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 wmso—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, ze. 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. 
