208 LAKES AND RIVERS. 



fresh-water mussel, is gregarious, and they attach 

 themselves by a strong byssus to extraneous sub- 

 stances. They can live a long time out of the water, 

 and are believed to have come from Tartary. 



D. polymorpha (Palla) is 1*4 long, and 0*6 broad. 

 It inhabits slow rivers, canals, and lakes. It has 

 been even found in the water-pipes in London. The 

 shell is oblong, rising in a sharp keel in the middle 

 of each valve and flattened below, pointed at the 

 end or beak, and gradually but obliquely widening 

 towards the front, solid but not glossy. Beneath 

 the epidermis it is purplish-brown. The beaks are 

 terminal. 



We shall now speak of the Univalves. 



The body is conical, the mantle forms a single lobe, 

 the head is usually distinct and furnished with tentacles, 

 of which the upper pair has two eyes, either at the tips 

 or base, on separate stalks, the foot is a muscular 

 disc by which the animal crawls or floats. Some 

 species are hermaphrodite. The respiratory system 

 consists of gills or lung-like organs, the former being 

 possessed by aquatic kinds. The shell is conical or 

 spiral, and covers the whole or most of the body. 



The order Pectinibranchiata has the body of spiral 

 form, and has a single gill within the mantle, the shell 

 is external. Three families of this order inhabit the 

 fresh waters of this country i. Neritida, 2. Palu- 

 dinidcz, 3. Valvatida. All these fresh-water snails have 

 two tentacles and eyes at their base. The shells have 

 an operculum and an epidermis. 



Of the family Neritida, there is one species of 

 the genus Neritina (Lam.); it is the Neritina fluviatilis 



