Ciliophora 337 



linidae, which also include two genera: Cepedea Metcalf (8, 152; Fig. 

 7. 1, J), with cylindrical or slightly flattened bodies; and Opalina Purkinje 

 and Valentin (43, 152, 153, 160; Fig. 7. 1, A-E, H), with much flattened 

 bodies. 



Geographical distribution 



The geographical distribution of the Protociliatia is interesting (152, 

 153). The genus Protoopalina, supposedly the most primitive, is widely 

 distributed and seems to be excluded only from the northeastern United 

 States and from southern India and neighboring islands. The genus 

 Zelleriella, represented in Central America, South America and southern 

 North America, apparently does not extend north of Australia in the 

 eastern hemisphere. Cepedea, although not represented in Australia, is 

 otherwise widely distributed, whereas the genus Opalina apparently has 

 not become established in Australia or South America. Metcalf (153) has 

 attempted to correlate these peculiarities in distribution of the ciliates 

 with the phylogeny of their hosts. 



Subclass 2. Euciliatia 



These are the typical ciliates with macronuclei and micronuclei. 

 Subdivision into orders and suborders is based largely upon the distribu- 

 tion of cilia and their derivatives and upon the differentiation of such 

 structures in the peristomial area. Following the practice of Kahl (100, 

 102, 104, 106), the subclass is now commonly divided into four orders — 

 Holotrichida, Spirotrichida, Peritrichida, and Chonotrichida — but taxo- 

 nomic treatment of the group has varied in different systems of classifica- 

 tion (Chapter III). One of the more recent proposals would divide the 

 Euciliatia into two groups: the "Spirotricha," the Spirotrichida as defined 

 below; and the "Holotricha," including the rest of the Euciliatia and the 

 Suctorea (62). 



The Holotrichida lack the strongly developed adoral zone of mem- 

 branelles so characteristic of the peristome in Spirotrichida. A rather uni- 

 form covering of somatic cilia is typical, although there are some genera 

 in which the cilia are restricted to certain zones or to one surface of the 

 body. 



The Spirotrichida show an extensive development of membranelles 

 and cirri which, in certain groups, have completely replaced simple cilia. 

 An adoral zone of membranelles arises at the left of the cytostome and 

 extends anteriorly, often winding around the anterior end of the body. 

 The group as a whole shows a strong trend toward reduction of the total 

 ciliated area. 



In the Peritrichida the epistome (peristomial area) is commonly a dis- 

 coid region bounded by two or more rows of cilia which, as viewed from 

 the oral end of the body, pass counter-clockwise around the epistome and 



