248 



METAZOAN PHYLA 



This is an abundant form living in burrows in the sand or mud of 

 the seashore. These burrows sometimes reach a depth of two feet 

 and the sand forming their walls is held together by mucus. Although 

 similar in many ways to the earthworm, the sandworm shows many 

 striking differences. The anterior metameres (Fig. 155) are distinct 

 from the rest and are recognized as forming a head, divided into two 

 parts, prostomium and peristomium. The prostomium possesses a pair 

 of feeUng organs or palpi, a pair of tentacles, and two pairs of eyes. 

 The peristomium contains the mouth, with a pair of chitinous jaws, 

 and bears four additional pairs of tentacles. The tentacles are organs 

 of touch, the palpi probably of taste and smell, and the eyes of light 

 perception. Along the sides of the body are fleshy projections, or 

 parapodia, a pair to each metamere except those of the head, each 

 parapodium bearing clusters of several setae. These parapodia also are 



abundantly supplied with blood ves- 



Jay/ 



Perisfornal 

 fenfacles 



sels and serve in respiration. The 

 sexes are separate in Nereis, which 

 is generally the case in all the poly- 

 chaets. 



Certain polychaets show a tend- 

 ency to the division of the body into 

 portions which have been likened 

 to the thorax and abdomen of arthro- 

 pods. Some of the polychaets reach 

 a large size, attaining a length of 

 three feet. These frequently con- 

 struct tubes, various in nature, in 

 which they live. The tube may be 

 limy, cylindrical, and attached to 

 rocks, over the surface of which it 

 follows a very irregular course, gradually growing in length and increas- 

 ing in diameter as the worm grows. In other cases it is made of grains 

 of sand cemented together, and in still other cases it is chitinous. In 

 these tube-dwelling forms the parapodia frequently become much reduced 

 in size. In worms known as sabellids the palpi become greatly developed, 

 complexly branched, frequently feather-hke, and serve as organs of respi- 

 ration (Fig. 156). These so-called gills are often vividly colored and 

 when expanded are objects of great beauty. Polychaet worms exhibit 

 all colors, and many are luminescent. 



The other subclass of the chaetopods is Oligochaeta, most of the 



members of which are either terrestrial or fresh- water forms. They 



,lack parapodia and tentacles, and the setae are single, though they may 



be near together in pairs. This group includes not only the earthworms 



Pafp 



Prostomium 

 with four eyes 



Setae in 

 parapodium 



Fig. 155. — Anterior end of a sandworm 

 {Nereis) with prostomium and peristomium 

 protruded and extended. {From Wieman, 

 "General Zoology," after Ehlers. by the 

 courtesy of McGraw-Hill Book ComiMny, 

 Inc.) 



