MOLLUSCA 



159 



sense 

 light 



^v; 



directly through the heart. In some forms it is so bent 

 on itself that it ends close to the mouth. A well-devel- 

 oped liver and very large sexual organs fill the remaining 

 space. 



Nervous System. — The nervous system consists of 

 three pairs of ganglia con- 

 nected by cords. These are 

 associated with three sets of 

 organs, sensitive to 

 or sound vibrations. 

 The eyes vary from mere 

 spots sensitive to light which 

 may be situated on the man- 

 tle, the siphon, the tips of the 

 tentacles, or the back, to the 

 well-developed eyes of the 

 squid which are situated on 

 the side of the head and are 

 almost as well developed as 

 the human eye. 



Economic Value. — Mol- 

 luscs have great economic 

 value. Clams, oysters and 

 scallops are highly prized for 

 food. The precious pearl is 

 formed by the oyster. The 

 oyster and the abalone furnish 

 the mother of pearl used ex- 

 tensively in the manufacture 

 of such articles as buttons 

 and knife handles. The ink that is found in squids 

 is used in the manufacture of sepia. It is useful to 





s^ 



Fig. 90.— Long clam buried in the 

 mud. (From Kingsley.) 



