DISTINCTIVE CHARACTERS OF THE PROTOZOA 11 



always by some form of fission that is to say, division of the body 

 into smaller parts ; that the phenomena known as " syngamy " and 

 " sex " occur, perhaps universally, throughout the group ; and that 

 it is very characteristic of 

 Protozoa, as compared with 

 other Protista, to exhibit in 

 their life-history a develop- 

 mental cycle, more or less 

 complicated, in the course of 

 which the organism may appear 

 under very different forms at 

 different stages in its develop- 

 ment. 



The Protozoa, as thus under- 

 stood, are commonly divided 

 into four main subdivisions, 

 termed "classes." Other 

 methods of classifying the 

 Protozoa have been suggested, 

 which will be considered later ; 

 for the present the old- 

 established subdivisions are 

 sufficient for our purpose. 



CLASS I., SARCODINA.* 

 Protozoa in which the proto- 

 plasmic body is naked or non- 

 corticate that is to say, 

 without a limiting envelope 

 in the form of a cuticle, 

 membrane, or stiff cortical 

 layer ; consequently the body 

 tends to be either more or less 

 spherical in floating forms, or 

 to have an irregular, con- 

 tinually changing shape in 

 creeping forms. Organs serving 

 for locomotion and capture of 

 food are furnished by tem- 

 porary extensions of the living 

 protoplasm, termed pseudo- 

 podia. ' A skeleton or shell 

 may be present. Examples 



* The name is derived from sarcode, the term coined by Dujardin to denote 

 the living substance, subsequently named by von Mohl protoplasm, the term now 

 universally employed. 



FIG. 10. Acineta grandis. si., Stalk ; th., 

 theca ; s., suctorial tentacles. After 

 Saville Kent. 



