320 THE FRESH-WATER MUSSEL LESS. 



THE FRESH-WATER MussEL. 1 



The body is bilaterally symmetrical, and is greatly com- 

 pressed from side to side. Its dorsal margin is produced 

 into paired flaps, the mantle-lobes (Fig. 78, A and B, Mant\ 

 which pass downwards one on either side of the body, 

 Closely applied to the outer surface of the mantle-lobes, and 

 formed as a cuticular secretion of their deric epithelium, ar j 

 the two valves of the bivalved, strongly calcined shell (B., S/i). 

 The ventral region of the body is produced into a laterally 

 compressed muscular structure, ihefoot (A and B, Foot], by 

 the contraction of which the animal can move slowly 

 through the sand or mud in which it lives partly buried. 



The possession of a mantle formed as a prolongation of 

 the dorsal region, of a calcareous shell secreted by the 

 mantle, and of a muscular foot formed as an unpaired 

 prolongation of the ventral region, are the most characteristic 

 features of the Mollusca generally. 



Posteriorly the edges of the mantle-lobes are greatly 

 thickened, and are connected with one another in such a 

 way as to form two apertures, a large ventral inhalent (Ink. 

 Ap\ and a small dorsal exhalent aperture (Exh. Ap). By 

 means of the cilia of the gills (see below) a current of water 

 is produced which enters at the inhalent aperture, carrying 

 abundant oxygen and the minute organisms used as food, 

 and makes its escape at the exhalent aperture, taking with it 

 the various products of excretion and faecal matter. 



The mouth (Mth) is anterior and ventral, lying just in 

 front of the foot : it is bounded on either side by a pair of 



1 For detailed descriptions of the fresh-water Mussel see Rolleston 

 and Jackson, Forms of Animal Life, pp. 124 and 285 : Huxley and 

 Martin, Elementary Biology ; p. 305 : and Marshall and Hurst, Practical 

 Zoology, p. 80. 



