[ EXTERNAL CHARACTERS 351 



The head is constituted by the prostomium and the 

 )eristomium (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 

 onger, stout appendages or palps. 

 The peristomium, which has some 

 resemblance to the segments of the 

 Dody, though wanting the parapods, 

 Dears 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 

 3ropelling it through the water. 

 When one of the parapods (Fig. 88) is 

 examined more attentively it is found 

 :o 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 T 



Fic.87. Nereis dumeri- 



[obes, and bears a bundle of setae, in. Natural size. 



T , . r 11- (From Parker and 



lodged in a sac formed by invagmation Hasweirs zoology, 

 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), 



