230 



HIGH SCHOOL ZOOLOGY. 



each side of the mouth, a pair of otocysts in the foot, sup- 

 plied by the cerebral ganglia, and a patch of olfactory neuro- 

 epithelium near the posterior adductoi- are the chief seats of 

 the special senses. In some forms, eyes are distributed along 

 the mantle-margin, but not in the Unios. 



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Tlie mouth lies under the anterior adductor and leads into 

 a stomach surrounded by a bulky liver, and possessing a ccecum 

 containing a " crystalline stylet" of unknown function. After 

 several turns within the foot, the intestine ascends, and in its 

 course to the upper surface of the posterior adductor, is envel- 



