Chap. III.] GROUPS OF PROTOZOA. 25 



I. PEOTOZOA. 



For our purposes we shall find it convenient to 

 divide the Protozoa into three great groups, one of 

 which has become degraded by parasitism ; these are 

 the Sporozoa, of which the best known division are 

 the Oregariiiida ; the others, one of which is dis- 

 tinctly higher than the other group, may be called the 

 Sarcodiiia and the Infusoria. 



Of the Sarcodina, the best type is the common 

 Amceba, which we have already studied ; like it, all 

 the members of the group move about and take in 

 their food by means of those movements of the proto- 

 plasm of the cell which result in the formation of 

 pseud opodia, and they reproduce themselves either by 

 division or by budding. 



In the Infusoria the amcebiform character is 

 lost, and the cell has and retains a definite form ; the 

 ectosarc ordinarily sheds out a structureless mem- 

 brane. This encloses the softer protoplasm which 

 makes up the rest of the organism, giving oft' 

 delicate processes which make their way through 

 the limiting membrane ; these processes, or cilia, 

 are typically developed, are portions of proto- 

 plasm which retain their contractile power, and form 

 the chief means of progression. Owing to the pre- 

 sence of the covering membrane or cuticle, it is neces- 

 sary that there should be at some point an opening in 

 the cell (cytostome), by means of which food may, 

 at any rate, enter ; this opening is ordinarily spoken 

 of as the mouth; in addition to it there is sometimes a 

 second orifice developed, which has the function of an 

 anus (cytoproct). 



The third division of the Protozoa are the de- 

 graded parasitic forms, of which the Gregariiie is 

 an excellent example. Though these cells are covered 

 in by a distinct membrane, there is no orifice or 



