450 ZOOLOGY sect. 



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. 346, B). 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 superseded by others which are considerably longer, more nume- 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 phas^/ 



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 irom 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 bodies (p. 19) — are thrown off at the upper pole, and the 

 process of segmentation begins (Fig. 355). Up to a fairly advanced 

 stage this corresponds very closely with the segmentation of the 

 Polyclad oosperm as described on page 273. 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 

 second set, somewhat larger than the others and differing from 

 them in its subsequent history, is termed the first somatoblast 

 (som. 1) ; a second somatoblast (som. 2) is soon given off from the 

 same megam ere that gave origin to the first. 



The germinal layers are now all established. The micromeres 

 constitute the ectoderm, destined to give rise to the epidermis and 

 all its derivatives, to the cerebral ganglion and nerve-cord, to the 

 oesophagus and rectum. The megameres eventually give origin 

 to the cells of the endoderm, forming the internal epithelium of 

 the alimentary canal. The second somatoblast gives rise to 



