158 



ANNELIDA 



[CH. 



having the reproductive organs few in number and definite in 

 position, by developing directly from the egg without the inter- 

 vention of any larval stage, and lastly by the absence of certain 

 structures which are very characteristic of the other great division 

 of the true worms orChaetopoda. 



Order II. Polychaeta. 



The Polychaeta differ from the Oligo- 

 chaeta, as their name implies, by possessing 

 a large number of chaetae on each segment. 

 The sides of each segment are further as a 

 rule drawn out into hollow flaps or lobes called 

 parapodia, which bear the chaetae. Each 

 parapodium maybe divided into a dorsal half, 

 the notopodium, and a ventral half, the 

 neuropodium (15 and 16, Fig. 66). Both 

 notopodium and neuropodium carry bunches of 

 chaetae, and each has as a rule one particu- 

 larly large chaeta, the aciculum, completely 

 concealed in a very deep chaeta-sac, which is 

 moved by muscles attached to its base and 

 serves as a kind of skeleton for the parapodium. 

 There is usually above the notopodium and 

 beneath the neuropodium a process called a 

 cirrus. The dorsal cirrus may be modified 

 into a gill, and both dorsal and ventral cirri 

 are absent in some cases. 



The coelom is often divided into three 

 longitudinal compartments by two muscular 

 partitions (5, Fig. 66) which run from the 

 dorso-lateral line towards the median ventral 

 line near the nerve-cord. The nephridia lie 

 beneath these muscular partitions. It is of 



great * nterest to not i ce taat i n a & w genera 

 they do not terminate internally in a funnel 

 which opens into the coelom but terminate like the nephridia of 

 Platyhelminthes, Nemertinea and Rotifera in a group of flame-cells 

 or solenocytes. The sexes are separated in Polychaeta, and the 

 genital cells of each sex are developed from extensive stretches of 

 the coelomic wall in every segment throughout a considerable region 



FIG. 65. Nereis pe- 

 Oersted L * After 



