136 



SYSTEMATIC ZOOLOGY 



around the oesophagus or anterior end of the alimentary canal, 

 and meet on the ventral side in a ganglionic enlargement ; 

 from this a nerve cord passes backward to the extreme poste- 

 rior end of the body, running to the ventral blood vessel, and 

 possessing in each segment a ganglionic enlargement from 

 which nerve fibers pass off laterally. Thus the segmentation 

 of a typical annelid is marked externally by rings, internally 

 by a repetition of compartments, nephridia, 

 germ-glands, lateral blood vessels, ganglionic 

 enlargements on the ventral nerve cord, and 

 lateral nerves. And inasmuch as the segments 

 are all essential!}' alike, these animals are said 

 to have homonomous segmentation. 



Like all worms, the Annelida live in moist 

 places, in the earth or in the water, fresh or 

 salt, or as parasites, though these last arc- 

 not numerous. Three readily distinguishable 

 classes may be recognized. 



CLASS I. CTLETOPODA 



Fio. 126. Nereis du- 

 merilii. Natural size 



The Chaetopoda ( ( ir. x a ^ T1 h bristle, and 7rou?, 

 foot), the bristle-footed worms, are typical An- 

 nelida like those described above. They are 

 distinguished from the other classes by the 

 presence of chitinous bristles, called seta-, 

 which are arranged in rows along the side ^<\ 

 the body. The locomotion is accomplished in 

 part bv these bristles, which act as levers, 

 (After Claparede, from moved by special muscles, and in part by the 

 Parker and Hasweiis general muscular system, which consists of two 



Manual.) & J 



sets of contractile fibers, one set, the outer, 

 arranged circularly just beneath the skin, and the other, the 

 inner, longitudinally just beneath the circular layer. Thus the 

 body-wall of the worm is said to consist of a dermomuscular sac. 

 The Chaetopoda are made up of two strikingly different groups 

 of worms ; in one the setae are much more numerous than in the 

 other, and this fact has supplied the names for the two sub- 

 classes, although there are other equally marked differences. 



