PHYLUM ANNULATA 413 



ereis dumerilii is an extremely variable species. If we 

 compare a number of specimens, we find numerous individual 

 differences 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 varies, together with the number of setae in the two 

 bundles ; 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 pharynx. 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. 318, B). The principal 

 changes which take place during this metamorphosis are a great 

 increase in the size of the eyes, and a great 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 becoming 

 superseded by others which are considerably longer, more nume- 

 rous, 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 setae. 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. 



Development. The egg of Nereis when first discharged is 

 enclosed in a transparent thick gelatinous envelope, within which 

 are two membranes an outer very thin and delicate, and an inner 

 (zona radiata) thicker and very distinctly striated in a radial 

 direction. The protoplasm of the ovum contains a number of 

 oil-drops and yolk-spherules. When fertilisation takes place 

 the yolk-spherules move away from what is destined to become 

 the upper pole of the egg, leaving a polar area composed of 

 granular protoplasm. The zona radiata disappears, and the 

 contents of the ovum undergo for a time amoeboid changes of 

 form. Then the spherical form is reassumed, two small bodies 

 the polar globules are thrown off at the upper pole, and the process 

 of segmentation (Fig. 326) begins. Up to a fairly advanced 

 stage this corresponds very closely with the segmentation of the 

 Polyclad oosperm as described on page 256. The oosperm divides 

 first into two parts, then into four. From these four cells the 

 megameres there are separated off in succession three sets of 

 micromeres, making twelve in all. One of these, belonging to the 



