ON ANNELIDA. 
59 
and the sabellas ; 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 from 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 lamina) or strata extremely thin, placed inside of and 
out-edging one another. From this result stria) 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 inol- 
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 
