98 



MOLLUSKS 



98. The Nervous System is not so well developed as in 

 the crayfish. There are three groups of ganglia (nerve 

 cells). One located far back in the body near the posterior 

 adductor is called the visceral ganglion because it largely 

 regulates the activities of the viscera (vis'se-ra), the inter- 

 nal organs of the body. Another in the foot region is 

 called the pedal (pe'dal) ganglion, and regulates the 

 movements of the foot. A third located in the region of 

 the gullet (esophagus) is the cerebral ganglion, which 

 regulates the activities of the part near the mouth. All 

 of these are connected by nerves. 



99. Digestive System. — The mouth, which is located 

 under the anterior adductor muscle, leads through the 

 short esophagus to the stomach. The intestine winds 



through the foot region forming 

 a loop, finally ascending and 

 passing through the pericardium 

 and between the chambers of the 

 heart itself and opening into the 

 upper siphon (Figure 96). 



100. Snails. — Snails having one valve are called uni- 

 valves as distinguished from clams, oysters, etc. which 

 are called bivalves because their shells are formed of two 

 valves. The greater number of snails 

 are marine (live in salt water), although 

 some live in fresh water and some on 

 land. Snails have a broad foot which 

 is used as a creeping disk. There is 

 a head region provided with eyes and 

 tentacles. The mouth of the snail is 

 provided with a rasping structure known 

 as the lingual ribbon (lin'gwal : Latin, 

 lingua, tongue) by means of which it 

 is able to cut and bore its way, even 



Figure 97. — Snail. 





Figure 98. — Tongue 



of Snail. 



(Magnified.) 



