The above structure of the foot, and conse quent mode of locomotion, although the most usual, is susceptible of considerable modifica tion. Thus in Scyllam, we find it only adapted for grasping the thin stems of fuci and other submarine plants, being for that purpose com pressed and grooved inferiorly into a deep sul cus. In the Tornatellafasciata, Lam. the struc ture of the foot is remarkable : beaten incessantly by the waves, in the cavities of rocks which it frequents, nearly on a level with the surface of the sea, to the violence of which it is always exposed, it has need of additional powers of retaining its hold; its foot is therefore divided into two adhering portions, placed at each extremity, and separated by a wide interval ; when it crawls it fixes the posterior disc and advances the other, which it attaches firmly to the plane of progression, and this being effected, the hinder sucker is detached and drawn for wards, locomotion being accomplished by the alternate adhesion of these two prehensile discs. In Cyclostoma the foot is likewise furnished with two longitudinal adhering lobes, which are advanced alternately. But the foot is not merely an instrument of progression on a solid surface, in many species being convertible, at the will of the animal, into a boat, by means of which the creature can suspend itself in an in verted position at the surface of the water, where by the aid of its mantle and tentacles, it can row itself from place to place. The Buli mus stagnalis, so common in our pools of fresh water, is a good example of this mode of sail ing ; and in the marine species, Aplysia and Gastropteron may be enumerated as exhibiting a similar structure.
Some of the naked Gasteropods, as Aplysia and Thethys, are able to move through the water in the same manner as the leech by au undulatory movement of the whole body, a mode of progression which in Thetliys is mate rially assisted by the membranous expansion of the mantle placed around the anterior part of the body, which forms a broad veil, and from the muscular fibres contained within it, must necessarily be an important agent in swim ming.
Particular secretions.—Many of the Gaste ropoda, in addition to the secretions which have been mentioned, furnish others adapted to peculiar circumstances, and produced from special organs.
In the Snail and the Slug tribes a slimy mucus is furnished in great abundance from an organ which has been denominated the " sae of the viscosity ;" this is a membranous bag sur rounding the pericardium, which when opened is found to be divided internally by delicate septa arising from its walls ; from this proceeds a capacious duct, which follows the course of the rectum, to which it is intimately united, to open externally in the neighbourhood of the respiratory aperture. The viscid secretion of
this gland spreading over the surface of the foot is most probably an assistant in progres sion, causing it to adhere more intimately to the surfaces over which the animal crawls.
Aplysia furnishes three distinct fluids issuing from different parts of the body. The first is a glairy mucus, which exudes in considerable quantities from the surface of the mantle, espe cially when the creature is irritated. The se cond i. a whitish liquor, which is thick and acrid, and has been reputed venomous; it is emitted in very small quantities, but its smell is strong and highly nauseous: the gland which produces it is a little reniform mass placed near the vulva, close to which is the orifice of its excretory canal. Blainvil le looks upon this as the representative of a urinary apparatus, but it does not appear to exist in all the species, and is never emitted except when the animal is tor mented.
The third secretion is much more abundant than the other two, and is generally of a beau tiful lake colour, except in Aplysia citrina, in which it is yellow. It is contained in a spongy substance, which occupies all those portions of the little mantle or operculum to which the shell does not extend. All the areolw of this tissue are filled with a purple matter, the colour of which is so intense, that when it is expressed it has a black violet hue, but when mixed with a large quantity of water, imparts to it the co lour of port wine. This colouring fluid seems to exude through the skin of the mantle, no ex cretory duct having been found specially ap propriated to its escape : it is apparently pro duced from a triangular glandular mass situated in the base of the mantle.
Several species of NIurex secrete a similar fluid, which, like the ink of the cuttlefish, serves as a defence from attack ; in all cases it is expelled with force, and in such abundance as to colour the water around to a considerable distance.
There is a species of Limax, ( Lima: nocti luctis,)deseribed by M. Orbigny,which produces a phosphorescent secretion capable of emitting a light of considerable brilliancy. The luminous organ is a small disc of a greenish colour by day-light, soft in texture, and slightly contractile. The light is only visible when the creature is expanded and in motion. The disc is always covered with a greenish mucus, which, if wiped off, is speedily renewed. It is found to be connected with the generative organs, and ap pears to be principally useful during the season of love.