OTHER ANNULATA 



311 



Oligochaeta. — The individuals comprising this sub-clasc 

 are found chiefly in moist soil and in fresh 

 water. A very few species inhabit the 

 ocean. They have a bodily organization 

 similar to that of the earthworm, although 

 they are of smaller size and variously modi- 

 fied. One thinks of the common earthworms 

 as land animals, but they are not fully 

 adapted for terrestrial life because of their 

 dependence upon moisture in the atmos- 

 phere. In fact, some species of earthworms 

 live in fresh water. Hence, the oligochsetes 

 as a ,7roup are primarily aquatic animals, 

 which, in a few instances like that of the 

 terrestrial earthworms, have become adapted 

 for life upon land under conditions of suffi- 

 cient moisture. As there are many species 

 of fresh-water oligochsetes, they are familiar 

 objects in laboratory aquaria. 



Polychaeta. — Although there are many 

 species and genera of Oligochaeta, the most 

 diversified and representative group among 

 the Annulata is the Polychaeta. These are 

 almost exclusively marine and are abundant 

 in all the oceans where they burrow in the sea 

 floor in shallower water; or, in some in- 

 stances, swim actively. In order to illustrate 

 the structure and activities of the Polychaeta, 

 for comparison with what has been seen in 

 the Oligochaeta, we may consider a single 

 representative example. 



A Representative Pohjchcete. — The clam- 

 worm. Nereis virens (Fig. 151), is a poly- 

 chaetous annulate, common along the Atlantic 

 Coast of North America from Long Island 

 Sound to Labrador. It lives buried in the 

 sand and fine gravel or under stones between 

 tide marks and just beyond low-water mark, 

 without, however, forming very permanent pair of parapodia that serve 



1 rr J- i. I ] -i- • a- i- 1 ^* locomotor organs (c/ 



burrows. It disturbed, it swims eiiectively Fig 152 B). 



Fig. 151. — Anterior 

 and posterior ends of 

 Nereis oirens, one of the 

 Polychtpta, from dorsal 

 view. 



In contrast to the earth- 

 worm (c/. Figs. 137 A and 

 \h'2. A), Xereis has well- 

 developed sense organs at 

 the anterior end or "head," 

 a pair of tactile cirri on 

 either side of the anus; 

 and on each somite, a 



