84 AN INTRODUCTION TO THE STUDY OF FOSSILS 



hard parts, and without differentiated tissue or organs. The 

 hard parts, secreted within or upon the surface of the proto- 

 plasm, consist of chitin in some of the Sarcodina, of cellulose 

 in the Dinofiagellata, of calcium carbonate in the Foraminifera, 

 and of silica in the Radiolaria. Protozoa are the most abundant 

 aquatic animals. 



Two extremes in size among living Protozoa are the organism 

 causing yellow fever, supposed to be a protozoon but so minute 

 that it has never been seen, and Porospora gigantea, a parasite 

 on the intestine of lobsters, which is two-thirds of an inch in 

 length. The same species may vary much in size " due solely 

 to the lack of food in the one case." A living species of Dilep- 

 tus varied from a proportional length of 80 when normal to 10 

 when starved (13). 



Derivation of name. — Protozoa > Greek protos, first, + 

 zoon, animal ; in this phylum occur the first or lowest animals. 



The Protozoa are divided into the following four classes, of 

 which only the Sarcodina and possibly the Mastigophora have 

 fossil representatives : — 



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A. Sarcodina , S8 



B. Mastigophora 94 



C. Sporozoa 95 



D. Infusoria 95 



Type of phylum. Amoeba proteus (living) (Fig. 30). 



This is a microscopic mass of jelly-like protoplasm, about 

 .2 mm. in diameter with a clear, usually thin, outer layer and 

 a granular inner portion, the granules being mostly proteid and 

 fat particles. Within this granular portion is a darker, rounded 

 body, the nucleus, and a rounded, pulsating, clear space, the 

 contractile vacuole. 



It secretes no skeleton, and is free-moving and abundant at 

 the bottom of fresh water pools and ponds among decaying 

 vegetation. It is irregular and variable in outline, continually 

 altering its shape by slowly pushing out and withdrawing 



