446 PHYLUM MOLLUSCA 



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 Limnceus, where 

 gastrula, trochosphere, and veliger can be readily seen. 



Second Type of Mollusca. The Fresh-w^ater Mussel 

 [Anodofita cygnea), one of the Lamellibranchiata 



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

 ponds. It lies wdth its head end buried in the mud, or 

 moves 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, 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 w-hich 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 



