ANNELIDS IN GENERAL 243 



(Fig. 143). It is only indistinctly metameric externally, but internally 

 it shows clear metamerism and a structure somewhat resembling that 

 of the earthworm. The prostomium bears a pair of fleshy tentacles 

 which are both sensory and respiratory. The other members of this 

 class are also small in size, simple in structure, and are marine. One 

 type, Dinophilus, has eyespots, moves by means of cilia, and has other 

 characters reminiscent of the planarians. 



283. Chaetopoda. — The chaetopods are divided into two subclasses, 

 one being Polychaeta, the species of which are mostly marine. The 

 type of this subclass usually studied is Nereis, the sandworm (Fig. 144). 

 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 differ- 

 ences. The anterior metameres (Fig. 145) 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 feeling organs or 

 palpi, a pair of tentacles, and two pairs of eyes. The peristomium con- 

 tains 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 several clusters of 

 setae. These parapodia also are abundantly supplied with blood vessels 

 and serve in respiration. The sexes are separate in Nereis, which is 

 generally the case in all the polychaets. 



Other polychaets show a tendency 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 construct 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, gradu- 

 ally growing in length and increasing 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-like, and serve as organs of respiration (Fig. 146). 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 



