PHYLUM MOLLUSCA 



271 



CEPHALOPODA, and marine gastropods breathe mainly by means 

 of gills. 



Reproduction. No cases of asexual reproduction have been 

 reported in mollusks. The sexes are usually separate, though 

 the members of one entire subclass of GASTROPODA (EUTHY- 

 NEURA) are hermaphroditic. The number of eggs laid by some 

 mollusks is very great ; for example, 9,000,000 in the oyster. In 

 all such cases the eggs are subjected to the dangers of the ocean 



si- 



FIG. 197. Stages in the development of a mollusk, Patella. A, trocho- 

 phore stage. /, foot; fl, flagellum; m, mouth; pac, postanal cilia; ve, velum. 

 B, veliger stage, 130 hours old. /, rudimentary foot; op, operculum; sh, shell; 

 v, v, velum. (A, from Lankester's Treatise, after Patten; B, from the Cam- 

 bridge Natural History, after Patten.) 



waves and to numerous enemies, and also pass through a meta- 

 morphosis after hatching. Other mollusks lay very few eggs, 

 for example, Lymncea, twenty to one hundred ; Helix, forty to one 

 hundred ; and Paludina, about fifteen. These are terrestrial or 

 fresh- water species whose eggs produce young in the adult form, 

 or, as in Paludina, the eggs hatch within the body of the parent. 

 The development of the eggs of most mollusks includes a tro- 

 chophore stage (Fig. 197, A) which becomes a veliger larva (Fig. 

 197, B), so called because of the presence of a band of cilia, the 

 velum (v) , in front of the mouth. The velum is an organ of loco- 



