STRUCTURE AND PHYSIOLOGY OF THE MOLLUSCA. 53 



have a pair of eyes, situated near the labial tentacles (fig. 30*, e) } 

 which are lost at a further stage, or replaced by numerous ru- 

 dimentary organs placed more favourably for vision, on the bor- 

 der of the mantle. 



Most of the aquatic gasteropoda are very minute when 

 hatched, and they enter life under the same form, that which 



Fig. 30 *. Try of the Mussel* 



has been already referred to as permanently characteristic of the 

 pteropoda. (Tig. 60.) 



The Pulmonifera and Cephalopoda produce large eggs, con- 



* Fig. 30*. Fry of mytilus edulis, after Loven. <?, eye; <?', auditory 

 capsule ; 1 1, labial tentacles ; s s f , the stomach ; 6, branchiae ; h, heart ; v, 

 vent ; /, liver ; r, renal organ ; a, anterior adductor ; ', posterior adductor ; 

 /, foot. The arrows indicate the incurrent and excurrent openings ; between 

 which the margins of the mantle are united in the fry. 



