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 anteidor third of the body, and 

 going towards the head, the gills and the setas 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. 

 MuUer 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 confasion. 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 



