M. A. de Quatrefages on the Classification of the Annelides. 11 
the division of the Annelides in general, it will be seen that 
these two genera form, in reality, a small and very remarkable 
separate group. In fact, the law of repetition so generally ap- 
plied in the Erratic, and so manifest in the Nerezdes proper, 
here undergoes a striking exception. In the Heteroneretdes 
especially, the foot, that fundamental organ, changes its form 
rapidly posteriorly in such a manner that the body presents two 
perfectly well-marked regions. Here, then, the essential cha- 
racter of the Sedentariz makes its appearance. Is it possible 
from this fact alone to transport the Heteroneretdes to that order? 
Or should we even isolate them from the Nerevdes? A more 
careful examination shows that both these conclusions would be 
equally unjustified. 
Thus anteriorly the Heteronereides are in all respects true 
Nereitdes, both externally and internally. The feet in particular 
are exactly the ordinary feet of the Neretdes, to their very least 
details; and these feet are essentially arranged for walking. 
Posteriorly the body itself presents no change; it remains the 
body of a Nerews. ‘The feet alone are modified so as to become 
powerful organs of natation. But while becoming adapted to 
this new function, they still retain their original type. We find 
in them all the elements of the anterior feet, occupying the same 
position under slightly different forms, and complicated only by 
a small number of accessory parts. 
The differences between the anterior and posterior regions are 
therefore more apparent than real; but the division of the body 
into two distinct parts exists none the less. There is here evi- 
dently as it were a reflexion of the type of the Sedentarize 
making its appearance in the midst of one of the families most 
clearly belonging to the Erratice. 
The Terebellea and the Serpulea present us with the exact réci- 
proque of the preceding fact. In both we find a certain number 
of species which, as regards the two anterior regions (the head 
and thorax), completely realize the type of their family, but in 
which the posterior region of the abdomen no longer presents in 
its rami and sete those changes which characterize it in the 
normal Sedentariz, in the Serpulea proper. In these exceptional 
species the abdominal feet remain similar to those of the thorax, 
so that from one extremity of the body to the other we find no 
more distinct regions than in the Erratic. Nevertheless, in all 
other respects these species remain faithful to their types. 
Thus these abnormal Sedentariz are true Terebellea, or true 
Serpulea in their anterior portion, as the Heteronereides are true 
Nereidea in the same part of the body. In the posterior region 
the Heteroterebellea and Heteroserpulea approach the Erratice, as 
the Heteronereides approach the Sedentariz in the same region. 
