ORDER DORSIBRANCHIATA ; NEREIS, ETC. 299 



ORDER I. DORSIBRANCHIATA. 



909. 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. 626), 

 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 

 no. 626. HEAD are to k e met w j t ^ O f comparatively gigantic pro- 



AND ANTERIOR ^ * 



SEGMENTS OF portions, having their bodies composed of 400 or 

 500 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 with muqps secreted from their 

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

 carnivorous, and live on various small marine animals. 



910. The NEREIDS, commonly known as Sea- Centipedes 

 (Fig. 621), 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 tu- 

 bercles, 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 

 EUNICID.E are nearly allied to the Nereidae; but have gills 

 composed of filamentous tufts (Fig. 622, b\ instead of leaf-like 

 plates. The Eunice gif/antea of West Indian seas sometimes 

 measures four feet in length ; and species of a much larger size are 

 said to occur in the Southern seas. There are smaller species 



