102 SUPPLEMENT 



setae; some finer and more flexible, to which this name may 

 be preserved, and the others stifFer, more resistent, and con- 

 stantly of a black colour, which M. Savigny has named 

 aciculi. The setse also exhibit differences in their mode of 

 termination ; but it requires magnifiers of considerable power 

 to enable us to perceive this character. 



The skin, or general envelope of these animals, in most in- 

 stances extremely thin, presents a character common to the 

 entire class, of exhibiting the diiferent colours of the prism, 

 according to the inclination of the luminous rays ; in other 

 respects it offers no great differences of thickness and of struc- 

 ture, according to the parts of the body to which it belongs. 

 It is, however, always thinner on the tentacular cirri, whether 

 branchial or not. 



These cirri, which constitute the only supposable organ 

 of touch in these animals, are of different sizes, often very 

 long, and placed throughout the whole extent of the body ; 

 the longest are in general the upper ones of each appendix, 

 and especially those which terminate it in fi'ont and behind, 

 when we have considered them as tentacula. They seem at 

 times to be as it were articulated, although they are never in 

 reality so, the skin being of the same thickness throughout. 

 This should seem to be owing to the manner in which the sub- 

 stance which fills them is divided, and it may be that this 

 sub-articulate disposition exists only in the nereides which 

 have been preserved in alcohol. 



The black points which the authors who have observed the 

 living nereides regard as eyes, are tolerably large, compara- 

 tively to the thickness of the cephalic ring which supports 

 them. Of number and position constant in each little group, 

 they are sessile, and rather independent of the skin. The eye 

 itself forms an elongated spheroid, altogether black on one 

 side, and shining on the other, leaving in the muscular stratum 



