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GENERAL ZOOLOGY 



strictly homonomous in their metamerism, there are many 

 other chsetopods which are heteronomous. Many of the 

 marine worms have a distinct head which may bear several 

 eyes, tentacles, and other sense organs (Fig. 84, B), thus 

 showing cephalization. Some have the metameres differ- 

 entiated into several groups, so that there are separate body 

 regions with more or less distinctive functions. 



FIQ. 84. Marine annelids on sea bottom. Some burrow through the mud, 

 others make tubes of leathery secretions, sand grains, bits of seaweed or lime, 

 which are buried in the mud or attached to rocks, piles, and seaweeds. A, 

 Hydroides in calcareous tubes on a rock; B, Nereis; C, Polynce; D, Spirorbis on 

 seaweed; E, Cistenides in sand tube; F, Chcetopterus in leathery tube; G, Arenicola. 



Chsetopods are divided mio two great groups. The 

 Oligochceta (few bristled) are mostly confined to land and 

 fresh water, the earthw r orms being common examples. The 

 small aquatic oligochsetes have rather long setae and are 

 often beautifully transparent. Under the microscope all 

 their internal organs may be seen through the body wall. 

 The Polychceia (many bristled) are mostly marine and show 

 great variation in structure and habits. Many species live 



