106 PHYLUM PROTOZOA THE SIMPLEST ANIMALS. 



at the present day are very important in the formation of calcareous 

 ooze ; in this respect Globigerina, with a chambered shell, is especially 

 important. Species of Gromia are found in both fresh and salt water ; 

 Halipkysema, a form utilising sponge-spicules to cover itself, was once 

 mistaken for a minute sponge, or for a very simple many-celled 

 animal. 



Most kinds of chalk consist mainly of the shells of Foraminifera 

 accumulated on the floor of ancient seas; Nummulites (Fig. 17) and 

 related fossil forms were as large as shillings or half-crowns. 



More complex are the (7) Radiolaria, which are divided by a mem- 

 brane into an inner central capsule (with one or more nuclei), and an 



FIG. 48. A Forarninifer (Polystoinella] showing shell and pseudopodia. 



After Schultze. 



outer portion, giving off radiating thread-like pseudopodia. The great 

 majority are pelagic. There is usually a skeleton in the form of a 

 siliceous shell outside the central capsule, but in some cases the shell 

 is formed of a horn-like substance called acanthin. Most Radiolarians 

 include unicellular Algre (yellow cells), with which they live in intimate 

 mutual partnership (symbiosis). They are abundant as fossils, and of 

 much importance in the formation of the ooze of great depths. 



Examples. Thalassicola, Eucyrtidium, and the colonial Collozoum 

 and SphtfTozouui. 



C. Predominantly active forms (ciliate and flagellate), 

 generally called Infusorians. Protozoa, with a definite rind and 

 with 1-3 undulating flagella, are included as (8) Flagellata, a very 



