972 FRESH- WATER BIOLOGY 



The Basommatophora are subdivided ijito three superfamilies 

 based mainly on the general character of their habitat: 



I. Terrestrial or semiamphibious, living in damp places or near 

 the margin of the sea, but not in the water, — Akteophila. 



II. Aquatic, living in fresh water and usually coming occasionally 

 to the surface for air, — Limnophila. 



III. Aquatic, living in salt or brackish water along the seashore 

 in the littoral zone, — Petrophila. 



The Streptoneura are divided into two orders: 



I. The Aspidobranchia, in which the nerve centers are not 

 closely concentrated, and the original bilateral symmetry has not 

 wholly disappeared, there being two auricles to the heart and two 

 kidneys. 



II. The Pectinibranchia, in which all trace of bilateral symmetry 

 in the circulatory, respiratory, and execretory systems has disap- 

 peared and the nervous system is more concentrated. 



The fresh-water aspidobranchs all belong to the suborder Rhipido- 

 glossa, in which the radula has very numerous marginal teeth 

 arranged like the sticks of a fan. 



The Pectinibranchia are divided into two suborders, of which 

 only one, the Taenioglossa, in which the radula has but one lateral 

 and two marginal teeth on each side of the central tooth, is repre- 

 sented in our fluviatile fauna. The fresh-water Taenioglossae are 

 all included in the superfamily Platypoda, in which the foot is flat- 

 tened ventrally for creeping purposes. 



The several superfamilies of the Euthyneura and Streptoneura 

 are subdivided into families, of which thirteen are represented in 

 the North American fauna. 



The class Lamellibranchia, so called from the form of the gills, 

 comprises all the fresh-water bivalve shells commonly called clams 

 or mussels. The name Pelecypoda is frequently applied to 

 this class from the hatchet-like shape of the foot. The lameUi- 

 branchs are aquatic mollusks, without a distinct head and with 

 the mantle divided into two lobes, which secrete a bivalve shell 

 united by a ligament, which covers the entire animal. The lobes 

 of the mantle are united by one or two transverse muscles, which 

 are attached to the inner surface of the valves and by their con- 



