18 



PROTOZOA 



D. corona Wallich (Fig. 7). Shell splieroid with a number of long 

 spines at hinder end; length up to .32 mm: very common. 



2. QuADRUELLA Cockerell. Shell 

 pear-shaped and composed of quad- 

 rilateral silicious plates, with oc- 

 casional spines at the hinder end: 

 several species ; in fresh water. 

 '\"%\ Q- symmetrica (F. E. Schulze) 



\ X (Fig. 8). Length up to .14 mm.: 



Fig. 9— Coc7jiiopodiuw6ih"m6osMm (Leidy). -^ swamps. 



3. CocHLioPODiUM Hertwig and Lesser. Shell minute, spheroid or 

 disc-like, without foreign bodies, and flexible, changing in shape: 3 

 species; in fresh water. 



C. bilimbosum Auerbach (Fig. 9). Diameter 

 up to .05 mm. ; opening large, the acute pseudopodia 

 protruding: among algae, etc., in fresh water. 



C. digitatum Calkins. Several openings through 

 which pseudopodia protrude. 



4. Arcella Ehrenberg. Shell yellow or brown 

 and smooth, not being covered with sand, convex 

 on one side, and flat or concave on the other, in 

 the middle of which is the opening; nuclei and 

 contractile vacuoles 2 or more: several species; in 

 fresh water, also in moist sand and moss. 



A. vulgaris Ehr. (Fig. 10). Diameter about .15 mm.; margin not 

 scalloped: very common. 



A. dentata Ehr. (Fig. 11). Diameter about .18 mm.; margin scal- 

 loped. 



Fig. 10 



Arcella vulgaris 



(Leidy). A, under 



view of shell ; 



B, side view. 



Fig. 11 



Fig. 12 



Fig. 13 



Yie.W— Arcella dentata (Leidy). Fig. 12— Cenfropj/xis aculeata (Leidy). 

 Fig. 13 — Euglypha alveolata (Leidy). 



5. Centropyxis Stein. Shell similar to Arcella, but with spines, 

 variable in number, and sometimes elongate: in ditches and pools. 

 C. aculeata Stein (Fig. 12). Diameter of shell .2 mm. 



