ON ANNELIDA. 
57 
We have already remarked, that even in the chetopoda, 
whose rings are most similar, there are, however, some differ- 
ences ; but in the genera, those differences are much greater. 
In general, on proceeding from the most complete segment, 
which is usually towards the anterior third of the body, and 
going towards the head, the gills and the setae diminish 
gradually in length and strength, becoming, as we have said 
above, more and more dorsal, while, on the contrary, the cirri 
acquire a greater development. This may be observed in a 
very manifest manner in the cirri of the cephalic rings, and 
even in those which surround the anus in the nereides. These 
latter preserve the name of cirri ; but such is not the case with 
those which accompany the rings which compose the head. 
Muller and Otho Fabricius have called them tentacula. M. 
Savigny designates them by the name of antennae, a denomina- 
tion which seems altogether improper, and calculated to pro- 
duce confusion. The name of tentacula is preferable, though 
after all there is nothing in those organs which can cause 
them to be compared to the tentacula of the cephalous mol- 
lusca, nor to the antennae of the hexapods, which rather ap- 
pear to be organs of olfaction. Let this be, however, as it 
may, the tentacula of the head in the chetopoda are usually 
perfectly in even numbers ; but it sometimes happens that 
there is one odd one, and that medial. It is, however, only in 
the aphroditae that this singular disposition takes place. In 
our descriptions, we regard as cephalic, or belonging to the 
head, not only the cirri which are found upon the first ring, 
but also those which spring from some of the following, and 
•which in general are very well distinguished from the cirri of 
the appendages by a greater degree of length. 
It may be sometimes remarked, that those organs appear to 
be divided into segments by transverse folds, which has caused 
them to be termed articulated by some writers. But it rather 
appears that this effect, which is real, often takes place from 
