A Typical Univalve Mollusk 



before as the radula. This flexible ribbon is closely set with rows 

 of horny teeth. It plays over the end of a central, stiff cartilage 

 rod, by means of muscles attached at both ends, and contracted 

 alternately. Thus holes are drilled in other shells by setting the 

 tip of the proboscis on the surface and drawing the ribbon back 

 and forth. 



Two large salivary glands lie near the base of the oesophagus, 

 with ducts to the mouth. The pancreas lies farther back on the 

 right of the oesophagus. The liver is the third gland, furnishing 

 juices that aid in the digestion of food. The stomach and intes- 

 tine complete the alimentary canal, which discharges its wastes 

 into the mantle cavity. 



The Nervous System. — A cluster of paired ganglia (little 

 brains), brown in colour, encircle the oesophagus, three-fourths 

 of an inch behind the base of the proboscis. One pair, the buccal 

 ganglia, sends nerves to the mouth parts. Another pair, the 

 pedal ganglia, supplies the foot. Both pairs are on the ventral 

 side of the oesophagus. On the dorsal side the pleural and cere- 

 bral pairs are fused and bound by commissures of nerve fibre 

 with the visceral ganglia; and the last are connected with the 

 abdominal ganglion, a brown mass visible just below the opening 

 of the kidney. The cerebral ganglia are the most centralised 

 "brains" of this mollusk, as they are not only joined, as a pair, 

 but directly connected, by commissures or by contact, with the 

 pedal, buccal and pleural pairs, and through the pleural with the 

 other two. Muscles all over the body are controlled by nerves 

 sent out from these ganglia. Sensations are brought to the nerve 

 centres along nerves from the foot, head, and especially the sen- 

 sitive mantle border. By these the mollusk learns all it knows 

 of what is going on outside its shell. 



The Special Senses. — Snails have eyes, but generally of a 

 low type. Sight is an unimportant sense. The sense of touch 

 is well developed in the mantle margin; the tentacles are touch 

 organs. The mouth has sensitive lips. The osphradium is an 

 organ of doubtful use. It may be the seat of the sense of smell. 

 It is believed to have a composite function of testing the water 

 that passes over it and enters the mantle cavity. Smell is a 

 well developed sense. The condition of the water and its fitness 

 to furnish food and oxygen to the body are broader considerations 

 probably determined by this generalised organ. 



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