SPECIAL MORPHOLOGY AND TAXONOMY OF INFUSORIA 389 



Order IV. HYPOTRICHIDA, Stein. 



The ciliates included in this Order belong to the most highly 

 differentiated forms of the Protozoa. Except for some species 

 belonging to the more generalized iyp^s (e. g., Uroleptus mobilis) 

 they are flattened dorso-ventrally and bear motile organs only on 

 the ventral surface. Tactile organs may be present on the dorsal 

 side but these are not used for locomotion. They may be easily 

 distinguished from the laterally flattened Holotrichida (Chlamydo- 

 dontidse) by the presence of a usually powerful adoral zone of 





Fig. 172. — .4, Stephanopogo/i colpoda; B, Peritromus emnice; C, Omjchodromus grandis. 

 c, Cirri. (From Calkins after Biitschli.) 



membranelles which is spirally wound to the left. The frontal 

 field is usually triangular and bears in the more complex types, 

 one or more undulating membranes. The genera and species offer 

 an excellent opportunity for the study of comparative anatomy 

 through homology. Thus the most generalized forms, represented 

 by types such as Peritromvs have no other motile organs than the 

 close-set rows of ventral cilia of uniform size, and the adoral zone 

 of membranelles (Family Peritromidae) (Fig. 172). In other types 

 localized areas of cilia are represented by cirri, the origin of which 

 is generally attributed to the fusion of adjacent cilia. Such areas 

 give rise to regional groups of cirri known as frontals, ventrals, 



