ORDER DORSIBRANCHIATA ; NEREIS, ETC. 303 



ORDER I. DORSIBRANCHIATA. 



838. Of the foregoing Orders, the Dorsibranchiata appear, 

 on the whole, to possess the most complete structure, as well as 

 the most varied faculties ; and they also exhibit the forms most 

 characteristic of the class. The head is almost always quite 



distinct from the trunk, and is furnished with one 

 or two pairs of minute eyes, which are seen as black 

 or reddish specks upon its upper side ; and it is also 

 provided with several pairs of appendages (Fig. 525), 

 as well as with complex instruments of mastication. 

 These Marine Worms do not attain any considerable 

 dimensions upon our coasts, rarely exceeding a few 

 inches in length ; but in tropical climates, species 

 FIG. 525. HEAD are to be met with of comparatively gigantic pro- 

 SEGRIENTS^OF portions, having their bodies composed of 400 or 

 NEREIS. 5QQ segments, and occasionally measuring four feet 



from one end to the other. They generally crawl with facility, 

 and swim rapidly ; their chief habitations are among rocks and 

 masses of shells ; but some of them bury themselves in the sand, 

 forming a sort of burrow, lined by mucus secreted from their 

 bodies, which they quit in search of their prey. They are all 

 carnivorous, and live on various small marine animals. 



839. The NEREIDS, commonly known as Sea-Centipedes 

 (Fig. 520), may be regarded as characteristic examples of this 

 group. They are distinguished by having the gills in the form 

 of leaf-like appendages or laminae, which are traversed by a net- 

 work of vessels ; and every segment has, on either side, two 

 tubercles, two cirrhi, and two tufts of bristles. They have none 

 of that venomous power, which their name might be supposed to 

 indicate ; but they are extremely voracious, and thread the 

 most intricate passages and crevices among rocks and stones, 

 in pursuit of their prey. In their turn, they are devoured by 

 Fishes. The Eunice is nearly allied to the Nereidse ; but has 

 gills composed of filamentous tufts (Fig. 521, &), instead of leaf- 



y 2 



