XI. 



THE FRESH-WATER MUSSEL 



(Anodonta Cygnaa). 



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

 of animals, which are not unfrequently abundant in our ponds 

 and rivers, are included; namely, the Anadonta 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 enclosed 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 un- 

 disturbed, the Anodonta will partially bury itself with its an- 

 terior end directed obliquely downwards; and the valves will 

 separate at their ventral edges for a short distance. At the 

 edges of this ' gape' of the shell the thickened margins of a 

 part of the contained body which is called the mantle, be- 

 come visible, and between them a large, whitish, fleshy, 

 tongue-shaped structure the foot not unfrequently pro- 

 trudes, and is used to perform the sluggish movements of 

 which the Anodonta is capable. If some finely dividing 

 colouring matter, such as indigo, is dropped into the water, 



