XII 



PHYLUM MOLLUSCA 



703 



CL 



p.pl 



me sent 



cin.c 



diverticulum from it becomes recognisable, 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 radulnr 

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

 the sides of the 

 mouth : these grow 

 inwards and become 

 sac-like, subsequent- 

 ly lying at the sides 

 of the foot, which 

 has meantime at- 

 tained a considerable 

 size. 



The trochosphere 

 stage, which is so 

 well marked in the 

 case of Patella, oc- 

 curs in other Gastro- 

 p< ds, though, as a rule, 

 presenting modifica- 

 tions perhaps trace- 

 able to the enclosure 

 of the embyro in an 

 egg-shell and to the 

 presence of much 

 food-yolk. The his- 

 tory of the blastopore 

 is not the same in all 

 cases ; in some, the 



mouth is developed from its anterior portion : in others the 

 stomodreal invagination arises after its complete closure. In most 

 of the Gastropoda the prse-oral circlet or velum (Fig. 610, 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. 610). 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 is symmetrically placed, but later becomes shifted 

 to the right side and forwards as well as dorsally. The foot (/.) 

 may attain a considerable development during the Veliger stage. 

 On its posterior and dorsal part appears the operculum. Two 

 little processes on the velar area develop into the tentacles (tent.), 

 and the eyes (ey.) appear at their bases. As the foot and other 



Fin. 609. Later trochosphere of larva of Patella in longi- 

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

 plate ; f. foot ; HH-S. mesoderm cells ; j/,.vui*. mesenteron ; 

 inn. mouth ; i-itii. rudiment of radula sac ; .sA. shell. (From 

 Korschelt and Heider, after Patten.) 



