328 ZOOLOGY sect. 



system consists of a single dorsal ganglion, with, in a few cases, a 

 smaller ventral or sub-oesophageal ganglion. The sexes are 

 separate, and the males are, in nearly all cases, smaller than the 

 females and degenerate in structure. 



The class is divided into five orders as follows : — 



Order 1. — Rhizota. 



Rotifera which are fixed in the adult state by the truncated end 

 of the non-retractile tail. 



Including Floscularia, Siephanoceros, Melicerta, &c. 



Order 2. — Bdelloida. 



Rotifera which both swim freely by means of the cilia of the 

 disc and creep after the manner of a Leech. The tail is telescopic 

 and forked distally. 



Including Rotifer, Fhilodina, &c. 



Order 3. — Ploima. 



Rotifera in which locomotion is performed by the ciliated disc 

 only. The tail is usually forked and more or less retractile. 



Suh- order a. — Uloricata. 



Ploima in which the trunk is not covered by a lorica. 

 Including Eydatina, Polyarthra Asplanchna, &c. 



Suh-order b. — Lorieata. 



Ploima in which a lorica is present. 

 Including Brachionus, EucManis, &c. 



Order 4. — Scirtopoda. 



Rotifera provided with setose appendages moved by striped 

 muscles : skipping movements are performed by the aid of these, 

 as well as swimming movements by the trochal disc. The tail is 

 either absent or is represented by a pair of ciliated processes. 



Including Pedalion and Hexarthra. 



Order 5. — Trochosphjjrida. 



Globular Rotifera having the trochal disc represented by an equa- 

 torial circlet of cilia ; tail absent. 

 Including Trochosphmra only. 



