IX 



MOLLUSCA 



329 



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 

 ?oon after the beginning of 



slom 



ap 



mes 



.ir 



ttr 



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 



velar region becomes relatively 



smaller whilst the post- trochal 



region grows very much in 



length, and then the velar 



region becomes finally com- 

 pletely invaginated, and in 



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 delicate hyaline 

 shell has been formed, and into 

 this the diminished prototroch 

 or velum can be withdrawn. 

 The foot has now made its 

 appearance as 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 aninnl. By 

 the fifth day the prototroch has 

 disappeared and the otocysts 

 and pedal ganglia can clearly 



FIG. 259. TraiLsverse section of the Trocho- ue seen the metamorphosis may 



FIG. 258. The Trochophore larva of Jtt'it- 

 talium twenty-six hours after fertilization. 

 (After Wilson.) 



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

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



mes 



pliore larva of Dentalium in the region 

 of the prototroch. (After Wilson.) 



Letters as in previous figure. 



now be said to be complete. 

 It is worthy of note that the 

 northern species of Dentalium, 

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



reach the same stage. 



