PHYLUM PROTOZOA 67 



than flagella; often they are fused together in groups forming 

 large cirri {Slylonychia, Fig. 43, B), or side by side, forming 

 membrancllcs as in the gullet of Paramecium. 



An external covering may be absent from the body of Pro- 

 tozoa (Atneba) or may be present as a distinct cuticle (Para- 

 mecium). Shells may also occur; these consist of material se- 

 creted by the animal, e.g. chitin by Arcella (Fig. 16), calcium 

 carbonate by Foraminifera (Fig. 21), and silica by Radio- 

 laria (Fig. 19), or are made up of foreign particles such as 

 grains of sand (Difflugia, Fig. 17). 



The cytoplasm of Protozoa is probably alveolar in structure. 

 It can usually be separated into a firm, clear, outer layer, the 

 ectosarc, and a more fluid, granular, inner mass, the endosarc. 

 Within the cytoplasm are embedded one or more nuclei, vacuoles 

 of several kinds, and frequently plastids., 



A nucleus is always present, although in some cases its essen- 

 tial substance, chromatin, is scattered throughout the cells, form- 

 ing a " distributed nucleus." Some Protozoa have two kinds 

 of nuclei, a macronucleus {Paramecium, Fig. 33, ma.), which is 

 supposed to have charge of the metabolic processes, and a micro- 

 nucleus (Fig. ^t,, mi.), which functions only in reproduction. 

 During binary division the chromatin of the nucleus may form 

 distinct chromosomes, but in many cases chromosomes have not 

 been observed. 



Vacuoles are of several kinds: (1) permanent globules of liquid 

 {Actinophrys, Fig. 18), (2) contractile vacuoles (Ameba, Fig. 9, 2), 

 and (3) food vacuoles {Paramecium, Fig. 33, f.v.). 



Many Protozoa possess plastids; these are usually bodies of 

 starchy food material, or colored bodies called chromatophores, 

 such as occur in Euglena. Besides these, many other substances 

 may be present, such as food material, indigestible matter, oil 

 drops, grains of sand, etc. 



Physiology. — Metabolism. - - The food of Protozoa con- 

 sists of organic matter both vegetable and animal. Bacteria, 

 diatoms, and other Protozoa form a large part of the bill of fare. 



