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 set® 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 praeanal. 
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 set®, 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 
