PROTOZOA. 77 



complete classification of the protozoa, for there are many orders and 

 genera which are unmentioned because they are parasitic neither in man 

 nor In animals. 



The form of a protozoon may vary greatly at different stages of its 

 development; for example, the adult trypanosome is an active organism 

 moving by means of a flagellum, the spherical developmental form of the 

 same parasite may have no flagellum. Consequently, the whole life history 

 of a protozoon must be known before it can be classified with absolute 

 certainty. The whole of the life history is known of only a few protozoa; 

 and, though the organisms mentioned in this classification are placed in 

 the position usually given to them, it must be understood that this classi- 

 fication is not final, and that the discovery of new stages in the life history 

 of some of these protozoa might make it necessary to remove them 

 from the classes in which they have been placed; for example, before 

 its flagellate stage was known, Herpetomonas donovani was known as 

 Leishmania donovani, and it was classified with the sporozoa. Now 

 it is grouped with the herpetomonads. 



The characteristics of the different genera and of the unimportant para- 

 sites are very briefly mentioned in the next few paragraphs; the important 

 parasites are treated more fully in the pages indicated by the references 

 given, in brackets, throughout the classification. 



- The RHIZOPODA include the simplest forms of animal life. A 

 rhizopod, such as an amoeba, consists of a single cell, without a protective 

 covering, and without organs of locomotion; it moves and ingests food 

 by means of pseudopodia. Very few of the rhizopods are parasitic; 

 most of those which are parasitic belong to the genus Amoeba. Amoebae 

 of different species may occur in any part of the alimentary canals of 

 animals of many different species (p. 667). 



The FLAGELLATA are distinguished by possessing one or more 

 flagella; they often have, also, a fin-like, undulating membrane. Most 

 flagellates are free-living. Comparatively few species are parasitic; 

 but some of these cause very serious diseases (p. 667). 



The SPiROCELETjE, as their name signifies, are thread-like organisms, 

 whid seem to be coiled in a spiral. It is probable that the curves of 

 many spirochastes lie in one plane and, consequently, that their bodies are 

 really waved and not spiral. They often have an undulating membrane 

 and a flagellum at either end. A Herpetomonas is an elongated organism 

 which possesses a nucleus and a blepharoblast. The blepharoblast is 



