i6o 



MOLLUSCA 



of the nature of the growth process from the analysis of measure- 

 ments of the growth of this clam. Other forms do not show these 

 rings clearly so that Richards has used x-ray pictures to show the 

 internal structure of the shell not visible to the unaided eye. The 

 growth of many shells takes place in an orderly manner and in some 

 cases in precise mathematical form such as the logarithmic spiral 

 formed by the chambered nautilus. ^^ 



Embryology. — The eggs of the MoUusca are extremely numerous. 

 In the oyster, it is estimated that there are 60,000,000; in the squid 

 as many as 40,000. The sexes are usually separate except in the 

 Gastropoda. (Figure 72.) 



\ 



■v, ■-. 





[ 





/ 



Fig. 72. First stages in embryonic development of the pond snail {Lymnaeus): 

 fl, egg cell; b, first cleavage; c, second cleavage; d, third cleavage; e, after numerous 

 cleavages (Morula);/, blastula (in section); |", gastrula just forming (in section); h, gas- 

 trula completed (in section). (After Rabl.) This may be taken as a type of the 

 earliest development of all many-celled animals (Metazoa). (From Jordan and Kel- 

 logg, Animal Life. Courtesy of D. Appleton and Co., Publishers.) 



Most larval molluscs include a trochophore stage, which becomes 

 a veliger larva. The velum is situated anteriorly to the mouth and 

 proves extremely important in the locomotion and dispersal of the 

 animal. In the fresh water mussels, a parasitic stage, the glochidium 

 (see page 148) attaches to the gills or fins of fishes. 



Care of the Young. — In certain of the Gastropoda, the eggs hatch 

 within the body of the parent. In one form the female {Galerus 

 chinensis) hatches her eggs by keeping them between her foot and 



" Thompson, D. A. W. 1917. Growth and Form. Camb. Univ. Press. 



