III. ANNELIDA: CH^ETOPODA. 



311 



somites, which contain the sexual organs, develop special bristles and para- 

 podia (fig. 284, A). Thus many species of Nereis pass into the * Hetero- 

 nereis ' stage. In other Polychaetes the sexual part (epitoke) separates 

 from the sexless atoke portion and swims freely, while the atoke produces 

 new epitokes. In the Samoaii Islands Eunice viridis reproduces in this 

 way, the epitokes coming to the surface at certain times in incredible 

 numbers, forming the 'palolo worm,' a delicacy in the Samoan diet. In 

 still other species the epitoke regenerates the head and thus becomes an 

 independent generation. Syllis and Heterosyllis are thus related. The 

 AutolytidaB furnish the most complication. Here the atoke, by budding 

 as in Myrianida, fig. 279) forms chains of dimorphic individuals which 

 later separate. The individuals of male chains were formerly described as 

 * Polybostriclms,' 1 the females as ' Sacconereis." 1 This same homonymy ex- 

 plains the regenerative powers of many worms. Thus if certain earth- 

 worms be cut in two, they will continue to live and will reproduce the lost 

 parts. 



Another important character of the Chaetopoda is the posses- 

 sion of bristles or chaetae. These arise in special follicles, singly 

 or several in a bunch, of which usually there are four right and 

 left, dorsal or lateral and ventral in each somite. Each follicle 

 (fig. 280) is a sac of epithelium opening on the surface and having 

 at the base a special cell for the development of each bristle. The 

 developed bristles project from the follicle and, moved by appro- 



FIG. 280. Arrangement of a bristle in an Oligochaete. (After Vejdowski.) e, epithe- 

 lium; rm, iw, circular and longitudinal muscles; w, muscle of the follicle; b,, 

 chaeta follicle, its chaeta in function ; b a , follicle for replacement, the formative 

 cell at its base. 



priate muscles, form small levers of use in locomotion. Their 

 numbers, shape, and support are of much systematic importance. 



Order I. Polychaetse. 



The Polychaetae owe their name to the fact that each group of 

 "bristles contains many chaetae; but more important is that the 



