12 M. A. de Quatrefages on the Classification of the Annelides. 
In the latter the resemblance is produced by the appearance of 
an exceptionally distinct region ; in the former bythe disappearance 
of anormally distinct region. In all it is in the feet that the unusual 
characters are manifested. Lastly, however striking these charac- 
ters may be, they are the result of modifications which are really 
very simple, and which in no respect alter the special type of 
the organs affected. 
It seems to me impossible to imagine a more complete fact of 
reciprocity, or one better fitted to ‘illustrate the nature of the 
relations resulting from modifications of this kind. It is evi- 
dent that we cannot place the Heteroneretdes among the Seden- 
tari, any more than we can arrange a Heterotercbelea among the 
Erratice. We cannot even isolate the former from the family 
of the Neretdea, or the second from that of the Terebellea, with- 
out the rupture of the most evident affinities. But these affini- 
ties are here complicated by relations of analogy. In the case 
before us the latter are much less marked than the affinities, and 
no one will hesitate as to the place belonging to the species under 
consideration. On the other hand, the analogies become stronger, 
and the affinitics less marked in the <Arenicole, Aricie, and 
Ophelia; and this has has led to the confounding of these two 
sorts of relations, and to the placing of these three last genera 
among the Hrraticze, whilst the Siphustomata (Chloremea) were 
removed to the Sedentariz. 
The reader will now understand, I hope, what I mean by the 
words reciprocal terms, and the nature of the relations which 
these terms present either with the group to which they some- 
times seem to belong, or with that to which they belong in 
reality. I believe that the investigation of facts of the same 
kind must, in certain cases, be of considerable importance, and 
that such will be discovered elsewhere than among the Annelides 
—for example, among the Acephalous Mollusca. 
It is not uninteresting to inquire which of the two orders into 
which the Annelides are divided makes the most efforts, so to 
speak, to establish these relations of reciprocity. The share is, 
in fact, very unequal: among the Erratice a single family 
betrays in its entirety certain characters which place it in the 
category of groups of which we are now speaking (Chloremea). 
Among the Sedentariz we find three (Arenicolea, Ariciea, and 
Serpulea), and perhaps a fourth (Leucodorea). In the first order 
a single family must be divided into tribes, in consequence of 
modifications which this type undergoes in the direction now 
under consideration (Nereidea). We find two of these in the 
second (Terebellea, Serpulea); moreover, in both of them the 
number of heteromorphous genera is much greater than in the 
lVereidea. 
