28 



SYSTEMATIC ZOOLOGY 



Order 2. Heliozoa 



The Heliozoa (Gr. r/\io<;, sun, and £<woi>, animal) constitute an 

 extremely small group which is confined to fresh water. They 

 are readily distinguished from the fresh-water Foraminifera by 

 their pseudopodia, which are permanent, delicate structures 

 radiating in all directions from the more or less spherical body, 

 — hence the name of the order (Fig. 7). In the axis of each 

 pseudopodium is a delicate, firm, supporting rod extending 

 toward the center. The protoplasm contains one or more 



FIG. 8. Actinophrys so!, a Heliozoon. (An icher, from Calkins' Proto, 



nuclei and one or more contractile vacuoles, and often pigment 

 granules or chlorophyll bodies as well as numerous non-con- 

 tractile vacuoles. The latter are often very numerous and 

 form a definite peripheral layer to the body of the heliozoon 

 (Fig. 8). Aside from the axial supports to the pseudopodia, a 

 skeleton may be entirely absent, or represented by a simple 

 gelatinous envelope, or it may consist oi a more or less elabo- 

 rate structure of silica. The latter may be composed of loosely 

 interlacing, simple, siliceous spicules (Fig. 7 >, or it may cor- 



