205 SUPPLEMENT 
almost exterior, as in certain monopleurobranches and cervi- 
cobranches. In the brachiocephala, the gills are contained in 
the sac formed by the mantle. 
In all the acephala, the gills are between the mantle which 
conceals them and the body. 
The place which the organ that we are now examining 
occupies also varies in a notable manner. ‘Thus it is some- 
times at the upper and posterior part of the body, as in doris, 
peronia, and even in the testacelle ; it is at other times on 
each side of the back, as in scyllea, the eolides, the tritoniz ; 
in other species it passes underneath, all around the edge of 
the mantle, between the foot and this last, as in phyllidia, um- 
bracula, and even a little so in oscabrio. But rarely the respi- 
ratory organ is on each side of the body, in the sac formed by 
the mantle, asin the brachiocephala, or only on the right side, 
as in all the monopleurobranches. In fine, most generally it 
is at the anterior and upper part of the origin of the back, and 
of the back itself, that we observe the respiratory organ, as in 
most cephalous mollusca, whether pulmonary or branchife- 
rous, and even in the dentales. 
Tn all the conchiferous acephalous mollusca, on each side of 
the body, between it and the mantle, are two large semi-lunar 
lobes, which are generally regarded as the gills of these animals. 
In the order of naked acephala, the respiratory organ is in 
a sort of tube, which conducts from the posterior part of the 
body to the mouth. 
As to the structure of the gills of the cephalous mollusca, it 
reminds us of that of these organs in the fish. Whether it be 
sorts of triangular lamine, ranged like the teeth of acomb upona 
common axis, or sorts of tubercles irregularly collected, in the 
manner of granulations, the skin which constitutes them is 
considerably attenuated, although it preserves its contractile 
quality. Its principal afferential vessels may be very well in- 
jected, whose ramifications, often very fine, proceed to unite in 
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a 
