50 PROTOZOA 



Key to the subclasses of Infusoria: 



a, Cilia present 1. CiLlATA 



«2 No cilia present in the adult, but long sucking tubes 2. Suctoria 



Subclass 1. CILIATA. 



Protozoa with cilia and usually with a definite mouth and gullet: 

 4 orders with about 1,000 species, of which about 400 are marine. 

 Key to the orders of Ciliata: 



Oi Cilia usually, but not always, present on all parts of the body, 



6i Cilia all approximately of the same length 1. Holotrichida 



fta Mouth surrounded by an adoral zone of large cilia 2. PIeterotrichida 



Oo Cilia present on only a part of the body. 



6i Cilia confined to the ventral side 3. Hypotrichida 



62 Cilia confined to one or more rings around the body 4. Peritrichida 



Order 1. HOLOTRICHIDA. 



Protozoa in which the cilia are usually evenly distributed over the 

 body, with a tendency to arrange themselves in straight lines, which 

 have often a spiral arrangement; in the vicinity of the mouth the cilia 

 are often longer than elsewhere and in a few forms are confined to this 

 region; trichocysts almost always present: 10 families. 



Key to the families of Holotrichida here described : 



tti Animals not parasitic. 

 bi Mouth closed except when taking in food, and without undulating membrane. 

 Ci Mouth terminal or subterminal. 



di Body usually oval or cylindrical 1. Enchelinidae 



da Dorsal side arched ; forward end often neck-like 2. Trachelinidae 



C2 Body ovoid ; mouth in middle or posterior region. . . .3. Chlamydodontidae 

 &2 Mouth always open and ventral with an undulating membrane around it or 

 in the gullet. 

 Cj Oral groove absent or slightly developed. 



di No equatorial zone of cilia 4. Chiliferidae 



di Broad equatorial zone of cilia 5. Urocentridae 



C2 Long oral groove present. 



di No undulating membrane along oral groove 6. Parameciidae 



di An undulating membrane along the oral groove 7. Pleuronemidae 



Oz Animals parasitic ; mouth absent 8. Opalinidae 



Family 1. ENCHELINIDAE. 



Usually oval Infusoria, sometimes with a long slender forward por- 

 tion, with a terminal mouth by which large objects are swallowed, food 

 not being introduced in currents, as the gullet is never ciliated: about 

 18 genera; principally in fresh water. 



