172 



THE PROTOZOA 



classes of Protozoa, yet in no other class are they all present in a 

 single cell. Each of the different elements thus brought together has 

 a definite function to play in the life of the organism, and intra-cellular 

 division of labor is developed to a high degree. 



Leading an active life and forced to seek food in all sorts of places, 

 from the clearest waters to the internal fluids of various hosts, the 

 Ciliata have acquired a very great diversity of form. The simplest 

 and probably the most primitive forms are monaxonic, the mouth 

 being anterior and the anus posterior (Fig. 91, A). Symmetry, how- 

 ever, is the exception and asymmetry the rule, the latter condition 

 arising by the gradual shifting of the mouth to a more or less well- 



C 



D 



Fig. 91. — Types ol Ciliata. [BuTSCHLI.] 

 A. Prorodon teres Ehr. ; an holotrichons form. B. Climacostomum virens Ehr. ; an hetero- 

 Irichous form. C. Pleurotruka grandis St.; an hypotrichous form. D. Vorticella umbellaria ; a 

 peritrichOLis form, z, adoral zone. 



defined ventral side, while the anus becomes more or less dorsal. 

 The functional anterior end may thus be either ventral, or superior to 

 the mouth, when the latter becomes sub-terminal. The simple 

 monaxonic ground-type is subject to other minor variations among 

 the Holotrichida, which point the way toward the more striking 

 deviations among the Heterotrichida and the Hypotrichida. A fre- 

 quent modification is the anterior prolongation of what might be con- 

 sidered the upper lip, as in Dileptus or Lionotus (Fig. 92), where 

 bilaterality and asymmetry are well established. The mouth becomes 

 more and more ventral in the family Trachelinidae (Holotrichida), 

 while in the order Hypotrichida it is always ventral and the original 

 monaxonic structure is replaced by dorso-ventral differentiation and 

 complete asymmetry. 



