430 



SECT. 



B 



ill the majority of the (Jhtetopoda, and organs of respiration in 

 the shape of gills or branchia3 are usually developed. The 

 excretory organs are in the form of segmentally arranged pairs of 

 tubes, the nephridia. The nervous system consists of a bilateral 

 principal ganglion or brain situated in the prostomium, and a 

 double chain of ganglia extending throughout the body. The 

 sexes are in some distinct, in others united. When a definite 

 larval form occurs it is a trochophore. 



I. EXAMPLES or THE CLASS. 



a. Nereis dumerilii. 1 



General External Features. Various species of Nereis 

 occur abundantly between tide-marks on the sea-shore, under stones, 



and among sea-weed, in all parts of 

 the world. The worm varies con- 

 siderably in colour even in the same 

 species, the differences being partly 

 due to differences in the stage of 

 development of the sexual elements. 

 In N. dumerilii the prevailing colour 

 is some shade of violet, with a blush 

 of red in the more vascular parts 

 due to the bright red colour of the 

 blood. In shape (Fig. 352) the 

 body, which may be about 7 or 8 

 centimetres in length, is long and 

 narrow, approximately cylindrical, 

 somewhat narrower towards the 

 posterior end. A very distinct head, 

 bearing eyes and tentacles, is recog- 

 nisable at the anterior end ; the rest 

 is divided by a series of ring-like 

 narrow grooves into a correspond- 

 ing series of segments or metameres, 

 which are about eighty in number 

 altogether ; and each of these ben rs 

 laterally a pair of movable mus 

 cular processes called the paro/>ilni, 

 provided with bundles of bristles or 

 setcB (cluiifp). The head (Fig. 355) 

 consists of two parts, the pros- 

 tomium (pnvst) and the peristomium (perist.). The former bears 

 on its dorsal surface four large rounded eyes, in front a pair of 



Though Nereis dumerilii is here named as the example, and the majority 

 of the figures refer specially to that species, the description given would apply 

 almost equally well to a considerable number of species of the genus. 



FIG. 352. Nereis dumerilii, 



natural size. A, Xi-irix pliasr ; 

 H, Heteronereis phase. (A Her 

 Clapar&de.) 



