208 SUPPLEMENT 
formed by small triangular plates, attached to the root of the 
first pair of appendages, as is the opinion of Baron Cuvier. 
We may also conceive that the filaments which bristle these 
appendages might take place of them, and then these appen- 
dages might be considered as gills. 
The oscabriones have their respiratory system formed almost 
as in the phyllidiz, of small triangular plates placed under the 
edges of the mantle. 
According to the structure, the form, and even the position 
of the respiratory organ, the apparatus by means of which the 
ambient fluid is brought in contact with the modified cuta- 
neous envelope, must of necessity be different. 
There was no need of any when the gills were external, 
either on the back or under the edges of the mantle. 
When, on the contrary, they have become internal, some 
particular modification was necessary in the edges of the 
cavity which contains them, and even in the shell which 
covers and protects them. Thus in a great number of the pec- 
tinibranch species the anterior edge of the mantle is prolonged 
into a tube, while others have but a sort of inferior auricle in 
place of this tube, or present but a broad cleft, which conducts 
into the branchial cavity. The pulmonary species have only 
a hole pierced into the thickened edge of the mantle. 
In almost all the acephala the water arrives at the gills 
through an aperture formed by the two lobes of the mantle, 
which are often prolonged behind by the addition of a long 
contractile tube, distinct, or united to that of the anus, as 
before observed. 
The theory of the function of respiration, independently of 
the organs, appears pretty nearly the same as in the more 
elevated types of animals. We know, in fact, that the mol- 
lusca absorb the oxygen of the air in which they are retained. 
But is this done alone by the organ of respiration? That is 
by no means probable, the general envelope being by its 
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