396 THE INFUSORIA 



identical. It is clear that there is partial somatic death during 

 conjugation ; it is not clear, however, that there is complete somatic 

 death. It is to the elucidation of this important question that we 

 may look with confidence to future investigations. 



CLASS CILIATA, EHRB. 

 ORDER HOLOTRICHA. 



Examples of a Genus. 



Sub-Order Gymnostomata. Prorodon. 

 Sub-Order Hymenostomata. Paramoecium. 



ORDER HETEROTRICHA. 



Sub-Order Polytricha. Stentor. 



Sub-Order Oligotricha. Ophryoscolex. 



ORDER HYPOTRICHA. Stylonychia. 



ORDER PERITRICHA. Vorticella. 



The Ciliata may be most conveniently divided into four orders 

 the HOLOTRICHA, the HETEROTRICHA, the HYPOTRICHA, and the 

 PERITRICHA. Of these orders the HOLOTRICHA undoubtedly con- 

 tain those genera which are the most primitive in their anatomical 

 characters, the other three orders contain genera which have 

 probably reached their present condition on independent lines of 

 evolution from a common holotrichous ancestor. 



ORDER Holotricha, Stein. 



The order HOLOTRICHA includes those free-swimming Ciliata in 

 which the cilia are all of approximately equal length and thickness. 

 There are never any structures of the nature of cirri. 



The order is divided into two sub-orders (1) the GYMNOSTOMATA, 

 in which the mouth is closed in the intervals between the acts of 

 the ingestion of food; and (2) the HYMENOSTOMATA, in which the 

 mouth is always open and provided with an undulating membrane. 



SUB-ORDER GYMNOSTOMATA, Biitschli. 



The GYMNOSTOMATA are usually of small size, rarely exceeding J mm. 

 in length. Several of the genera (Holophrya, Enchelys, etc.) occur both in 

 the sea and in the fresh-water, others are found only in fresh-water (Pro- 

 rodon, Lacrymaria, Didinium, etc.), others only in sea-water (Stephanopogon, 

 Onychodactylus), and others are parasitic (Butschlia in the stomach of 

 Ruminants). 



In the more primitive forms (Holophrya, Enchelys) the mouth is a 

 simple passage from the medulla to the exterior, situated at the anterior 



