AND DISTRIBUTION OF THE FIROLIDA. 265 
to increase the lateral range of vision; while by those which curve round and form a kind 
of orbicular, the eyeball may be pressed on, so as to change the arrangement, relative 
position, and curve of the lenses, and hence alter the focus of the eye and give a more- 
extensive antero-posterior range, which enables the animal to perceive objects at greater 
or shorter distances. An organ so complex, and with a large special nerve-supply and 
a well-pronounced nervous system, all indicate an equal development of the sense of 
sight, and furnish presumptive evidence that this function is not confined to the appre- 
ciation of light and darkness, but probably extends to the discrimination at least of form, 
colour, and distance. "When swimming, the head and body, as far back as the tentacles, 
are often bent nearly at right angles, as well seen in Plate XLIV. fip. 13. By this arrange- 
ment the tentacles (which possess no muscular fibres and are therefore immobile in 
themselves) and the eyes become more prominent; the former enjoy free play, and the 
latter a far wider range of vision; so that sight, touch, and taste, the senses most 
employed in the search for food, are materially aided. To the frequent contraction and 
dilatation of the orbicular fibres, and the rapid alteration of the focus of the ocular lenses 
in vision, are doubtless due the appearance of winking sometimes observed. 
The auditory apparatus consists of the two nerves already described, which run out- 
ward and backward to the auditory vesicles (PlateX LIIL fig.8, a, %) behind the optic nerves, 
usually about their middle or outer third. The nerve appears their only attachment to 
surrounding tissues. The vesicles are globular, pellucid, and probably ciliated internally, 
two-thirds of their cavity being filled by a smooth, round, opaque otolith. The minute 
structure of this apparatus and, especially, the mode of distribution of the nerve still 
require observation. Its rudimentary nature, however, proves that the sense of hearing 
must be imperfect, and probably does not extend beyond a limited appreciation of sound. 
Olfactory apparatus.—Neither this nor a special nerve of smell can be detected. Both 
are supposed to be wanting in the mollusca generally; but they have been discovered by 
Owen in their highest form, the Cephalopoda, and may possibly exist in the Firola and 
other Heteropods, which rank next in the animal scale, in a rudimentary condition and 
combined with the sense of taste, their nerves being united. 
The sense of touch doubtless exists in Firola and the Firolide generally. Minute 
filaments from the ganglia on their intercommunicating cords, doubtless purely sensory, 
and perhaps derived originally from the cephalic ganglion, are frequently seen penetrating 
the muscular coat to end in the outer tunic. The non-retractile tapering tentacula, 
which correspond to the inferior or anterior tentacles of the Helix, appear more highly 
endowed as special organs of sensation, and are each supplied with a large branching 
nerve from the cephalic ganglion (Plate XLIII. figs. 1, 8). Plate XLIV. fig. 13 shows 
the animal applying these organs to the vessel in which it was kept, as if to ascertain its 
nature. In some of the Firolidæ they are absent. 
Circulatory System. 
Our knowledge of the anatomy of the circulatory organs and breathing-apparatus of 
the Heteropoda, and the connexion which necessarily exists between them, is by no 
means so complete as could be desired. 2:2 
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