ON MOLLUSCA. 207 
a principal efferential trunk, which is so directed as to come 
forth from the organ in an inverse direction from that of the 
afferential vessel. ‘The projection of these branchial combs, 
or of these tubes, is sometimes not inconsiderable. 
The gills of the acephalophora are composed, in most cases, 
of two pairs, more or less unequal, of semi-lunar plates, placed 
vertically between the abdomen and the lobes of the mantle, 
and applied one against the other. Separated or united more 
or less in the extent of their lower edge, that of one side is 
joined to its correspondent on the other, through a part more 
or less considerable of its superior or dorsal edge. But it is 
attached on the sides of the belly by its anterior extremity, the 
other being often free. Each of these four gills is itself formed 
of two plates, which leave between them a free space, divided 
into a great number of vertical lodges, open at the dorsal edge, 
by numerous triangular partitions. These plates are consti- 
tuted by two strata of parallel vessels, vertical, and united by 
other transverse vessels. One of these strata is formed by the 
ramifications of the branchial artery, and the other by those of 
the vein. These ramifications unite in two thick trunks, 
which border the back of the branchial plate, and which are 
in communication, one with the auricle, on its side, and the 
other with the venous system of the rest of the body. 
In the lingule and approximating genera, it appears that 
the gills are a little different in their structure and in their 
position, since they are in the form of a comb, applied to the 
internal face of each lobe of the mantle. 
In the naked acephala, the branchial apparatus is still more 
anomalous. In fact, in the ascidiz, it is formed by a net-work, 
with quadrangular meshes, which invests the cavity of the tube 
as far as the mouth; and in the biphore it is a kind of narrow 
plate, almost free, and directed obliquely. 
The nematopods have their respiratory apparatus approxi- 
mating to that of the entomozoariez, if it be certain that it is 
