14 M. A. de Quatrefages on the Classification of the Annelides. 
Nowhere, I think, can we point out so complete a manifestation 
of the law of economy upon which M. Milne-Edwards has very 
justly insisted in his ‘ Essai de Zoologie générale.’ 
Reciprocal terms also often make their appearance in the fami- 
lies, and from tribe to tribe; but it will be understood that ex- 
amples of them are rare, precisely because, the families being very 
natural, there are but few that I have been obliged to subdivide. 
Indeed, properly speaking, I only know of one truly worthy of 
attention, namely that presented by the family Serpulea. Here 
the small group of Sabellea with a caleareous tube, compared 
with the other representatives of the Sabella-type, presents a 
remarkable exception, which assimilates it to the true Serpulea, 
all of which have tubes of this nature. Hence many authors 
have arranged the Protule by the side of the latter and far from 
the Sabelle, with which they have such evident relations in their 
organization. On the other hand, the genus Filigrana, although 
composed of species which inhabit a calcareous tube, does not 
possess true opercula, and is related in other respects to the 
Sabellea. Although not so evident as in the cases previously 
cited, the reciprocity cannot be overlooked here. 
It may be remarked that, as regards the form and arrange- 
ment of the branchiz, the Protule and the Psygmobranchi (Sa- 
bellea with calcareous tubes) precisely repeat the two arrange- 
ments presented by the Serpule, the Vermilia, and the Cymospire 
(true Serpulea), so that they play the double part of reciprocal 
and corresponding terms. 
In glancing over the various tables of the families, the reader 
will easily remark that the characters placed in the first rank 
are far from being always derived from the same organs. Most 
frequently the feet, or the totality of the body, have served me as 
a starting-point ; but sometimes the cephalic appendages, some- 
times the number and arrangement of the branchie, &c., the 
proboscis, or even the eyes have furnished me with the most 
general characters. This is because, in fact, in the class of An- 
nelides as in the animal kingdom in general, the same apparatus 
does not retain throughout an identical and constant value as a 
means of characterization. It is evident, for example, that when 
in the whole of a family, as in the Hunicea, the feet are uni- 
formly uniramose, furnished with two cirri, and armed with 
setee modelled on the same type, we cannot find in them the 
characters of groups. or genera; at the utmost they will serve 
for the distinction of the species. On the contrary, in the Sy/- 
lidea, in which the same organs become progressively degraded 
until they only present a small setigerous mamilla, the natura- 
list finds excellent characters in their successive modifications 
affecting one of the most essential parts of the body. 
