IX 



MOLLUSCA 



329 



slom 



is not yet formed. At the sides of the intestine are seen two short 



mesodermal bands. In the pre trochal region two masses of cells 



are seen lying to the right and 



left. These are proliferated from 



the ectoderm, and are almost 



certainly the beginnings of 



the cerebral ganglia. Very 



soon after the beginning of 



larval life the rudiment of 



the shell gland can be made 



out, and the everted edge of 



this already foreshadows the 



future mantle fold, which is 



at first double, like that of a 



Pelecypod. 



During the course of the 

 next day the larva sinks to 

 the bottom ; the pre-trochal or s f 

 velar region becomes relatively 

 smaller whilst the post-trochal 

 region grows very much in 



length, and then the Velar FIG. 258. The Trochophore larva of Den- 

 region becomes finally com- talhim twenty-six hours after fertilization. 



pletely invaginated, and in (After Wilson.) 



this way the larva attains the 



stage of a veliger. 



By the end of the second day not only is the shell gland 



everted but a delk-ate hyaline 

 shell has been formed, and into 

 this the diminished prototroch 

 or velum can be withdrawn. 

 The foot has now made its 

 appearance its a median ridge. 

 At the end of the third day the 

 foot has become large, protrusible, 

 and bilobed at its free end ; and 

 the mantle lobes have partially 

 united beneath the animal. By 

 the fifth day the prototroch has 

 disappeared and the otocysts 

 and pedal ganglia can clearly 

 be seen ; the metamorphosis may 

 now be said to be complete. 

 It is worthy of note that the 



ttr 



a.p, apical plate ; mes, mesoderm ; p.tr, prototroch ; 

 st, stomach ; stom, stoniodaeum ; t.tr, telotroch. 



mes 



FIG. 259. Transverse section of the Trocho- 

 pliore larva of Dentalium in the region 

 of the prototroch. (After Wilson.) 



Letters as in previous figure. 



northern species of Dentalium, 

 which was studied by Lacaze-Duthiers, took twenty-five days to 

 reach the same stage. 



