x PHYLUM ANNULATA 439 



twenty-fifth. But in other species of Nereis they are much more 

 numerous. These, during the season of their active development, 

 give off groups of cells which become disseminated throughout the 

 ccelomic fluid. The original cells (mother-cells) undergo division 

 into smaller cells, each of which develops into a sperm with a 

 minute rod-shaped head and a long vibratile flagellum or tail. In 

 the female the ovaries (Fig. 357, ov), formed by a similar process 

 of proliferation, take the form of rounded masses of cells, meta- 

 merically arranged, surrounding the principal vessels throughout 

 the length of the body. The young ova become detached from 

 the ovaries, and attain their full development while floating about 

 in the ccelomic fluid. Both ovaries and testes dwindle after they 

 have given off the sexual cells, and at the non-breeding season of 

 the year are not to be detected. 



Ova and sperms, when fully ripe, are discharged, reaching the 

 exterior probably through apertures temporarily formed by 

 rupture of the body-wall (cf. above), and impregnation takes place 

 by contact between the two sets of elements while floating freely 

 in the sea-water. 



Nereis dumerilii is an extremely variable species. If we compare 

 a number of specimens, we find numerous individual differ- 

 ences between them. The most striking of these are differences 

 of colour and of the number of segments in the body ; but a careful 

 examination reveals many other points in which individuals differ. 

 Thus the precise form of the lobes of the parapodia, together with 

 the number of setse in the two bundles, varies ; so also do the relative 

 length of the tentacles, the number of teeth on the jaws, and the 

 number and arrangement of the denticles in the buccal cavity. 

 Not only are such individual differences common, but the species 

 occurs in two distinct forms or phases, which differ from one another 

 so widely that they have been referred to distinct genera. One 

 of these is the Nereis phase, which is that described in the preceding 

 paragraphs. A Nereis dumerilii may become sexually mature in 

 this form, or may first undergo a series of changes by which it 

 becomes converted into the second or Heteronereis phase (Fig. 352,5). 

 The principal changes which take place during this metamorphosis 

 are a great increase in the size of the eyes, and a marked modification 

 of the parapodia in the posterior portion of the body, the lobes 

 becoming larger and more leaf-like, and the setae of the Nereis 

 being superseded by others which are considerably longer, more 

 numerous, and somewhat oar-shaped. The Heteronereis, instead 

 of creeping about on the bottom, swims about actively through the 

 water by wriggling movements of the body combined with active 

 paddling movements of the parapodia with their long setse. After 

 a time the Heteronereis, like the Nereis, becomes sexually mature, 

 developing ova and sperms, the latter of which differ remarkably 

 in shape from those of the Nereis phase. 



