THE MOLLUSCA 



971 



the surface of which is covered with numerous rows of small chitin- 

 ous teeth. 



The Gastropoda are either ovo viviparous or oviparous. The sexes 

 are separate in some groups (Dioecia) and united in the same indi- 

 vidual in others (Monoecia). All of the fresh- water gastropods are 

 provided with an external shell, which covers the animal completely 

 when retracted, and which is spiral, discoidal, or conical in shape. 



Owing to the torsion of the body, caused by the spiral shape of the 

 shell, the animal of all the fresh-water gastropods, while externally 

 bilaterally symmetrical, is internally asymmetrical. There is but 

 one lung or one functional gill, as the case may be, and the termina- 

 tions of the digestive and genital systems, instead of being posterior 

 and central, corresponding to the mouth, are on the side near the 

 respiratory chamber. 



The Gastropoda are further divided into subclasses, accordingly 

 as the torsion of the viscera has or has not been accompanied by a 

 similar twisting of the visceral nerve loop. In the Euthyneura, 

 the visceral nerve loop lies beneath the intestinal canal, and was 

 consequently not affected by the torsion to which that organ was 

 subjected, while in the Streptoneura, the visceral nerve loop lies 

 above the intestines and became involved in the twisting of the 

 viscera and was consequently made to assume the form of the figure 

 8. The aquatic Euthyneura, which comprise practically all our 

 air-breathing or pulmoniferous fluviatile mollusks, are included 

 in the order Pulmonata; while the Streptoneura comprise all the 

 gill-breathing or branchiferous species, which are furnished with 

 a pecuUar chitinous or calcareous structure attached to the upper 

 surface of the posterior extremity called the operculum, and which, 

 when the animal retires within the shell, completely closes the 

 aperture. Species thus provided are termed operculate. The 

 Pulmonata, on the other hand, have no operculum, and are there- 

 fore called inoperculate. 



The Pulmonata are divided into two suborders: the Stylomma- 

 tophora, in which the eyes are borne on the extremities of retractile 

 tentacles, and which are all terrestrial species; and the Basom- 

 matophora, in which the eyes are placed at the base of contractile 

 tentacles, and which are aquatic or amphibious in habit. 



