126 ELEMENTARY BIOLOGY. [CHAP. xi. 



anterior retractor, the latter of the protractor 

 pedis muscle. 



Extending from each adductor impression 

 towards the umbo is a fainter, gradually ta- 

 pering impression, which may be followed 

 into the cavity of the umbo, and indicates 

 the successive attachments of the adductor 

 muscles, as the animal has increased in size. 



13. In the breeding season, examine the contents of the 

 testis for spermatozoa, and those of the ovary for 

 ova. Note the micropyle of the latter. If the outer 

 gill appear to be thick and distended, it will be found 

 full of the larvae of the Anodon, Glochidium. Note 

 the characters of their shells and the entangled fila- 

 ments, or byssus, with which they are provided. 



