144 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



commencement of this chapter, by far the greater part of the bulk of the 

 body — in fact everything except the thin layer of endoderm lining the 



alimentary canal, the thin ecto- 

 derm or epidermis covering the 

 outer surface, and the nervous 

 system. In the worm and in the 

 large variety of animals grouped 

 together as the Coelomata the 

 mesoderm is excavated to form 

 the coelomic body-cavity. This 

 may be defined as "a body- 

 cavity lined by mesoderm, com- 

 municating with the exterior by 

 nephridia and developing the 

 gonad from its lining epithelium." 



The phylum Annelida is sub- 

 divided into three sections (i) 

 Polychaeta, (2) Oligochaeta, (3) 

 Hirudinea. 



I. Polychaeta 



The general features of the 

 Polychaeta are well illustrated by 

 Nereis — the Ragworm (Fig. 68) — 

 of which several species are 

 common round our coasts under 

 stones or amongst sandy mud. 

 The first striking difference in 

 appearance from Lumbricus is 

 afforded by the presence of 

 numerous rude leg-like projec- 

 tions — the parapodia ij}p) — 

 arranged down the sides of the 

 body — a pair to each somite. If 

 a thick transverse section of the 

 worm is made so as to show a 

 parapodium from its anterior 

 that the parapodium is bilobed — 

 and the ventral the neuropodium. 



Fig. 58. 



Nereis, A, View of entire worm from the dorsal 

 side (after M'Intosh) ; B, enlarged view of head 

 region of a different species of Nereis to show 

 details, ch, Chaetae ; dx, dorsal cirrus ; e, eye ; 

 p, palp : pe, peristomium ; pp, parapodium ; ps, 

 prestomium ; t, prestomial tentacles ; v.c, ventral 

 cirri. 



or posterior side (Fig. 69) it is seen 

 the dorsal lobe being the notopodium 



