404 



ZOOLOGY 



SECT. 



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

 Chsetopoda ; 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 nepliridia. 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 Trochosphere. 



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. In 

 shape (Fig. 318) the body, which 

 may be about 7 or 8 centimetres 

 in length, is long and narrow, 

 approximately cylindrical, some- 

 what narrower towards the pos- 

 terior end. A very distinct head, 

 bearing eyes and tentacles, is re- 

 cognisable at the anterior end ; 

 the rest is divided by a series of 

 ring-like narrow grooves into a 

 corresponding series of segments or 

 metamcres, which are about eighty 

 in number altogether; and each 

 of these bears laterally a pair of 

 movable muscular processes called 

 the parapodia, provided Avith 

 bundles of bristles or setcc. The 

 head (Fig. 321) consists of two 

 parts, the prostomium (prccst.} and the pcristomium (perist.}. 



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. 318. Nereis dumerilii. Natural 

 size. A, -A"i. /<:/'.< phase ; B, Net u so nereis 

 phase. (After C'laparede.) 



