ON ANNELIDA. 101 



their presence or absence, their number and disposition, ap- 

 pear to furnish tolerably good zoological characters. 



We may term cervical appendages, the first, in greater or 

 less number, which do not present the complete composition 

 of those of the rest of the body, and especially are destitute of 

 gills. 



We may reserve, on the contrary, the denomination of 

 thoracic appendages, for those which are perfectly complete, 

 at least in relation to the particular species which may happen 

 to be under consideration ; for it may occur, that in a group 

 of species, no appendage is absolutely complete, that is to 

 say, formed of its two parts, divided by a lateral line, and both 

 composed of a fasciculus of setae and of aciculi, of tentacular 

 filaments, and particularly of a primate or branchial ten- 

 taculum. 



Finally, the name of abdominal appendages suits those of 

 the rings, which have lost something of their complication, 

 from the most perfect to the pair which precedes the anus, 

 and which may be termed prseanal. 



In the different parts of the appendages, some differences 

 may be found, on which it is proper to remark. Those of the 

 absolute, or proportional length of the tentacular filaments, or 

 of the nipples which support them, are of but little importance. 

 Such, however, is not the case respecting the simplicity, or 

 the complication of form of the filaments, or superior tentacu- 

 lar cirrus. In the first case, there is no gill, properly so called, 

 but in the second there is, and then the number of digita- 

 tions and their forms become characteristic of the species, and 

 often of the rings. 



The fasciculus of stiff, corneo-calcareous setae, which com- 

 pletes the appendages, is sometimes like the latter, divided 

 into two fasciculi, more or less distinct, by a lateral line. 

 But, moreover, they are themselves formed of two sorts of 



