MOLLUSCA 



147 



side of the esophagus) with cerebral commissures forming a nerve 

 ring. Each cerebropleural ganglion sends a nerve cord ventrally, 

 ending in a sin^t pedal ganglion in the foot. Each cerebropleural 

 ganglion gives off a cerebrovisceral connective (sometimes enclosed 

 by the kidneys) leading to a single visceral ganglion. 



Fig. 63. Different stages of cyst. Proliferation in glochidia, fin margin of carp. 

 (After Lefevre and Curtis. Jour. Exp. Zool.^ 19 10.) 



Sense Organs. — A patch of yellow epithelial cells, the " osphra- 

 dium," covers each visceral ganglion. They are supposed to be 

 olfacto-gustatory organs. Paired otocysts (statocysts) with calcareous 

 statoliths behind the pedal ganglia are organs of equilibrium. 

 Yves Delage removed the otocysts and caused lack of balance. 

 The edges of the mantle have sensory cells — especially on the 

 inhalant siphon — which are sensitive to light and touch. Pecten, the 

 scallop, has from 80 to 120 ocelli at the edge of the mantle. They 

 are connected with the branchial ganglion and each has cornea, 

 lens, and optic nerve. Apparently visual organs are absent in the 

 fresh water bivalves, but they are better developed in the marine 

 forms along shore. There is no satisfactory evidence for color 

 vision in Mollusca. Certain eyeless species react to sudden darken- 

 ing very quickly, but soon get used to stimuli and cease to respond. 

 The sense of geotropism ^ is determined by obscure conditions. 

 Reactions are influenced by size of illuminated or darkened surface, 

 as well as by intensity of light. 



Class I. Lamellibranchiata. — Fresh water mussels or clams 

 ( Unionidae, Anodontidae, Lajnpsilidae) have assumed considerable 

 importance in America. The United States Bureau of Fisheries has 

 established a Biological Station at Fairport, Iowa, for the artificial 



^ See page 25 for definition of tropisms. 



