ANNELIDS IN GENERAL 



247 



a series of nervous units. These may act individually but are so related 

 in a central nervous system as to be capable of acting in concert. This 

 makes possible both unity and variety of action. Differ- 

 ent metameres may carry on differing activities, while in 

 case of necessity all may be brought into play in an action 

 involving the body as a whole. Still another advance is 

 seen in the greater degree of specialization exhibited by 

 the digestive system, which is here divided into a larger 

 number of regions than heretofore, each region having 

 a special function to perform. 



283. Classification.— The phylum Annehda (a nel' i 

 da; L., anellus, a little ring, and G., eidos, form) is 

 divided into four classes: 



1. Archiannelida (ar kl a nel' i da; G., archi-, first, + 

 annelida). — Primitive annelids which possess neither 

 setae nor parapodia. 



2. Chaetopoda (ke t5p' o da; G., chaite, horse's mane, 

 and podos, foot). — Annelids with setae, and in one of 

 the two subclasses with parapodia, which are fleshy 

 lateral outgrowths of the body wall. 



3. Hirudinea (hi roo din' e a; L., hirudo, leech). — 

 AnneUds possessing neither setae nor parapodia but 

 having suckers, which are an adaptation to parasitic 

 life. 



4. Gephyrea (je fi re' a; G., gephyra, bridge). — A 

 small and heterogeneous group, apparently more 

 appropriately placed in Annelida than in any other 

 phylum. 



284. Archiannelida. — A type of this class is Poly- 

 gordius, a small worm about an inch and a half long, 

 Hving in the sand of the seashore (Fig. 153). It is only 

 indistinctly metameric externally, but internally it 

 shows clear metamerism and a structure somewhat 

 resembling that of the earthworm. The prostomium 

 bears a pair of fleshy tentacles which are both sensory 

 and respiratory. The other members of this class are 

 also small in size, simple in structure, and are marine. 

 One type, Dinophilus, has eyespots, moves by means of 

 cilia, and has other characters reminiscent of the 

 planarians. 



285. Chaetopoda. — The chaetopods are divided into two subclasses, 

 one being Polychaeta, the species of which are mostly marine. The 

 type of this subclass usually studied is Nereis, the sandworm (Fig. 154). 



Fig. 154.— a 

 sandworm, Nereis 

 virens Sars. From 

 a specimen. The 

 parapodia bear foli- 

 aceous lobes which 

 are conspicuous in 

 the figure. X %. 



