404 



ZOOLOGY 



SECT. 



is a well-developed blood -vascular system in the majority of the 

 Chastopoda; and organs of respiration in the form 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 extend- 

 ing throughout the body. The sexes are in some distinct, in others 

 united. When a definite larval form occurs it is a Trochospherc. 



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 ele- 

 ments. In N. dumerilii the pre- 

 vailing 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. Ii 

 shape (Fig. 318) the body, whi< 

 may be about 7 or 8 centimeti 

 in length, is long and narrow 

 approximately cylindrical, som< 

 what narrower towards the pos 

 terior end. A very distinct keac 

 bearing eyes and tentacles, is 

 cognisable at the anterior en( 

 the rest is divided by a series 

 ring-like narrow grooves into 

 corresponding series of segments 

 metameres, which are about eight] 

 in number altogether; and eacl 

 of these bears laterally a pair 

 movable muscular processes call( 

 the parapodia, provided wit! 

 bundles of bristles or setce. The 

 head (Fig. 321) consists of 

 parts, the prostomium (prcest.) and the peristomium (perist.] 



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

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

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



FIG. 318. Nereis dumerilii. Natural 

 size. A, Nereis phase ; B, Heteronereis 

 phase. (After Claparede.) 



