XL 



THE FRESH-WATER MUSSEL 



(Anodonta Cygncea). 



UNDER the name of ' Fresh- water Mussel ' two distinct kinds 

 of animals, which are not ^infrequently abundant in our ponds 

 and rivers, are included ; namely, the Anodonta and two or 

 three kinds of Unio. The Anodonta is chosen for special 

 study here, but what is said about it applies very well to all 

 parts of Unio except the shell. 



The animal is inclosed in a shell composed of two pieces 

 or valves, which are lateral, or right and left, in relation to the 

 median plane of the body. The more rounded and broader 

 end is anterior, the more tapering, posterior. If placed in a 

 vessel of water, at the bottom of which there is a tolerably 

 thick layer of soft mud or sand, and left quite undisturbed, 

 the Anodonta will partially bury itself with its anterior end 

 directed obliquely downwards; and the valves will separate at 

 their ventral edges for a short distance. At the edges of this 

 1 gape ' of the shell the thickened margins of a part of the 

 contained body which is called the -mantle, become visible, 

 and between them a large, whitish, fleshy, tongue-shaped 

 structure the foot not unfrequently protrudes, and is used 

 to perform the sluggish movements of which the Anodon is 

 capable. If some finely divided colouring matter, such as 

 indigo, is dropped into the water, so as to fall towards the 



