THE BRISTLE-WORMS. 107 



which may be going. When the hermit is alarmed and 

 retreats, the worm does the same, and then retires to the 

 topmost whorl of the shell, entirely out of sight. So far 

 does it retreat that it is by no means easy to extract it from 

 a shell quitted by the hermit, and a very vigorous shake is 

 required before the attic tenant will show himself. There 

 is usually only one worm present, but I have found two in 

 one shell. The percentage of cases in which the worm 

 occurs also varies greatly according to the locality ; off the 

 Isle of Man it is said to be present in 90 per cent, of the 

 whelk shells inhabited by the hermit-crab, while in other 

 places it is relatively rare. It is not entirely confined to 

 hermit-crab shells, but occurs occasionally free, and occa- 

 sionally in empty shells. 



The living animal is very easily recognised by its colour 

 and markings. It is of a beautiful red tint, with two pure 

 white bands on the dorsal surface. After death, however, 

 the colouring soon fades, whatever the preservative em- 

 ployed. In structure the worm differs from the two 

 preceding in that the parapodia are not all similar, the 

 posterior differing from the anterior. In the posterior 

 segments especially the uppermost lobe of the parapodia is 

 elongated, arched, and swollen. It is highly vascular, and 

 no doubt functions as an efficient respiratory organ. 



We shall describe only one other species of Nereis, and 

 that is the large and handsome Nereis virens. It is a green 

 worm, differing from the other species described in the 

 presence of large leafy plates in the dorsal region of the 

 parapodia. The plates are not expansions of the dorsal 

 cirrus, like the plates of Phyllodocids or the scales of 

 Polynoids, but are expansions of the dorsal lobe itself (cf. 

 N. fucata). The structure of the parapodium altogether 

 suggests to some extent the modified parapodia of the 

 "Heteronereis" of other species, and it is interesting to note 

 that though N". virens does become converted into a Hetero- 

 nereis, the changes are relatively slight. The worm reaches 

 a length of over a foot (up to eighteen inches), and when 

 the large black jaws are fully protruded has quite a formid- 

 able appearance. It is said to be called the " Creeper," and 

 to be used as bait on some parts of the coast. The leafy 

 plates, like those of Phyllodocids, secrete an abundant 



