440 



ZOOLOGY 



SECT. 



the segments. There is a well-developed blood-vascular system 

 in the majority of the Cha3topoda, and organs of respiration in 

 the shape of gills or branchiae 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 trcchophore (cf. p. 322). 



1. EXAMPLES OF 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 consider- 

 ably 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. 347) 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 mctameres, 

 which are about eighty in number 

 altogether ; and each of these bears 

 laterally a pair of movable mus- 

 cular processes called the parapodia, 

 provided with bundles of bristles or 

 setce (chcetcB). The head (Fig. 350) 

 consists of two parts, the pros- 

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



1 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. 347. Nereis dumerilii. natural 

 size. A, Nereis phase ; B, Hetero nereis 

 ph ase. (After Claparede.) 



