CLASSIFICATION OF THE PROTOZOA 413 



contact with the material environment, and the endoplasm, with- 

 in. Within the endoplasm there is generally a nucleus (in some 

 forms two, a large and a small) , and frequently inclusions of other 

 sorts. 



The ectoplasm in some cases cannot be differentiated sharply 

 from the endoplasm. These are the exception, however, and not 

 the rule. The ectoplasm may become variously modified, and, by 

 secretion, form shells or a heavy membrane for protection. Many 

 of the pathogenic protozoa become encysted by the formation of a 

 heavy, chitinous wall. 



The endoplasm is made up of a very delicate, foam-like or 

 alveolar structure, the density of which varies greatly. It may 

 inclose amorphous granules or crystals of different kinds, green 

 granules or chromatophores, vacuoles, etc. 



None of the protozoa are certainty known to be entirely des- 

 titute of nuclear material. The nucleus may be devoid of a 

 nuclear membrane or distributed; generally a membrane is pres- 

 ent. A single homogeneous nucleus is present in most forms, 

 with the exception of the infusoria, which have, in general, a large 

 or macronudeus and a small or micronucleiLS. At some stages in 

 the life-history many nuclei may be present in a single cell. This 

 is particularly true just before or during spore formation. 



The protozoa with few exceptions are motile, at least during 

 certain stages in the life history. The exceptions are among 

 certain of the parasitic Sporozoa. Special organs of locomotion 

 are frequently found. Pseudopodia are changeable processes of 

 the protoplasm which are thrown out by the cell, the cell contents 

 frequently flowing forward into the pseudopodia. Many or few 

 may be present at one time. They may be short and blunt or 

 long and slender. Usually the endoplasm as well as ectoplasm 

 takes part in their formation. The protozoan flagella are derived 

 from the ectoplasm; are usually unchangeable in shape, long, 

 thin, and pointed; in general, longer than the cell itself. 

 One, two, or many may be present. They propel the cell by a 

 series of undulations or spiral or rotary motions. Cilia, when 

 present, are found in considerable numbers. They are shorter, 

 generally blunt, and, by striking the water in unison, resemble 

 oars in their motion. 



