242 



METAZOAN PHYLA 



/ 



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

 makes possible both unity and variety of action. Different 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. 



281. 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 ki a nel' i da; G., archi-, first, + 

 annelida). — Primitive annelids which possess neither 

 setae nor parapodia. 



2. Chaetopoda (ke top' 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). — 

 Annelids 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 



The parapodia bear 



Fig. 144. Fig. 145. 



Fig. 144. — A sandworm, Nereis virens Sars. From a specimen, 

 foliaceous lobes which are conspicuous in the figure. X ^3. 



Fig. 145. — Anterior end of a sandworm (Nereis) with prostomium and peristomium 

 protruded and extended. (From Wieman, "General Zoology," after Ehlers, by the courtesy 

 of McGraw-Hill Book Company, Inc.) 



appropriately placed in Annelida than in any other phylum. 



282. Archiannelida. — A type of this class is Polygordius, a small 

 worm about an inch and a half long, living in the sand of the seashore 



