ANNELIDA. 239 



and most primitive of the Annelida^ as is shown by the 

 ectodermal nervous system, the persistence of radial septa 

 and longitudinal mesenteries, the simple nephridia and the 

 absence of appendages. It contains two or three other small 

 worms. 



Class II. — PoLvcHiEXA. This class has a great number 

 and variety of types. Many live in tubes and burrows and 

 the anterior end bears a mass of tentacles and gills, whilst 

 the free-swimming forms often have a great development 

 of lateral appendages which are in many cases used for 

 swimming. They are called Polychceta because they usually 

 have great numbers of setae. 



Fig. 157. — Foot or Parapodium of a Nereis. {Ad nat.) 



Dorsal Cirrus. 

 Notopodium. ^ 



Setje. 



Ventral Cirrus. Neuropodium. 



Class III. — OLiGOCHiEXA. As in Lumbruus^ the body 

 is usually without appendages or gills and has only com- 

 paratively few setae. They are usually divided into the 

 mud dwelling (freshwater) forms and the terrestrial. Their 

 hermaphrodite and complex sexual organs and protected 

 embryonic development are characteristics. 



Class IV. — Hirudinea. In many respects this class 

 resembles the last, especially in the absence of appendages, 

 the hermaphrodite sexual organs and the development. 

 It is, however, clearly characterised by the reduced con- 

 dition of the coelom and its continuity with the blood- 

 vascular system, by the suckers and the mode of life. 



The most important features of the sub-phylum and the 

 classes are summarised in the subjoined table : — 



