62 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 setae 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 serpulae. 



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 final genera, are in fact capable of being withdrawn almost 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 of 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 ai'rangement 



