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SUPPLEMENT 
muscular bands, separated only by the dorsal vessel, are con- 
tinued without interruption from one extremity of the body 
to the other, not, however, attaching themselves successively 
to the contraction of each ring, or to each transverse furrow. 
When arrived in front, they contract, and terminate at each 
tentaculum of the head. 
The appendages, in their active, or contractile parts, are 
really composed like the rest of the skin, with the difference 
that the muscular stratum is there of necessity much less 
thick ; but it has not appeared that those parts are provided 
with special muscles. 
The passive parts of the appendage, or the seta3 of what 
kind soever they may be, are always rigid and fragile. Their 
chemical composition would appear to be a mixture of cal- 
careous and corneous matter. Each seta is hollow through 
its entire extent, at least if we are to judge of all, from the 
demonstration which has been made in some species. Ordi- 
narily pointed, and harder on the summit, they are on the 
contrary truncated and soft at the base. We have already 
seen, that, according to their uses, they are straight, aciculated, 
or curved into hooks, at their extremity, in which last case, 
they are always much shorter. Sometimes they are even 
denticulated, as in the serpuhe. 
These parts, usually retractile, and intractile, except in the 
final genera, are in fact capable of being withdrawn almost 
completely through proportional holes pierced in the skin. 
They do not appear, however, either particularly, or in fasci- 
culi, to be provided with muscles which should produce 
movement. But their extremity, after having traversed the 
skin, pushes as it were within the longitudinal and lateral 
muscular fasciculus, which produces sorts of shrouds as to 
the masts ol a vessel. By the contraction of the fibres, the 
seta is pushed outwards, more or less strongly, without which 
it re-enters into a state of repose. This is an arrangement 
