236 



COLLEGE ZOOLOGY 



The two principal groups of the Polycteeta are the Phane- 

 rocephala and Crypto cephal a. 



Order 1. Phanerocephala. — Polycteeta with most of the 

 segments similar, a distinct head (prostomium) and a protrusible 

 pharynx usually provided with chitinous jaws. Examples: 

 Nereis (Fig. 163), Aphrodite, Autolytus (Fig. 166). 



Order 2. Cryptocephala. — Polychaeta with head (prosto- 

 mium) usually small and indistinct; segments differentiated, 

 forming two or more regions, the thorax and abdomen, and 

 palpi often divided into a crown of gills. Examples: Am phi- 

 trite (Fig. 167), Spirorbis, Terebella, Sabella. 



Subclass 2. Oligochceta 



The earthworm illustrates the chief characteristics of this 

 subclass. There are usually only a few setae, and no parapodia 



nor tentacles. The sexes are united, 

 i.e. hermaphroditic. Most of the 

 Oligocileta are either terrestrial 

 or live in fresh water. Two orders 

 are recognized: (1) the Microdrili, 

 and (2) the Macrodrili. 



Order 1. Microdrili (Limicola). 



— These are mostly small fresh- 

 water animals. Examples: Tubifex, 

 Dero, Nats (Fig. 168). Many of 

 them reproduce by transverse fission 



mouth; &g wg ^ ag sexua U y 



Order 2. Macrodrili. (Terricola). 



— This order contains the terrestrial 

 Examples: Lumbricus (Fig. 154), Allolobophora, 



Fig. 168. — Nais. a 

 b, anus ; c, intestine. (From 

 Davenport, after Leunis.) 



Oligocieeta. 

 Diplocardia. 



5. Class III. Hirudinea 



The animals included in this class are commonly called leeches 

 (Fig. 169). They are usually flattened dorso-ventrally, but 



