236 COLLEGE ZOOLOGY 



The two principal groups of the POLYCH^TA are the PHANE- 

 ROCEPHALA and CRYPTO CEPHALA. 



Order i. Phanerocephala. POLYCH^ETA 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: Amphi- 

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



Subclass 2. Oligoch&ta 



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 

 OLIGOCH^TA are either terrestrial 

 or live in fresh water. Two orders 

 are recognized: (i) the MICRODRILI, 

 and (2) the MACRODRILI. 



Order i. Microdrili (Limicola). 



- These are mostly small fresh- 

 water animals. Examples: Tubifex, 

 Dero, Nais (Fig. 168). Many of 

 them reproduce by transverse fission 



FIG. i6S.-Nais a, mouth; ^ weU ^ sexually 

 o, anus; c, intestine. (rrom 



Davenport, after Leunis.) Order 2. Macrodrili. (Terricola). 



- This order contains the terrestrial 



OLIGOCH^ETA. Examples: Lumbricus (Fig. 154), Allolobophora, 

 Diplocardia. 



5. CLASS III. HIRUDINEA 



The animals included in this class are commonly called leeches 

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



