248 



POLYCHAETA 



CHAP. 



The head consists of a preoral portion above the mouth, the 

 " prostomium," and a postoral region surrounding the mouth, the 

 " peristomium " (Fig. 125). The prostomium varies in shape 

 in different species of Nereis; but it always carries on its 

 dorsal surface two pairs of eyes. From its narrower anterior end 

 there arises a pair of short, somewhat conical, sensory processes 

 known as the " prostomial tentacles." A second pair of processes 

 springs from the under surface, and rather to the side of the 

 prostomium ; these are known as the " palps/' and in Nereis are 

 much more conspicuous than the tentacles ; each is composed of 

 two parts, a large basal piece and a smaller terminal joint, capable 

 of being withdrawn into the former. The palps are highly mus- 



Fig. 125. Nereis diversicolor Mii.ll. x 4. Head, with buccal region everted. A, Dorsal 

 view ; B, ventral view, a, Prostomium ; B, everted buccal region ; c, c,' peristomial 

 cirri, 1, 2, 3, 4 ; d, denticles or paragnaths ; e, eyes ; E, lower lip ;' P f palp in A, 

 entrance to pharynx in B ; J, jaw ; T, tentacle ; I, peristomium ; II, foot of 

 apparent second segment. 



cular, and though they are sensory organs, act also as great 

 lateral lips. 



The peristomium is in many species of Nereis (as in N. pda- 

 gica) considerably larger than the trunk segments ; it carries at 

 its anterior edge four filiform cirri on each side, which are directed 

 forwards and used as feelers. They are arranged in couples ; 

 a more anterior couple of dorsal and ventral cirri, and a more pos- 

 terior couple of dorsal and ventral cirri. 



The Tail. As the most anterior segment is perforated by 

 the mouth, and is modified as described above, so the last or anal 

 segment, which carries the anus, differs from the rest. It is more 

 or less elongated, cylindrical, and without parapodia or chaetae. 

 It retains, however, its pair of ventral cirri, which are very 

 long. 



Internal Anatomy. In correspondence with the external 





