In this stage the shell (sh.) 

 increases in size, loses its simple form, and begins to develop a 

 spiral. A cleft-like depression in the border of the mantle on the 

 right-hand side forms the rudiment of the mantle-cavity in which, 

 later, the gills are developed. The anus when it first appears may 

 be symmetrically placed, but later becomes shifted to the right 

 side and forwards as well as dorsal ly. A pair of larval nephridia 

 are developed, having a remarkable resemblance to the excretory 

 canals of the Flat- worms. Each consists of a longer or shorter tube 

 sometimes intra-cellular, sometimes inter-cellular, opening on the 

 exterior at one end, and at the other terminating in one or several 

 flame-cells. The foot (/.) may attain a considerable development 

 during the veliger stage, and on its posterior and dorsal part appears 



ctn.c 



Pig. 646. — Later trochophore of larva of Patella in longi- 

 tudinal section, an. c. anal cells with cilia ; ap. pi. apical 

 plate ; /. foot ; vies, mesoderm cells ; mesent. mesenteron ; 

 too. month ; rail, rudiment of radula-sac ; xh. shell. (From 

 Korschelt and Heider, after Patten.) 



