THE MONAD AND OTHER INFUSORIANS. 19 



and of these inconceivably minute beings there are many 

 thousand species which act as scavengers to purify the sea 

 and fresh waters. Certain kinds live in the bodies of in- 

 sects, etc., and are called Gregarinida (Fig. 15). 



a 



FIG. 14. Process of fission in Stentor. b, a new Stentor budding: out; e, ready 

 to separate from the original one; /, the two in a contracted state. 



CLASSES OF PROTOZOA. 



1. Body jelly-like, formless, usually shelled. , . . Rhisopoda. 

 2 Body cylindrical ; parasitic ........ . ....... Gregarinida, 



3. Body ciliated ............................ Infusoria. 



These beginnings of life, simple as they appear, seem to 

 find food, to overcome the perils of life 

 and to survive the attacks of their ene- 

 mies, though they have no outer organs, 

 such as legs, jaws, eyes; or organs of de- 

 fence. Their strength lies in their won- 

 derful plasticity of form, mobility, 

 and minuteness, hence their striking 

 power of adapting themselves to untoward 

 circumstances. They flourish in such 

 hosts that were their growth not checked 

 there would be no room in the waters of 

 the land or the sea for other animals. 

 They serve as food for sponges, worms, Fl <*- 



J fr 



shell-fish, and the smaller shrimps, which 



. 

 Swallow thousands OI them at a Single 



,, . , ,, , . 1V 



mouthiul; and thus, though millions are 



born each hour, millions die each hour, 

 and hence their numbers are kept within 

 due limits. As it is, the Protozoans are wonderfully 



15 :, 



from the alimentary 



canal of a beetle, i, 



younger state, with a 

 beaklike continuance 

 (o') of the head; a. old- 

 er:ri, anterior end;/*, 



bod/f ?-? r nucleus 6 

 (Highly magnifle ' 



