10 



THE PROTOZOA 



The attempt, therefore, made in the following paragraph to give 

 a diagnosis of the Protozoa must not be regarded as a definition 

 of the group in the rigidly logical sense, but merely as the construc- 

 tion of a general type, the characters of which are liable to a 

 certain amount of variation in special cases a compromise between 

 the claims of logic and the versatility of Nature. 



The Protozoa, then, are Protista 

 in which the organization is of the 

 cellular type, with nucleus distinct 

 from the cytoplasm. They are uni- 

 cellular, in the sense that the cell 

 constitutes an entire individual, 

 which may exist singly and in- 

 dependently or in the form of cell- 

 colonies ; but in the latter case the 

 cells are not subordinated to the 

 individuality of the entire cell- 

 aggregate by the formation of 



n-. 



- ! . 



iH 



. -.& 



1 i :ty$ 

 ~ "* 



Ht 



f^&i^M^ 



FIG. 8. Stentor roesdii, fully expanded. 

 oes., (Esophagus; N, band-like macro- 

 nucleus ; c.v., contractile vacuole, con- 

 nected with a long feeding- canal (/.c.) 

 stretching down the body ; H, gelat- 

 inous house into which the animal 

 can retract itself completely ;/., fibres 

 attaching the extremity to the stalk 

 to the house. After Stein. 



c.v- 



an. 



FIG. 9. Nyctotherus cordiformis, 

 parasite of the rectum of the frog. 



"" N, Macronucleus ; n, micronucleus ; 

 gr., mass of granules in front of the 



^ macronucleus ; ces., oesophagus; c.v., 



f^ contractile vacuole ; an., anus 



L_ ( c ytopyge). After Stein. 



tissues. The body protoplasm is naked or clothed with a firm 

 envelope, which is usually not of the nature of cellulose. Proto- 

 plasmic organs are usually present for purposes of locomotion and 

 for the capture and ingestion of food. Chlorophyll is usually 

 absent as a cell -constituent, and the metabolism is usually of the 

 animal type. To these characters it may be added, though not 

 as special peculiarities of Protozoa, that reproduction takes place 



