APPENDIX B 



633 



The Protozoa are the simplest of all animals. Because of their microscopic size, 

 they do not come within our everyday experience, and one who for the first time 

 sees them swarming in a drop of pond water under the microscope feels as though 

 he were discovering a new world. Protozoa are unable to live under permanently 

 dry conditions, but they may occur in fabulous numbers in moist or aquatic 

 situations. Fresh- water ponds, lakes, and streams have many of them; uncount- 

 able numbers live in the sea; and there is a great protozoan fauna in the soil. 

 Many species of Protozoa are parasitic, living on or within the bodies of other 



Fig. B.2. Model of a marine protozoan, Lychnosphaera regina. This is a member of the class 

 Sarcodina, order Radiolaria. The skeleton is composed chiefly of silicious spicules. (Courtesy 

 American Museum of Natural History.) 



animals. Among the more interesting and important parasites are the species 

 that live within the red blood cells of man and produce malaria. 

 Approximately 15,000 species of Protozoa have been described. 



"pore/' 



and ferre, 



Phylum II. PORIFERA (po rif' er a; Latin, porus, 

 "to bear"). 



The sponges. Primitive aquatic Metazoa, mostly marine and invariably 

 sessile; usually colonial or consisting of many individuals indistinguish- 

 ably fused; exhibiting the following characters (Figs. 13.4 and B.3): 



Body wall diploblastic, consisting in the adult of an outer dermal 

 epithelium and an enclosed gastral epithelium, between which there is 



