ARTICULATA. 219 



Order 10. Chcetopoda, or Bristle-footed worms. Some 

 of these occur under the stones of the sea-shore, as the 

 lug-bait of fishermen. (Fig. 121.) Others se- 

 crete a glutinous material from the surface, 

 which cements sand and other foreign bodies 

 into a tube. Others secrete calcareous matter, 

 which forms a tubular residence, as the com- 

 mon Serpula, whose white, snake-like concre- 

 tions abound on the stones and shells of the 

 shore, and the Spirorbis, whose minute whorled 

 shells dot the surface of many sea-weeds. 

 Some of the Nereids, or Sea-centipedes, attain 

 to a considerable size, one species being four 

 feet long. The Sea-mouse (Aphrodite] also be- 

 longs to this order. The latter is clad with 

 iridescent scales and bristles, or barbed spines. 

 Those who bear the gills along the back have 

 been called Dorsibranchiates. These gills are 

 found close to the root of the dorsal oar, or 

 bristle, and the blood is purified by being ex- 



FlG. 121. 



posed to the oxygen held in solution in the Lob-worm, 



r~, i 1 1- . (Arenicola 



sea-water. Those worms which live m tubes 

 (Tubicola) have the gills developed only on 

 the foremost segments of the body, and the 

 dorsal and ventral oars of the other joints are or the ex j e 1 r - 



J nal gills. The 



rudimentary, but they have branching tentacle- large head is 



without eyes 



like processes about the head. In Serpula one or jaws. 

 of the tentacles is formed into a lid, or operculum, with 

 which the open mouth of the tube can be closed at will. 

 (Fig. 122.) 



The common Earth-worm (Lum&rtcus) has few and 



