60 
SUPPLEMENT 
the extremity of a long anterior elongation of the body, then 
without distinct articulations, without any other appendages 
than sorts of jaws, varying in number, and it is eminently 
retractile. This has been designated under the name of pro- 
boscis. We shall see that the mouth, when it is simple, is 
often accompanied with barbies, or cirruli very numerous, as 
in the serpulse. 
As for the anus, always very large, and transverse, it pre- 
sents nothing very characteristic, except, perhaps, in the 
cirri, which may accompany it, but which belong to the ap- 
pendages. 
The internal organization of these animals has been much 
less studied than the form of the external parts. It is certain 
that it presented difficulties of a very different nature. 
The envelope, which constitutes the external cylinder of 
which the body of the chetopoda is composed, is entirely soft, 
or is never supported either externally or even internally, by 
any solid part of a calcareous or corneous nature. We merely 
observe, that it is furrowed transversely, by striae more or less 
deep, where the larger part of the body of the animal is 
situated ; but it cannot be said that the envelope is there less 
hard, than on the rings themselves. This envelope is com- 
posed, as usual, of a skin, and a contractile, muscular stratum. 
The skin does not, as in other animals, appear to be divisible 
into the usual parts. It seems to be formed only of a sort of 
mucous and transparent epidermis, doubtless decomposing the 
light, by a physical disposition of its parts, or perhaps even 
by its folds, so very fine that they form fissures. Underneath 
is sometimes found a true pigmentum, on which the natural 
coloration of the animal depends, which is very various. It 
has been found impossible to discover any nervous structure 
in this, though its sensibility, as we shall see by and by, is 
very great. 
We are not acquainted with any special organ of sensation 
