PHYLUM COELHELMINTHES {ANNELIDA) 



149 



ported by numerous bristles or setae which are arranged in two 

 bundles. The lobe of the parapodium surrounding the ventral 

 bundle of setae is called the neuropodium, while that surrounding 

 the dorsal bundle is the notopodium. In Nereis, the notopodia 

 are supplied with numerous blood vessels and function as gills. 

 Among other polychaetes, gills are frequently developed as long 

 filamentous outgrowths, either along the sides of the body or 

 restricted to certain areas. Each lobe of the parapodium 

 frequently bears a fleshy sensory pro- 

 jection termed the cirrus. In some 

 instances, dorsal scales covering the 

 back of the worm represent modified 

 dorsal cirri. 



Dimorphism. — Among the poly- 

 chaetes, two different types of indivi- 

 duals are frequently encountered in the 

 same species, the atoke or sexless and 

 the epitoke or sexual individual. Before 

 their relationships were understood, 

 these phases were frequently considered 

 as distinct species and even as different 

 genera. In some instances, the atoke 

 (Fig. 75) has the power of budding to 

 produce sexual epitokes which become 

 separated as free individuals. This is 

 the condition found in the palola worm 

 {Eunice viridis) the epitoke of which 

 appears in extreme numbers in the tropical South Seas and is 

 relished by the Samoans as a food. The atoke remains in the 

 corals at the bottom of the sea where it proceeds to regenerate a 

 new posterior end. 



In some of the common species of marine annelids there are 

 striking differences in appearance of the immature worms and 

 those filled with mature germ cells. These differences are so 

 conspicuous that before they were understood, mature and 

 immature worms of the same species were ascribed to different 

 genera. In some species of the common clam worm (Nereis) the 

 peculiarly modified mature specimens were given the generic 

 name Heteronereis. When the young worms approach maturity, 

 the segments in the posterior half of the body become storehouses 

 for the germ cells, taking on an entirely different appearance 



Fig. 75. — A Palola worm, 



Eunice viridis (Gray), show- 

 ing differentiation of body 

 into an enlarged atoke and 

 a posterior epitoke. (After 

 W oodworth) . 



