36 



LAMELLIBEANCHIA. 



Restricting ourselves for the present to the ectodermal structures, 

 we find, in the centre of the cephalic area, beneath the strong flagella, 

 where such are present, an ectodermal thickening which, in form and 

 position, corresponds essentially to the neural plate of the Annelid 

 larvae (Fig. 18, sp ; cf. also Vol. i., Fig. 118, p. 265, and Fig. 120, 



mim 



jntt-- 



PS 



..of. 

 -9 



-Tnes. 

 -a.. 



10, 



FIG. 18. Larva of Teredo (after HATSCHEK). a, anus; dm, dorsal retractor muscle ; 

 g, ventral (pedal) ganglion ; I, liver ; m, mouth ; ma, stomach ; mes, mesoderm ; mu, 

 retractor muscles ; n, head-kidney ; ot, otocysts ; s, shell ; sp, neural plate ; vm, ven- 

 tral retractor muscle ; w, post-anal ciliated tuft ; w /t pre-oral, w lt , post-oral ciliated 

 ring. 



p. 269, &c.). From this thickening, which must be regarded as the 

 neural or apical plate, and which later yields the cerebral ganglia, 

 a system of peripheral nerves is said to extend. [In Ostrea there is 

 a distinct but shallow depression formed in this thickened area during 

 the development of the cerebral ganglia.] 



