10 M.A. de Quatrefages on the Classification of the Annelides. 
able to show that the converse is equally true. But here the 
most important type, that of the Ariciea, is wanting, and our 
data are sufficient only as regards the Arenicolea. Now, to judge 
from this example, we may say that the species which present 
this peculiarity depart in certain respects from the general type 
of the class, and are sufficiently removed from the type of the 
order to have led to their having been often removed from it. 
Savigny placed the Aricie among his Néréidés (Erratice). 
He has been imitated by Cuvier, Blainville, Audouin and 
Edwards, Grube, &e. Most of these authors have referred the 
Arenicole and the Ophelie to the same type. On the other 
hand, the Siphostomata, the Pheruse, &c., species of the family 
Chloremea, have generally been placed by the side of species 
which enter into our order of Sedentariz as established here. 
Whilst acting otherwise than my predecessors, I can easily 
understand how they were led to the conclusions which I dis- 
pute. It is impossible to deny the resemblances which ally the 
Chloremea to the best-characterized Sedentariz. On the other 
hand, the Arenicole, the Ophelia, and especially the Aricie, have 
certainly something which approximates them to the Erratice. 
But these relations in both eases are due to analogies, and not to 
affinities. The Chloremea are the representatives of the type of 
the Sedentarie in the midst of the true Erraticee. The Opheliea, 
the Arenicole, and the Aricie in the same way are the repre- 
sentatives of the Erratice among the Sedentarie. There is, 
so to speak, reciprocity between the two orders—each of them 
having in the other some species which recall it to mind. 
These species, up to a certain point, are reciprocal terms of one 
another. * 
The preceding examples perhaps will not suffice to lead all 
naturalists to admit the fact, here of fundamental importance, of 
this reciprocity of representation, and the consequences which 
flow from it for the appreciation of true relations of affinity. The 
following is another and a more conclusive one, because it bears 
in both orders upon families as well marked as possible, because 
the inverse modifications bearing upon the same organs are at 
once very simple and very striking, and because, whilst influ- 
encing one of the most essential characters of the order, they do 
not authorize the formation even of new families, but only of 
tribes. 
The family of Neretdea as circumscribed by me is certainly one 
of the most natural and best defined. Hssentially it includes only 
the genera Lycastis and Nerets of the old writers. rsted in 
describing the Heteronercides, and Blainville in founding the 
genus Neretlepas, effected mere dismemberments relatively to 
Savigny. But from the point of view which has served me for 
