III. ANNELIDA: CH^TOPODA 



275 



In many polychretes which reproduce exclusively by the sexual process the sex- 

 less, slowly-moving young {atoke) becomes so altered at sexual maturity as to 

 have been described under another name. It becomes very active in its move- 

 ments, and the hinder somites, which contain the gonads, develop special bristles 

 and parapodia (fig. 263, A). Thus many species of Nereis pass into the 'Hclcr- 

 onereis' stage. In other Polychsetes the sexual part (e/ijto^e) separates from the 

 sexless atoke portion and swims freely, while the atoke produces new epitokes. 

 In Samoa Eunice viridis reproduces in this way, the epitokes coming to the sur- 

 face 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 Autolytidas are most complicated. Here the atoke, by 



Fig. 25S. Fig. 259. 



Fig. 258. — Budding in Myrianida (after Milne-Edwards). The sequence of letters 

 shows the ages of the individuals. 



Fig. 259. — Arrangement of a bristle in an Oligocha^te (after Vejdowski). e, 

 epithelium; rm, Im, circular and longitudinal muscles; m, muscle of the follicle; J,, 

 cha:-ta follicle, its chjeta in function; 6,, follicle for replacement, the formative cell at its 

 base. 



budding as in Myrianida (fig 258), forms chains of dimorphic individuals which 

 later separate. The individuals of male chains (fig. 263) were formerly de- 

 scribed as 'Polybostrich'US,' the females as 'Sacconereis.' This same homonomy 

 explains the regenerative powers of many worms. Thus if certain earthworms 

 be cut in two, they will live and reproduce the lost parts. 



Another important character of the Chxtopoda is the possession of 

 bristles or chcclcE. These arise in special follicles, singly or several in a 

 group, there usually being four groups — right and left, dorsal or lateral and 

 ventral — in each somite. Each follicle (fig. 259) is a sac of epithelium open- 

 ing to the surface and having at the base a special cell for the development of 

 each bristle. The developed chfetae project from the follicle and, moved by 

 appropriate muscles, form small levers of use in locomotion. Their num- 

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



