RHIZOPODA 17 



FAMILY 2. ARCELLIDAE. 



Shell membranous and consisting of a single chamber, the surface 

 of which is either smooth or covered with sand or other foreign par- 

 ticles; single opening usually present from which blunt pseudopodia 

 protrude: fresh-water animals of minute size; 10 genera and about 30 

 species. 



Key to the genera of Arcellidae here described : 



! Shell covered with sand or other foreign bodies 1. DIFFLUGIA 



o. Shell not covered with foreign bodies. 



&! Shell composed of quadrilateral plates 2. QUADBUELLA 



6 a Shell of one piece, not composed of plates. 



c x Shell flexible and more or less disc-like ; 1 to 3 openings. .3. COCHLIOPODIUM 

 c. Shell disc-shaped and not flexible. 



dj Rim of shell without spines 4. ARCELLA 



d 2 Rim of shell with spines 5. CENTROPYXIS 



1. DIFFLUGIA Leclerc. Shell covered with sand-grains or other 

 foreign bodies, pear-shaped or spherical, frequently with spines at the 

 sides or hinder end, and with a large opening at the other end from 



Fig. 6 Fig. 7 Fig. 8 



Fig. 6 Difflugia lobostoma (Conn). Fig. 7 Difflugia corona (Leidy). 

 Fig. 8 Quadruella symmetrica (Leidy). ^ 



which a number of blunt pseudopodia may project; nucleus and con- 

 tractile vacuoles always present: about 20 species, which are very vari- 

 able in form; in fresh-water pools, usually on plants or on the bottom. 



D. lobostoma Leidy (Fig. 6). Shell spherical or ovate, the oral pole 

 truncated; mouth usually 3 to 6 lobed; aboral end rounded; length .12 

 mm. : common. 



D. pyriformis Perty. Shell bottle-shaped with a cylindrical neck 

 more or less distinct ; length up to .6 mm. : common. 



D. urceolata Carter. Shell jug-shaped with short neck and often a 

 rim; hinder end pointed or rounded; length up to .52 mm. 



D. constricta Ehrenberg. Shell ovoid; mouth oblique; aboral end 

 rounded, often with spines; length up to .3 mm. 



D. globulosa Dujardin. Shell spheroidal or oval; mouth circular; 

 length .2 mm. or less: rather common. 



D. acuminata Ehr. Shell oval; aboral end acute, often prolonged, 

 rarely with 2 or 3 points; length ,4 mm r 



