ON PTEROPODA. 321 
right side, and the other exists also at the right, but is 
situated much more forward, and in the emargination which 
separates the two parts of the body, in front of the orifice of 
the branchial cavity of this side. 
We have already viewed the exterior envelope of the hyalza, 
in the relation of its disposition and form, let us now consider 
the mantle in its uses. 
It would appear that it is not of a very mucous texture, for 
the shell is very thin; but it is evidently very contractile, and 
probably very sensible in its exsertile parts. The local, or 
special sensibility is carried on by the tentacula, of which we 
have spoken above, and which are but little developed, and 
probably by the eyes. 
The exsertile parts of the mantle, are, moreover, capable of 
great extension. They have particular muscles which draw 
them inwards, and they may easily be seen on the back, and 
under the belly. These white fibres, which are evidently 
muscular, proceed transversely, and in directions almost 
parallel to each lip of the lateral lobes. These our author 
considers as branchial vessels. 
The foot of the hyalea being the principal, if not the only 
- organ of locomotion, the muscular arrangement there becomes 
more complicated. ‘These animals belong to the ocean, and 
are seldom found near the shore. ‘Their manners and habits 
are little known with the exception, that they swim with 
great swiftness. On the rest of this class we have nothing to 
add. 
VOL. XII. Y 
