482 



BIOLOUY OF THE PROTOZOA 



mouth is at the anterior end of the body and such forms are still 

 regarded as the most generalized types of eiliates. In the sub-order 

 Pleurostomina the mouth is no longer terminal but occurs as an 

 elongated slit (Amphileptus, Lionotus) or as a circular opening at 

 the base of a more or less pronounced proboscis (Dileptus, Trache- 

 lius, etc.). In the sub-order Hypostomina the mouth is on the 

 physiologically ventral side as in Nassula, ( nilodon, etc. 



The orders Trichostomida and Hymenostomida include forms in 

 which the mouth is provided with cilia or with membranes, free 

 cilia in Trichostomida and undulating membranes in Hymenosto- 

 mida. There is no great difference between these two orders, and 

 it is frequently difficult to determine whether a particular form 

 belongs to one or the other. Lines of cilia in the gullet, as in Para- 

 mecium, often give the impression of an undulating membrane. 



Fig. 199. — Types of eiliates. A, Cyclidium glaucoma; B, Lembadion bullinum; 

 C, Pleuronema chrysalis. (A, C, after Calkins; B, after Butsehli.) 



In Hymenostomida the mouth, as a rule, is more complex than in 

 Trichostomida. Undulating membranes surrounding it (peristo- 

 mial) are frequently enormously developed (Pleuronemidae, Fig. 199), 

 forming sail-like traps for food bodies. In other cases the mem- 

 branes are inside an oral pit or vestibule and such mouth parts 

 are very complicated (Fig. 8, p. 29). 



In the sub-class Spirotricha we find the most spectacular types of 

 eiliates; some are huge (Bursariidae, Condylostomidae, Stentoridae 

 of the order Heterotricha) ; some are spirally twisted (Metopidae) ; 

 some highly flexible (Lichnophoridae) . 



Cilia, in additon to the adoral zone of membranelles, cover the 

 body in the majority of Heterotrichida — but are greatly reduced or 

 absent in the Oligotrichida and Hypotrichida, where in the latter 



