PROTOZOA THE SIMPLEST ANIMALS. 



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 

 outer portion, giving off radiating thread-like pseudopodia. 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 Algse (yellow 



Fig. 45.— A pelagic Foraminifer — Hastigerina {GloHgerina) 

 Murrayi, — After Brady. 



Note central shell, projecting calcareous spines with a protoplasmic 

 axis ; also fine curved pseudopodia and vacuolated protoplasm. 



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, Eiuyrtidium, and the colonial Collozoum 

 and Sphccrozoum. 



O. Predominantly encysted Protozoa-Sporozoa.— Forms like 

 Gregarina and Monocystis are included as (8) Sporozoa or Gregarinida. 



