EXTERNAL CHARACTERS 



351 



The head is constituted by the prostomium and the peris- 

 tomium (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 

 bears 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 elongated 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 creeping or acting as a series of 

 oars for propelling it through the 

 water. When one of the parapods 

 (Fig. 88) is examined more atten- 

 tively it is found to be biramous, or 

 to consist of two distinct 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 



FIG. 87. Nereis dumerilii. 

 Natural size. (From 

 Parker and Haswell's 

 Zoology, after Claparede.) 



