8 M.A. de Quatrefages on the Classification of the Annelides. 
development of the trunk and of the stomatogastric nervous 
apparatus). Still less can we refer the Neretdea to the Nerinea 
or the Cirratulea by considerations of the same nature. We 
are even led to see that, whilst in the Erraticze with a com- 
plicated buccal armature the superiority belongs incontestably 
to the branchiate family, in those with a simple buccal arma- 
ture the superiority reverts, on the contrary, to one at least of 
the abranchiate families (Nerezdea). Nevertheless, in both di- 
visions, the branchiate and abranchiate species very evidently 
occupy the position of mutually corresponding terms, if we place 
ourselves at the systematic point of view of respiration. 
From what has just been said, it follows that the Erratic with 
a complicated buccal armature form a remarkably natural divi- 
sion, inasmuch as the type, remaining the same, presents itself 
to the naturalist sometimes as being realized very completely, 
sometimes as degraded. The two families resulting from these 
different conditions are, moreover, very homogeneous. In the 
first, that of the Hunicea, which possesses branchiz, these vary 
as regards their form and complication, without its being possi- 
ble, however, to separate the gencra from each other. The same 
intimate relations exist between the genera belonging to the 
abranchiate family (Lumbrinerea). 
Nothing of this kind occurs among the Erraticee with a sim- 
ple buccal armature. Here, in the branchiate species, the least 
variation in the respiratory organ coincides with other modifica- 
tions of sufficient importance for the multiplication and distinet 
separation of the families, and these modifications affect even the 
most central organs, the nervous system. The composition of 
the cerebrum and the mode of distribution of the nervous trunks 
are quite exceptional in the Nephthydes, which, in other respects, 
would closely approach the Neretdea and the Phyllodocea; the 
Nerinea have the abdominal chain double, and in this respect 
resemble the best-characterized Sedentariz (Serpule and Sabelle); 
the Cirratuli, on the contrary, present abdominal ganglia which 
appear as if fused into a ribbon, which, again, reminds us of what 
exists in other Sedentarize (C/ymene). All these facts, and many 
others, indicate the existence of several distinct secondary types 
in this totality of branchiate Erraticee with the buccal armature 
simple. 
We find rather more heterogencity in the species of the same 
division which are destitute of branchiee. Here the Neretdea may 
be regarded to a certain extent as the highest expression of a 
type to which belong the Sy/idea, the Hesionea, and the Phyllo- 
docea. Nevertheless the resemblance is not strongly marked, 
either internally or externally. 
The Syllidea, a great number of which would perhaps depart 
