M.A. de Quatrefages on the Classification of the Annelides. 13 
It will be seen, I hope, from what precedes, that the reciprocal 
terms are very distinct from corresponding terms, although the 
existence of the latter depends equally upon considerations de- 
rived from analogy, and not from affinity. There is correspondence 
when, in two great groups more or less remote, we find similar 
and not inverse modifications being produced. For example, the 
branchiate and abranchiate Sedentariz are in a general way and 
in certain respects the corresponding terms of the branchiate and 
abranchiate Erraticee. Nevertheless in this case the organic and 
morphological differences are sometimes great enough at least to 
dissemble these analogies. And yet, on close examination, it is 
difficult not to be struck by the fact that in both orders the 
respiratory organs present extremely similar modifications. Thus, 
at the first glance, the cephalic branchie of the Chloremea 
resemble those of certain low Sedentariz* ; the arborescent so- 
matic branchie of certain Amphinomea evidently correspond with 
the branchiz situated in the same region of the body, and pre- 
senting the same form, in the Arenicolea; and I may say the 
same of the branchize of the Nephthydea and Nerinea as com- 
pared with those of the Ariciea and Hermellea. 
But it is especially in the details of certain families, and when 
the genera become numerous, that we see numerous correspond- 
ing terms make their appearance. We may judge of this by a 
mere glance at the table of Syllidea. Here the number of well- 
characterized genera rises to thirty-one, and from group to group 
we see repeated the absence or the presence of frontal lobes, the 
same number of antenne, tentacles, eyes, &c. These groups 
and genera are, in every acceptation of the word, the analogues, 
or the corresponding terms of each other. 
The frequency of this kind of relations results from a remark- 
able fact, presented by no class of the animal kingdom in so 
marked a manner as by the Annelides. In them the immense 
variety of secondary characters is obtained in the most simple 
manner, by modifications of the same nature, or even very often 
completely identical, repeating themselves in groups which are 
otherwise distinguished by well-marked differences, in such a 
manner that a very considerable number of results is usually ob- 
tained with a truly marvellous economy of processes. The 
Syllidea, the Terebellea, and the Serpulea offer us remarkable 
examples of this fact. In the Zeredel/ea in particular, the three 
known heteromorphous genera are the exact repetition of three 
normal genera, and are distinguished only because they have in 
common the kind of modifications which I have indicated above. 
* These resemblances are, however, more apparent than real; for the 
branchize of the Chloremea issue from the buccal ring, and not from the 
head properly so called. 
