STRrCTURE AND PHYSIOLOGY OF THE MOLLUSCA. 



43 



similarity, as in tlie case of the marine and fresh,- water forms 

 of the mytilidse. The following account refers to the type to 

 which the young of Crenella belong. At first they have a 

 swimming disk, fringed with long cilia, and armed with a slen- 

 der tentacular filament [flagellum). At a later period this disk 

 disappears progressively as the labial palpi are ^l 

 developed ; and they acquire a foot, and with it 

 the power of spinning a byssus. They now have 

 a pair of eyes situated near the labial tentacles 

 (Fig. 30* e), which are lost at a further stage, or 

 replaced by numerous rudimentary organs placed 

 more favourably for vision, on the border of the 

 mantle. The development of the young has 

 been noticed in many of the genera of Pteropods. 

 They are divisible into two groups : those in 

 which the body is surrounded with one or more rings of cilia, 

 and those in which these rings are absent. 



n 7> 



Fig. 30*. Fry of the Mussel.t 



Most of the aquatic gasteropoda are very minute when 



* Fig. 30. Very young fry of crenella marmorata, Forbes, highly magnified; 

 d, disk, bordered with cilia ; /, flagellum ; v v, valves ; m, cUiated mantle, 

 t Fig. 30*. Fry of mytilus edulis, after Loveu. e, eye ; e\ auditory capsule ; I t. 



