XII 



PHYLUM MOLLUSCA 



659 



as an imagination of the ventral surface, and the larva of most 

 forms, unlike that of Anodonta or Unio, passes into a stage in 

 which it closely resembles the trochosphere of Worms (Fig. 556) 

 having a pre-oral and a post-oral circlet of cilia, a tuft of cilia 

 round the anus, and an apical tuft in the middle of the prostomium. 

 There is also an ectodermal thickening on the prostomium which 

 becomes the cerebral ganglion, and a similar ventral thickening 



,;. 555. Vertical section of eye of Pecten. 1, cornea; 2, lens; 3, external epithelium, 

 , blood-sinus ; 5, retina ; 6, pigmentary layer ; 7, optic nerve. (From Korschelt and Heider.) 



rhich gives rise to the pedal ganglion and corresponds with the 

 idiment of the ventral nerve-cord in Worms. The pelecypod 

 trochosphere is, however, distinguished from the corresponding 

 stage in Worms by the presence of the shell-gland^ which soon 

 secretes a delicate unpaired shell. The prostomial region grows 

 it into a thickened retractile rim, bearing the pre-oral circlet 

 )f cilia, and called the velum (Fig. 557 vel.) : the larva at this stage 

 is distinguished as a veliger a very characteristic molluscan, 



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