EXTERNAL CHARACTERS OF NEREIS. 



353 



are 



small prostomial tentacles at its anterior end, and near its 



base two pairs of eyes, of which the anterior pair 



farther apart than the other pair. 



At the sides of the prostomium 



project obliquely two conspicuous 



cylindrical structures called the 



palps. These are also preoral, and 



may be regarded as outgrowths of 



the prostomium. At the end of 



each palp is a rounded papilla, 



which is retractile. The palps are 



well supplied with muscular tissue 



and nerves ; they are sense organs, 



perhaps more especially olfactory. 



The first segment of the body 

 behind the mouth and prostomium 

 is called the peristomium. It is 

 longer than the ordinary somites, 

 and bears no parapodia. Attached 

 to its anterior border, however, on each side are four long 

 slender cirri, called the peristomial tentacles. They are 

 arranged in two sets a dorsal and ventral anteriorly, and 

 a dorsal and ventral posteriorly. Each cirrus is slightly 

 enlarged at its base. 



The pharynx, when protruded, consists of two layers an 

 external thinner layer, which bears the chitinous teeth, 

 and an inner thick muscular tube which is continued back 

 into the intestine. The pair of large chitinous jaws are 

 attached to the end of the muscular part, so that when 

 the pharynx is fully protruded they are terminal, and 

 serve for seizing the prey, for Nereis is carnivorous and 

 predatory. The jaws are curved and toothed on their 

 inner edges. The external surface of the outer layer of 

 the pharynx is marked by grooves into distinct areas, in 

 which are situated groups of small chitinous teeth. There 

 is an anterior zone and a posterior zone. Dorsally the 

 anterior zone has three areas, two large teeth in the central 

 area, a group of small teeth in each of the lateral. The 

 posterior zone shows two areas, with a group of rather 

 large teeth in each. Fig. 195 shows the ventral view of the 

 ZOOL. 23 



Fig. 184. EVERTED PHARYNX 

 AND ANTKRIOR SEGMKNTB 

 or NKKKIS FROM YKNTRAL 

 SIDE. 



