6 M.A. de Quatrefages on the Classification of the Annelid 
means of considerations of the same nature, and derived like- 
wise from exceptions presented to the law of repetition. 
Thus in the first order (A. erratice) the greatest number of 
the species are entirely composed of similar segments ; in other 
words, the repetition is manifested from segment to segment. 
In some others the repetition only takes place from pair to pair 
of segments, at least on the greater part of the body. The 
former constitute for me the suborder of Erratice proprie; the 
latter that of the Hrratice aberrantes. 
In the same way, among the Sedentariz, a very small group, 
including only the Chetopterea, shows us the law of repetition 
failmg in the segments of a single region; it constitutes for 
me the suborder of Sedentarie aberrantes. In the second sub- 
order of this division the law of repetition is observed in the 
different regions of the body; it includes the S. propria. 
As a matter of course, in the establishment of the families, I 
have taken into account anatomical and physiological as well as 
external characters. But in the table which I have the honour 
to place before the reader, I have had recourse solely to the 
latter, in order to facilitate the zoological study of the species. 
The armature of the mouth, the absence or presence of branchie, 
the position and form of the latter, the absence or the presence 
of certain appendages of the head or of the feet, the modifica- 
tions of these latter, &c., have been employed successively in 
the order just indicated. This order itself was the consequence 
of the principle of the relative constancy of the characters. 
It has enabled me to characterize each family with precision, 
and to group them in such a manner as to bring into relief a 
certain number of general results, well fitted, it appears to me, 
to justify the method followed. 
Thus, on glancing at the accompanying table, every natu- 
ralist will perceive that the divisions resulting from considera- 
tions derived solely from external characters are equally homo- 
geneous from an anatomical point of view. He will also perceive 
that the totality of the families in the two orders subdivides into 
secondary groups corresponding to so many more or less im- 
portant subtypes, of which the representatives are united ; and, 
lastly, that the exceptional or aberrant types are also quite na- 
turally brought to the notice of the reader. I may be permitted 
to dwell a little upon these considerations. 
Leaving out of consideration for the present the Suborders I. 
and III., including the general aberrant types of the two great 
fundamental divisions, there remain, as composing the Frratice 
propria, thirteen families, and ten for the Sedentarie propria. 
Let us first notice the former. 
The presence of cephalic rotatory apparatus serving for loco- 
