MOLLUSCA 147 



The digestive system is well defined. Near the anterior 

 adductor muscle between the two pairs of triangular flaps, 

 called labial palps, :'s the mouth opening into a short esopha- 

 gus leading into the stomach surrounded by a dark tender 

 mass of tissue, the liver, from which ducts empty into the 

 stomach. The intestine after leaving the stomach makes 

 a turn on itself in addition to several bends, and terminates 

 dorsal of the posterior adductor muscle in the exhalant 

 siphon. 



The nervous system is difficult to demonstrate. It con- 

 sists of three pairs of ganglia or bunches of nerve cells 

 connected by nerve cords. The visceral ganglia he on the 

 under side of the posterior adductor muscle. From these 

 a pair of threadlike nerves extend forward to the cerebral 

 ganglia at the base of the labial palps and behind the an- 

 terior adductor muscle. A circular nerve cord joining 

 these two ganglia surrounds the mouth and from each a 

 nerve cord leads to the pedal ganglion near the center of 

 the foot. 



Gastropoda: Snails and Slugs 



The univalve mollusks are unsymmetrical and have but 

 one shell which is often in the form of a spiral. The pond 

 snail (Limnea), abundant in ditches and stagnant ponds, 

 has a right-handed shell, i.e., starting from the apex the 

 coil turns toward the right. Physa, the other pond snail, 

 has a left-handed shell. The eggs of pond snails appear as 

 minute white spots in a mass of transparent albumen. 

 During the summer months they are easily found adhering 

 to boards, stones, and other objects in the quiet waters. 



