The tentacles vary in number in different genera : thus in Planorbis we find two, in the generality of cases four ; in a few, as some spe cies of (liolis, six; and in Polycera even eight of these appendages are met with. The structure of the tentacles is by no means the same in all the individuals belonging to this class. In the aquatic species they are to a greater or less ex tent retractile, but can in no case be entirely concealed within the body, as is usual in the terrestrial division ; they are therefore not hol low, but composed of various strata of circular, oblique, and longitudinal muscular fibres, by means of which they are moved in every direc tion, and applied with facility to the objects submitted to their examination. In all instances they are plentifully supplied with nerves arising immediately from the brain. Their shape is subject to great variation ; they are usually simple processes from the surface of the body more or less elongated, and in some cases even filiform, as in Planorbis. In 31arex (Jig. 193) each tentacle is a thick and fleshy stem, near the extremity of which a smaller one is appended. In Tritonia each tentacle is corn posed of five feathery leaflets, and is enclosed in a kind of sheath which surrounds its base. In Doris the two inferior are broad, flat, and fleshy, while the superior are thick and club shaped. In Scyllera they consist of broad fleshy expansions attached by thin pedicles to the anterior part of the body. In net hys they are placed at the base of the veil which characterises the animal, but in all cases they are solid and incapable of entire retraction. In the terrestrial Gasteropoda, in which from many causes the tentacles are more exposed to injury, a much more complicated structure is needed, by which these important organs are not only moved with facility in different directions, but which allows them to be perfectly withdrawn into the interior of the body, from which posi tion they may be made to emerge at the will of the animal : the mechanism by which this is effected will be understood by referring to fig. 192, representing a dissection of the common snail, and exhibiting the tentacles in different states of protrusion. Each tentacle (c, d,) is here seen to be a hollow tube, the walls of which are composed of circular bands of muscle, and capable of being inverted like the finger of a glove; it is in fact, when not in use, drawn with in itself by an extremely simple arrangement.
by far too small to admit of any satisfactory examination of their internal structure; and even in the largest forms of the organ which are met with in the more bulky marine genera, it is with difficulty that their organisation can he explored. In fig. 193 we have delineated the position and structure of the eye in a large Murex.
From the common retractor muscles of the foot four long muscular slips are detached, one for each horn ; these run in company with the nerve to each tentacle, passing within its tube when protruded, quite to the extremity (g). The contraction of this muscle dragging the apex of the organ inwards, as seen at c, of course causes its complete inversion, whilst its protrusion is effected by the alternate contrac tions of the circular bands of muscle of which the walls of each tentacle are composed. There is, however, another peculiarity rendered neces sary by this singular mechanism, by which the nerves supplying the sense of touch may be enabled to accommodate themselves to such sudden and extensive changes of position ; for this purpose the nerves supplying these organs are of great length, reaching with facility to the end of the tubes when protruded, and in their retracted state the nerves are seen folded up within the body in large convolutions. In the figure, a a indicates the origins of the retractor muscles of the foot from the columella ; 1), the right superior tentacle fully protruded ; c, the left superior tentacle partially retracted ; d, the left inferior tentacle extended, and e, the right inferior tentacle fully retracted and concealed within the body; f, the nerve supplying the superior tentacle elongated by its extension ; g, the retractor muscle of the same tentacle arising from the common retractor muscle of the foot and inserted into the extremity of the tube ; h, the nerve of the opposite side thrown into folds ; i, the retractor muscle of the same tentacle contracted ; k, the aperture through which the nerve and retractor muscle enter the tentacle d ; 1, the brain ; to, the subcesophageal ganglion ; a, the eye.
l'ision.—The eyes of Gasteropoda are tremely small in comparison with the bulk of the animals, and seem more to represent the rudiments of an organ of sight than to be adapted to distinct vision. In many species indeed they appear to be absolutely wanting. found, they resemble minute black points, The natural size of the organ is seen in the upper figure, in which on the right side the organ is represented untouched, while on the left a section has been made to exhibit its interior. This section when magnified, as in the lower figure, skews us that it consists of a spherical cavity lined posteriorly with a dark choroidal membrane,and containing a large spherical lens; the position and structure of the retina we have been unable satisfactorily to determine, although the visual nerve may be readily traced to the back of the choroid, where it seems to expand ; but whether, as in the Cephalopods,its sentient portion is spread out behind the pigment which lines the eye-ball, or whether, as in the forms of the organ common to the vertebrate orders, the retina is placed anterior to the choroid, is a question which we are at present unable to solve. But however this may be, we see anteriorly a distinct pupil surrounded by a dark radiating zone, apparently ao iris, to which it corresponds at least in position, although that it is really capable of contracting or en larging the pupillary aperture is more than our observations warrant us in affirming. Finding, therefore, the eye of the Murex to offer a struc ture which indubitably entitles it to be regarded as an organ of sight, we are justified in consi dering the more minute specks of smaller Gas teropoda as similarly formed and subservient to the same office. In the aquatic species the eyes are generally placed at the base of the su perior or larger tentacles, although not unfre quently they are supported upon short pedicles appropriated to them, as is the case in and others. In Muter we have seen that the tentacles which support them are large anti fleshy, and by the position of the eyes at the extremity of so long a stem these can be readily directed to different objects. In no case, how ever, can they be retracted within the body so as to be quite enclosed in the visceral cavity. In the terrestrial Gasteropods the eyes are gene rally placed at the extremity of the superior horns, a position which manifestly extends the range of vision, and moreover, in consequence of the structure which we have described when speaking of the organs of touch, may be com pletely drawn within the body. Infig. 189, b, the eye of Vaginulus is seen at the extremity of the upper tentacle, and the origin of the optic nerve (c) from the brain (d), as well as the convolutions which it makes to allow of its adaptation to the varying length of the tentacle, and the bulb in which it terminates behind the eyeball (b*), are sufficiently displayed. In fig. 192, b, the eye of the snail exhibiting the same circumstances has been represented, and the apparatus by which the movements of the whole organ are effected is so clearly shewn as to render further description superfluous.