THE NATURAL HISTORY SURVEY. 45 
2. Shell large, globose; spire short and obtuse; 
aperture and spire about equal in length..... Viviparide 
3. Shell small, variable in form. 
1. Shell globose or elongated.............. Amnicolide 
Ao Shellsnatancs discoidally.\),,..2.!<s1<to cise aon Valvatide 
A. CRASS PELECYPODA, 
“Aquatic, bilaterally symmetrical, acephalous mollusks, pro- 
tected by a pair of shelly valves secreted by the lateral portions 
of the mantle, connected by a ligament, and moved by the con- 
traction of muscles which connect the inner faces of the valves; 
feeding by ciliary action and destitute of a radula or jaw; breath- 
ing by lateral gills, the type of which is a midrib or stem, with a 
row of transversely oriented leaflets or filaments depending from 
each side of the stem, single, or mutually combined to form a 
direct or reflected plate; imperfectly sensible to light and rarely 
I) ape) 
) Nae 
Ohh 
|] 
Diagram showing position of Unio while ploughing its way through the 
bottom of a lake or river. (After Morse.) cl, cloacal siphon; br, branchial 
siphon; f, foot; s, shell; b, surface of mud at bottom. ~<—— direction in 
which the animal is moving. ——>, currents of water to and from the gills. 
provided with peripheral visual organs; possessing olfactory 
organs (osphradia), auditory and equilibrating organs (otocysts), 
tactile papille and a nervous system composed of (usually 
three principal pairs of) ganglia united by nerves, but withouta 
pedovisceral commissure; provided with an extensive tactile or 
locomotor organ (foot); a closed, though partly lacunary, circu- 
latory system, containing (usually colorless) hemolymph, and 
operated by a single or paired cardiac ventricle and two auri- 
