202 



THE PROTOZOA 



of cilia, which are here fused to form the characteristic membranelles ; 

 the mouth, as in Heterotrichida, remaining on the ventral side. In the 

 most generalized forms, such as Peritromus or Oxytricha (/?), the cilia 

 are well distributed over the ventral surface, but in most of the other 



Hypotrichida they are 

 reduced, and many 

 are obliterated or 

 fused into character- 

 istic cirri. The cirri 

 in the Oxytrichinae 

 are primitively ar- 

 ranged in six rows, 

 but in the various 

 genera the number 

 becomes reduced, and 

 frequently only iso- 

 lated cirri mark the 

 original position of 

 the row (C). 



The Peritrichida, 

 finally, show the most 

 far-reaching devia- 

 tions from the holo- 

 trichous type, from 

 which they are prob- 

 ably derived through 

 the Heterotrichida 

 and the Hypotrichida. 

 In all members of 

 this group the adoral 

 zone is continued into 

 > E a spiral, which may 



Fig. 114. Illustrating Biitschli's view of the origin of the Vor^ have as manv as five 



ticellidae. [BUTSCHLI.] 



The Trichodina form C is supposed to have arisen from the com P lete tVJTlS (Cam- 



Lichnophora-Mte form A by the outgrowth of the lower ciliated panella). The chief 



area, first forming an intermediate stage B. This ring of cilia j n f e j-pst concerning" 

 becomes lost in the Vorticellidae, appearing only when the indi- . 



viduals are free-swimming. D, E. Side and front of Lichnophora this adoral zone IS 



tfoAwwClap. t h at J n some forms 



the spiral is turned 



to the left, similar to that of all of the other groups of Ciliata, 

 while in other forms, belonging to the great family of the Vorticel- 

 lidas, the spiral is turned to the right. The sinistral type of the 

 Peritrichida originates, according to Butschli, from an hypotrichous 

 form, becoming attached at the posterior half of the ventral surface, 



