ON ANNELIDA. 59 



and the sabellge ; but in the latter, surrounding this mucosity, 

 is attached externally a stratum, more or less thick, composed 

 merely of mud or very fine grains of sand, or, in fine, of debris, 

 more or less thick, of shells and larger grains of sand. These 

 tubes are constantly open at both extremities ; there are also 

 some of them more regular, which are completely calcareous. 

 The double opening is a character whereby they are distin- 

 guished fi-om tubular shells, the summit of which, on the con- 

 trary, is constantly imperforate. These last-mentioned tubes, 

 however, appear constantly to grow after the manner of those 

 shells, by laminae or strata extremely thin, placed inside of and 

 out-edging one another. From this result strise marking the 

 growth, more or less apparent outside, but we never remark 

 longitudinal striae on their surface, nor any thing indicating 

 the delicate working of the edges of a mouth, as in the mol- 

 lusca. This character alone might suffice to distinguish them 

 from the true tubular shells ; but to this we may add, that the 

 constant perforation of the summit of the tube of the chetopoda 

 never allows the animal, in growing and advancing in its tube, 

 to form partitions there, whereas in the tubular shells the re- 

 verse is invariably the case. A final character which dis- 

 tinguishes the tubes of the chetopoda is, that they are adherent, 

 and fixed flatly through a greater or less portion of their 

 extent, on foreign bodies, which never takes place with the 

 tubular shells. 



The two last external parts which we have to examine in 

 the Chetopoda, and which naturally conduct to the consider- 

 ation of their internal organization, are the orifices of the in- 

 testinal canal, which constantly exist, and which are pretty 

 nearly always terminal, but sometimes, however, a little 

 oblique, under each extremity, especially the mouth. 



This mouth is sometimes immediately at the origin of the 

 body, or at the anterior extremity of the head, as in the lumbrici, 

 and a great number of nereides ; but also, in many cases, it is at 



