2 ENTOZOA FOUND IN MAN. 



have no complete distinction between their different 

 organs. In some the simple organization is reducible 

 to the type of a cell, whilst in others, which are more 

 complex, the functions are still performed by simple 

 organs, and not by special parts. 



The integuments of the protozoa are sometimes 

 soft, contractile, and not distinct from the substance 

 of the body ; sometimes more distinct, and reti- 

 culated ; sometimes firm and non-contractile, or hard 

 and horny, and remaining after the destruction of the 

 animal. They are usually provided with various 

 appendages which serve for the purposes of taking 

 food, of locomotion, and possibly of respiration ; 

 these are the contractile expansions, sometimes short 

 and broad, sometimes long and filiform, which certain 

 protozoa have the power of frequently emitting or 

 retracting ; or, in others, these are the constantly 

 agitated vibratile cilia, and the little filaments which 

 appear to be under the control of the will of the 

 animalcule. 



The body is composed of a soft, transparent, 

 diffluent, and contractile substance. One or more 

 reddish vesicles, which appear and disappear at uto- 

 gular intervals, represent a rudimentary system of 

 circulation. The digestive tube, like the other 

 organs, is usually altogether wanting. 



No well-defined limits have, as yet, been assigned 

 to the class of protozoa, both because it is easy to 

 include within this class the larvae of other animals 

 which are higher in the scale of organization, and 

 because it is difficult to distinguish the protozoa from 

 plants which are endowed with motion, or even 

 from particles newly separated from a living structure, 



