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 serpulae. 



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 fuiTowed transversely, by strige 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 



