132 



PHYLUM PROTOZOA— THE SIMPLEST ANIMALS 



of Gromia are found in both fresh and salt water ; Haliphysema, a 

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

 minute sponge. 



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 chitinoid 

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



Fig. 60. — Formation of shell in a simple Foraminifer. 

 — After Dreyer. 



In A and B the shell has one chamber ; C and D show the formation 

 of a second. Note outflowhig pseudopodia and the enclosure of 

 the shell by a thm layer of protoplasm ; note also the nucleus 

 in the central protoplasm. 



an outer portion, gelatinous and vacuolated, giving off radiating thread- 

 like pseudopodia, which very rarely interlace. There is usually a 

 skeleton in the form of a siliceous lattice-work or regularly disposed 

 spicules outside the central capsule, but in some cases the shell is 

 formed of a horn-like substance called acanthin, which is probably a 

 complex silicate. Radiolarians multiply by fission, which sometimes 

 includes a halving of the skeleton, and by spores, which in some cases 

 are dimorphic. Most Radiolarians include unicellular Algae (yellow 

 cells), with which they live in intimate mutual partnership (symbiosis). 

 Most Radiolarians float on the surface of the sea ; others live below 



