STRUCTURE OF ANODONTA. 305 



ploughs its way along slowly by means of its wedge-like foot. 

 Its food consists of minute plants and animals, which are 

 wafted in at the upper (or more strictly posterior) end by 

 the currents produced by the ciliated gills. When the 

 mussel is observed during life, it will be seen that small 

 particles in the water move away from and towards the 

 posterior end. Inside the mussel there are usually little 

 water-mites and sometimes other parasitic animals. ' Crows 

 occasionally catch these mussels, when they have ventured 

 too near the edge of water. 



External Appearance. — -The bivalve is four to six inches 

 long ; its valves are equal and united in a dorsal hinge by 

 an elastic ligament, which is an uncalcified part of the shell ; 

 on the ventral surface when the valves gape the foot pro- 

 trudes ; the anterior end is rounded, the posterior end is 

 more pointed, and it is there that the water currents flow in 

 (ventrally) and out (dorsally). The greenish-brown soft 

 (" horny ") layer of the shell is often worn away near the 

 dorsal knob or umbo on each side (which is obviously the 

 oldest part), and then displays the median " prismatic " layer 

 of lime. Internally there is a pearly layer. Lines of growth 

 on the shell mark the position of the margin in former years. 



The shell is a cuticular structure, i.e., it is made by the 

 epidermis of the mantle. It consists mostly of calcium 

 carbonate plus an organic stuff called conchiolin. The 

 mussels may be found in water which has extremely little 

 carbonate of lime, and the thickness of the shell depends, 

 according to Hatchett Jackson and RoUeston, not upon 

 the amount of lime in the waters, but upon the working of the 

 tissues, modified by surrounding influences whether chemical 

 or non-chemical. 



Internal Appearance. — When the right half of the shell is 

 folded back, the anterior and posterior closing muscles being 

 carefully cut close to the gently raised valve, the following 

 parts are readily seen — the mantle folds line the shell and 

 form posteriorly the ventral inhalent and dorsal exhalent 

 lips ; internal to the mantle lies on each side a double gill ; 

 projecting from between these is the foot, muscular ven- 

 trally, softer dorsally ; the median dorsal heart cavity is just 

 beneath the hinge ligament ; the ventricle shines through its 

 walls, and the dark-coloured kidneys are seen through its 



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