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setse ; 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 setee 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 different 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, w r hether 
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 front and behind, 
w r hen w r e 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 wdio 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 
