320 SUPPLEMENT 
It is that of the male apparatus of generation. We cannot 
say that it is really central. 
Underneath, the middle projection of the trunk, which is 
always of avery deep colour, and which can only be well 
seen in front, is terminated by a small inferior cleft, disposed 
in the direction of the axis of the body. This is the mouth. 
It is placed in the summit of the angle formed by two small 
labial decurrent bands, and which, separating considerably, 
are lost under the locomotive appendages. 
These appendages, which remain to be described, are 
nothing but what is called the foot, in the gasteropods, and 
which assumes its development, not in the middle and inferior 
part, but especially on the sides, and in front, so as to pass 
considerably beyond the head; and as these two lateral parts 
to proceed thus forward, must approximate to each other, a 
sort of deep emargination has been the result on the lateral 
parts of the head. From this character is derived the name 
of the class pteropods. Behind, however, there is no trace 
of separation, and there the external edge, after having formed 
a fold in front of the lateral aperture of the mantle, is con- 
tinued without interruption, and is curved in front of its in- 
ferior edge, forming a sort of bulky transverse fold, from 
which it results that this foot, much thicker at the root, and 
at the middle of its dilatation, and rather thin in its circum- 
ference, may not only serve for swimming, by acting like 
sorts of wings, but that it can very probably creep a little on 
the surface of submarine bodies, and particularly form a sort 
of sucker when the animal is in a state of repose. 
To complete the external description of the hyalea, we 
have only to speak of the orifice which terminates the diges- 
tive apparatus, and of that of the apparatus of generation. 
The one is formed at the posterior extremity of the furrow 
which separates the two lateral lips of the mantle, on the 
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