FRESH-WATER MUSSEL. 393 



consists of minute plants and animals, which are wafted in 

 at the posterior end by the currents produced by the cili- 

 ated gills. What is noted here in regard to Anodonta will 

 also apply, for the most part, to Unio and other fresh-water 

 mussels. 



External appearance. The bivalve is 4 to 6 in. long ; 

 its valves are equal and united in a dorsal hinge by an 

 elastic ligament, an uncalcified part of the shell ; on the 

 ventral surface when the valves gape the foot protrudes ; 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). In bivalves the ligament is generally 

 posterior to the dorsal knob or umbo the oldest part of the 

 shell and the umbo generally points towards the anterior 

 end. The greenish brown soft (" horny ") layer of the shell 

 is often worn away near the umbo on each side, and then 

 displays the median layer of lime. This is called prismatic, 

 since the lime salts are deposited in prisms, transversely 

 varicose or striated, like those which form the enamel 

 of our teeth. Internally there is a pearly layer. Lines 

 of growth on the shell mark the position of the margin 

 in former years, the newest part being obviously at the 

 edge. 



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

 epidermis of the mantle. It consists, as in the snail, of 

 calcium carbonate plus conchiolin or conchin. Thus the 

 composition of a Pinna shell is : Lime salts, 89*2 ; organic 

 matrix, 1*3; water, 9*5. 



Internal appearance. When the right half of the shell 

 is folded back, the anterior and posterior closing muscles 

 having been carefully cut close to the gently raised valve, 

 the mantle folds are seen lining the shell, and forming 

 posteriorly the ventral inhalant and dorsal exhalant lips. 

 The ventral lips have papillary processes. Internal to the 

 mantle there are two gill-plates on each side ; projecting 

 from between these is the foot, muscular ventrally, softer 

 dorsally ; the median dorsal pericardium is just beneath the 

 ligament; the ventricle shines through its walls, and the 

 dark-coloured kidneys are seen through its floor. Below 

 the anterior adductor muscle is the large mouth, bordered 

 beneath by two lip processes (labial palps) on each side, 



