VI 



EXTERNAL CHARACTERS 



351 



The head is constituted by the prostomium and the 

 peristomium (p. 327). The former bears on its dorsal 

 surface four large, rounded eyes, in front a pair of short 

 cylindrical tentacles, and further back a pair of somewhat 

 longer, stout appendages or palps. 

 The peristomium, which has some 

 resemblance to the segments of the 

 body, though wanting the parapods, 

 bears laterally four pairs of long, 

 slender, cylindrical tentacles ; on its 

 ventral aspect is a transversely elon- 

 gated aperture, the aperture of the 

 mouth. The segments of the body 

 differ little in external characters from 

 one another throughout the length of 

 the worm and there is no clitellum ; 

 each bears laterally a pair of parapods, 

 which in the living animal are usually 

 in active movement, aiding in creep- 

 ing or acting as a series of oars for 

 propelling it through the water. 

 When one of the parapods (Fig. 88) is 

 examined more attentively it is found 

 to be biramous, consisting of two dis- 

 tinct divisions a dorsal, which is 

 termed the notopod (noto), and a ventral, 

 termed the neuropod (neuro}. Each of 

 these is further subdivided into several 

 lobes, and bears a bundle of setae, 

 lodged in a sac formed by invagination 

 of the epiderm and capable of being 

 protruded or retracted and turned in various directions 

 by muscular fibres in the interior of the parapod. 

 In each bundle there is, in addition to the ordinary 

 setae, a stouter, straight, dark-coloured seta (ac), 



Fio.87. Nereis dumeri- 

 lii. Natural size. 

 (From Parker and 

 Haswell'S Zoology, 

 after Claparede.) 



