Prof. E. Claparéde on the Structure of the Annelida. 349 
Perivisceral Cavity and Circulatory System. 
We are indebted to M. de Quatrefages and Dr. Williams, but 
especially to the former, for a profound investigation of the 
perivisceral cavity and of the lymph which it contains. These 
naturalists, more than any one else, have pointed out the physio- 
logical importance of this liquid, which cannot be too highly 
estimated. Some details, only, require a slight rectification 
here. The perivisceral cavity is lined by a delicate membrane, 
which is not easily demonstrable, except in the larger species— 
a membrane the discovery of which M. de Quatrefages ascribes 
to himself, and to which he gives the name of peritoneum. Had 
he thoroughly explored the works of Delle Chiaje and Rathke, 
he would have found in them both the membrane and the name. 
The structure of this peritoneum (tunica sterosa, tunica peri- 
toneale of Delle Chiaje) is subject to considerable variations, as 
I shall show in the course of this memoir. At any rate, the 
perivisceral cavity is clothed, in some species, with vibratile ‘cilia 
borne by the peritoneum. ‘Tf I am not mistaken, Dr. Sharpey 
was the first to describe these, in Aphrodita; Dr. Williams then 
detected them in the branchie of the Glycere; and I described 
them as occurring in the whole of the perivisceral cavity of the 
latter worms. They have also been seen in the Tomopteridea. 
M. de Quatrefages, who only notices in passing the observation 
of Dr. Williams, adds that this ciliary movement was long since 
known to him in a great number-of Annelida, and that it will 
be met with in all the species, if we take the trouble to look for 
it. This opinion is not well founded. The immense majority 
of the Annelida present no ciliary movement in the perivisceral 
cavity, except at the entrance to the segmental organs. For 
my own part I am acquainted with the perivisceral ciliary coat 
only in the following groups :—in all the Aphroditea, Glycerea, 
and Polycirrida, in the Tomopteridea, and in a small and rather 
abnormal Terebella (T. vestita). It is a striking circumstance 
that all these Annelida, with the exception of the little Terebella 
_ and Aphrodita aculeata, are completely destitute of vessels. 
Now, of these two exceptions, one (the Aphrodita) is an animal 
with a rudimentary vascular system, helonging to a family which 
is otherwise entirely anangian ; the other, the Terebella, belongs 
to a family which is generally vascular, but one tribe of which, 
that of the Polycirrida, is anangian. Considering these facts, 
I must regard the perivisceral ciliary movement as a function 
vicarial of the circulation in Annelida deprived of a true circu- 
latory system. 
The circulation of the Annelida has been most carefully de- 
scribed by M. de Quatrefages, who at the same time renders full 
