x PHYLUM ANNULATA 455 



the parapodia are usually equally developed throughout and 

 provided with cirri. The branchiae, when present, are not confined 

 to the anterior end. 



ORDER 3. CRYPTOCEPHALA. 



Polychaeta devoid of protrusible pharynx and of jaws or teeth. 

 The head is frequently very small, and sometimes is devoid of 

 eyes or of tentacles, the prostomium sometimes much reduced and 

 covered over by the peristomium. The body is distinguishable, by 

 differences in the form of the segments, parapodia, and setae, 

 into two or even three regions. The parapodia are little prominent 

 in the posterior parts, and usually without cirri. The branchiae, 

 when present, are usually confined to the anterior end, and are 

 sometimes represented by modified cephalic palpi. 



Sub-Class II.-OLIGOCH^ITA. 



Chaetopoda with the sexes united, the reproductive system com- 

 plicated, the ovaries and testes compact and never more than two 

 pairs of each. No definite paropodia are developed and no cirri, 

 and only a small number of simple setae on each segment. The 

 head is not distinct. A clitellum is usually present. There is no 

 metamorphosis. Mostly terrestrial or fresh-water forms. 



ORDER 1. MICRODRILI. 



Small Oligochaeta with relatively few segments, often multiply- 

 ing asexually. The male genital pores are on, or in front of, the 

 seventh segment. The vasa deferentia are short, opening on the 

 segment immediately behind that in which the internal apertures 

 are situated. The anterior part of the body is often distinguished 

 from the rest by a difference in the form and arrangement of the 

 setae. The clitellum, which is composed of only one layer of cells, 

 is situated comparatively far forward. Eye-spots are frequently 

 present. 



ORDER 2. MEGADRILI. 



Mostly large Oligochaeta with many segments, never multiply 

 ing asexually. The male genital pores are behind the seventh 

 segment. The vasa deferentia are elongated, passing through two 

 or more segments. The anterior part of the body is never special- 

 ised as regards its setae. The clitellum, which consists of two 

 layers of cells, never begins in front of the twelfth segment. 

 Eye-spots are not developed. 



Systematic Position of the Examples. 



Nereis dumerilii is one of many species of Nereis differing from 

 one another in certain minor details of their structure such as 

 the relative length of the palpi and tentacles, the size and form of 



