466 ZOOLOGY 



Order 1. — Microdrili. 



Small Oligochseta 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 

 setse. 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 Oligochseta 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 seta;'. 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 dwmerilii 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 

 the eyes, the shape of the parapodia, the form of the seta3, and the 

 like. The genus Nereis differs from the other genera of the 

 family Nereido}, to which it belongs, in having the parapodia 

 biramous and the cirri simple, and in the presence of a series of 

 denticles in the buccal cavity in addition to the pair of jaws. The 

 family Nereidaa differs from all the other families of the sub-order 

 Nereidiformia of the Phanerocephala in the union of the following 

 characters : — The body is always elongated and made up of a con- 

 siderable number of segments. The prostomium is well developed, 

 and bears a pair of tentacles, a pair of palpi, and four eyes. The 

 peristomium is devoid of parajoodia, and has four pairs of tentacles. 

 The parapodia are either uniramous or biramous ; both dorsal and 

 ventral cirri are present ; the setas are compound (articulated). 

 There is a pair of anal cirri. In the pharynx there is always a 

 pair of horny jaws, and usually a number of denticles in the 

 buccal cavity. 



The members of the sub-order Nereidiformia are all character- 

 ised by the possession of well-developed tentacles and palpi, and 

 usually peristomial cirri. There are highly developed parapodia 

 with acicula, jointed setae, and dorsal and ventral cirri. The buccal 



