FRESH-WATER MUSSEL. 347 



Segmentation is total but slightly unequal. As the snail 

 is a terrestrial Gasteropod, there is no trochosphere larva, 

 nor more than a slight hint of the characteristic Molluscan 

 velum. A miniature adult is hatched in about three weeks. 

 The study of development may be more profitably followed 

 in the pond-snail Limnaus, where gastrula, trochosphere, and 

 veliger can be readily seen. 



Second Type of Mollusca. The Fresh-water Mussel 

 {Anodonta cygned), one of the Lamellibranchiata. 



Habit. — The fresh-water mussel lives in rivers and ponds. 

 It lies with its head end buried in the mud, or ploughs 

 slowly along by means of its ploughshare-like foot. Its food 

 consists of minute plants and animals, which are wafted in 

 at the posterior end by the currents produced by the ciliated 

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

 apply almost equally 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 on prisms, transversely 

 varicose or striated like those which form the enamel of the 

 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. 



