THE BRISTLE-WORMS. 



113 



they can be freely exposed to the purifying action of the 

 water. They further differ from those of Cirratulus in 

 being branched and comparatively short. Some other 

 adaptations to life within a tube are almost equally obvious. 

 Thus the parapodia are greatly reduced, and the bristles 

 modified so as to suit the needs of a tube -inhabiting 

 worm. 



On the anterior segments, from four to twenty-one, there 

 are in all seventeen 

 pairs of papillae 

 bearing fan -shaped 

 tufts of bristles. 

 The papillae repre- 

 sent the dorsal lobes 

 of the parapodia, 

 and are absent from 

 the narrow posterior 

 region of the body; 

 they no doubt assist 

 the animal in mov- 

 ing up and down 

 its tube. The ven- 

 tral lobes of the 

 parapodia are repre- 

 sented by elongated 

 vascular bands at 

 the sides of the 

 segments, each of 

 which bears from 

 eighty to one hun- 

 dred mirmtp Viookq FlG - $I.Terebella removed from its tube. Note 



area minute n >OKS. t ^ e long tentacleS) the tufted gills> and the 



Ihese nooks are difference between the anterior and posterior 



modified bristles regions of the body - 



and are present in various forms in the majority of tube- 

 inhabiting worms. Very little observation on the rocks 

 will acquaint you with the fact that in most cases a 

 tube-inhabiting worm can withdraw into its tube on an 

 alarm with extraordinary rapidity. In a worm like Terebella 

 the process is assisted by many thousands of hooks, each 

 bearing secondary teeth. The hooks are very small, and 

 can just be made out in a good light with a strong lens. 



