316 THE FRESH-WATER MUSSEL 



the fresh-water crayfish the young is hatched in a condition 

 closely resembling the adult, but in the lobster and the sea- 

 crayfish there is a metamorphosis. 



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. 79, A and B, Mant\ 

 which pass downwards one on each side of the body. 

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

 formed as a cuticular secretion of their deric epithelium, are 

 the two valves of the bivalved strongly calcified shell (J3., Sh). 

 The ventral region of the body is produced into a laterally 

 compressed muscular structure, the foot (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 impaired 

 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 united to 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 pro- 

 duced which enters at the inhalent aperture carrying 



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. 76. 



