example; and as the Notarchus (molluscans nearly allied to the Aplysia) are weaker of frame, they have, in addition to this furrowed foot, a small sucker in front of it, analogous in form and use to the sucker of the Nucleobranches,* which they otherwise resemble in their manners. Other allied Gasteropods, still more pelagic, float even on the wave; and the foot, having become useless, does either not exist, or exists merely in a rudimentary condition; while, on the contrary, the pallial appendages deserve the name of fins from their developement and use. Thus in the apodal Pterosome a thin membrane surrounds the whole body, which it sustains by the extent of its expansion, and displaces by its muscular movements; but in the Glaucus and Briareus, the expansions are divided and extend horizontally on each side. You will remember that, like the more normal species, all these swim constantly in a reversed position, having the foot, or ventral surface, applied against a thin layer of water which intervenes between it and the atmosphere.† But unquestionably the most remarkable apparatus for locomotion among the Gasteropods, is offered for our contemplation by the Ianthina, a snail-like mollusk, with a light and purple-coloured shell, the native, it is believed, of tropical seas, though one species has been so frequently found on our own coasts, that its claim to be considered a native is now generally admitted. Its proper habitat is the open sea, where it has the power of swimming at the surface at a slow rate. To the posterior part of the foot there is attached a large vesicular appendage, very aptly named spuma cartilaginea by Fabius Columna, for the vesicles are as transparent as the air-bubbles in foam, while the skin is cartilaginous or membranous.‡ Buoyed up on these air + Ibid. p. 24. * Rang's Man. P. 28. Dr. Reynell Coates gives the following description of the mode in which this organ is constructed or repaired: "Individuals being placed in a tumbler of brine, and a portion of the float being removed by the scissors, the animal very soon commenced supplying the deficiency; the foot was advanced upon the remaining vesicles, until about two-thirds of the member rose above the surface of the water; it was then expanded to the uttermost, and thrown back upon the water, like the foot of a Lymneus when commencing to swim; in the next place it was contracted at the edges, and formed into the shape of a hood, enclosing a globule of air, which was slowly applied to the extremity of the float. A vibratory movement could now be perceived throughout the foot, and when it was again thrown back to renew the process, the globule was found enclosed in its newly constructed envelope. From this it results that the membrane which encloses the cells is secreted by the foot, and that it has no attachment to the animal other than the close adhesion resulting from the nice adaptation of proximate surfaces." -Zool. Journ. iii. 264; Ann. Phil. n. s. x. 385. a bladders, the Ianthina floats at ease,-not left, however, to be driven at random by every current or breeze which may sweep across its path, for its course is guided by means of small fin, which runs along each side of the foot a little above its edge. It is only when the "tempest's breath" blows hard that the snail yields to its violence, and suffers wreck on the unfriendly shore. Of this interesting mollusk Dr. Browne says, that "it probably passes the greatest part of life at the bottom of the sea, but rises sometimes to the surface."* On the contrary, all recent observers assure us that it has no power of even sinking in the water. The opinion of Browne long prevailed, and we were told that the air-bladders could be thrown off and renewed at pleasure, as the animal wished to sink or rise. Nor am I convinced that this is an erroneous supposition, for certainly the vesicles are often found swimming detached. Cuvier supposed that the snail might be able to compress the apparatus to such a degree as to allow its withdrawal within the shell, when the body would sink by its own weight, and that it would rise again when the snail relaxed its muscular efforts by the natural elasticity of the gaseous contents expanding the vesicles to their full volume.† This conjecture Mr. Bennet has disproved. He found that when the Ianthina was purposely irritated, it had no power of retracting its float:"On the animal being touched, in ever so slight a degree, it produced the effect of causing it immediately to withdraw itself into the shell, and even at first, on any person moving near the glass of water in which it floated; but then the frothy appendage always remained stationary."‡ Nor is the float, although undoubtedly the buoyancy depends in a great measure on it, essential to the support of the snail at the surface, for Dr. Browne and Mr. Bennet frequently captured specimens there which had it not; whence the latter infers" that they can float on the surface of the water equally with or without it, although it must be naturally supposed that the animal and its shell cannot be retained so long on the surface without as with it." The connexion of this organ with the foot does not appear to be organical; it is merely fixed in its place by some albuminous secretion, and can be detached without any laceration or wound. The nature of this connexion between the parts greatly supports the conjecture that the animal may cast it off after a season by a natural exfoliation, * Hist. of Jamaica, p. 400. + Mem. xv. 5. Med. Gazette for 1834, p. 233. Also Grant in Proceed. Zool. Soc. iii. 14. when it may sink to the bottom for a certain time; for, notwithstanding all that has been lately said, I am inclined to go into Dr. Browne's belief of its living there; and am not unwilling to believe further, that its floating may be during the breeding-season only, for, as we shall afterwards see, the vesicular appendage is likewise a sort of ovarian receptacle. It is no substitute for an operculum, as Cuvier states," because it does not adhere in the same manner, nor in the analogous place; for instead of being situated above the posterior part of the foot, it is below:† the organ is, in fact, a special contrivance made for a specific use.‡ The habits of the Litiopa are not less worthy of your notice. This is a small snail, born amid the gulf-weed, where it is destined to pass the whole of its life. The foot, though rather narrow and short, is of the usual character, and, having no extra hold, the snail is apt to be swept off its weed; but the accident is provided against, for the creature, like a spider, spins a thread of the viscous fluid that exudes from the foot, to check its downward fall, and enable it to regain the pristine site. But suppose the shock has severed their connexion, or that the Litiopa finds it necessary to remove, from a deficiency of food, to a richer pasture, the thread is still made available to recovery or removal. In its fall, accidental or purposed, an air-bubble is emitted, probably from the branchial cavity, which rises slowly through the water, and as the snail has enveloped it with its slime, this is drawn out into threads as the bubble ascends; and now, having a buoy and ladder whereon to climb to the surface, it waits suspended until that bubble comes into contact with the weeds that everywhere float around! From the observations of Mr. Gray, it appears that the Rissoa parva of our coasts had somewhat similar habits; and the Cerithium truncatum, which is generally found in brackish water in mangrove swamps and the mouths of rivers, does sometimes suspend itself from the boughs and roots of the mangrove by a glutinous thread. T We have also a native lacustrine species, the Physa fontinalis, which can let itself down gradually, like the Litiopa, § * Mem. xv. 4. See also Proc. Bost. Soc. N. Hist. i. 21. + Rang, Man. p. 25. "Mr. Parkinson rightly conjectures that the shells resembling the Helix, or snail, in the older strata, were constituted for swimming, like the Ianthina: they could scarcely have used a foot for crawling at the bottom of a deep and agitated ocean."-Bakewell's Geology, p. xxxv. § Rang's Man. pp. 26, 198. Kiener in Ann. des Sc. Nat. xxx. 223. Proc. Zool. Soc. iii. part i. 116. Mag. Nat. Hist. n. s. iii. 127. by means of a glutinous thread affixed to the surface;* and many land slugs have been seen to spin a line from the gummy secretion of their skin, and thus descend, from trees and precipices, by a shorter route than that by which they had ascended.t Our second class of Crawlers embraces the bivalved mollusca, or at least the great majority of them, for the Monomyairians in general have no foot, and are consequently incapable of locomotion. The foot, where it exists, varies greatly in size and figure, accompanied always, of course, with corresponding variations in the mode or velocity of progression, but in general it is of an oblong shape, often with a bend in the middle, and more or less compressed. (Fig. 22.) Attached to the abdominal and middle part of the body, more or less in front, it can be moved in nearly every direction, Fig. 22. more especially forward, shortened or lengthened, bent or made straight, by the action of its interlaced texture of muscular fibres; and it can be drawn at will within the valves, or protruded beyond them, by other muscles, which run towards different points of the shell where they are inserted. Its length is often surprising. I have seen a small individual of the Crenella discors put forth a foot at least six times longer than the shell, which, nevertheless, when not in action, was so neatly folded up and contracted within it that no part was visible. Bivalve mollusca proceed at a rate even slower than that of any snail, and, perhaps, seldom attempt the exercise, unless driven by some urgent want. One species only (Psammobia aurantia, Lamk.) is certainly known to creep like the Gasteropods, although, from the structure of the foot, it has been conjectured that some Arcæ likewise do so.§ The rest, when bent on change, leisurely protrude the motive organ, extend it to the utmost, apply it with hesitation and care to a solid surface, and then, by contracting it, as with a painful effort, they jerk forwards the body and its testaceous envelope. Now the foot is again extended in the same cautious manner, and the shell again dragged forward * Montagu, Test. Brit. p. 227. + Megalomastoma suspensum has derived its trivial name from the habit the animal has of suspending itself by glutinous threads. It is a native of the woods of St. Vincent. See Swainson's Malacology, 186, fig. 29. Proc. Zool. Soc. 1847, p. 21. Blainville's Man. p. 151. The Nucula, a genus of the Arcadæ, is now known to creep like a Gasteropod.-FORBES and HANLEY, Brit. Moll. ii.217. to the point of fixture. Such is the manner in which I have seen the Cyclas, an inhabitant of our ponds, and some of the lesser Bivalves which inhabit our shores, move along; and, I presume, it is in a similar manner that the other and larger species proceed. Reaumur has happily compared their mode of progression to that of a man who, having laid himself flat on his belly, desires to move onward by the sole aid of his arms: he stretches the arm to a point of support which he can just reach with the hand, and take hold of, when, by shortening the arm, he drags his body on; and the foot of the Bivalves differs from the arm only in this, that the shortening is effected by a general contraction of the muscular fibre, and not by muscles bending a joint.* Reaumur also informs us, that some Bivalves, as the Myæ, can move retrogradely along the ground in this manner. They plant the point of the foot just beyond the margin of the valves in the clay or mud, and then by elongating the foot, they push themselves backwards, in the same way that a sailor pushes off a boat by leaning against the oar which he has planted in the sand on one side. When the bend in the foot of a Bivalve is considerable, forming a sort of elbow, the animal is projected forward by a succession of short leaps. Such a structure characterises the Tellinæ and Donacidæ, and you may see it well marked in the Donax trunculus of British authors, a species which is abundant on most of our sandy shores. When it is about to make a spring, it firstly, by appropriate motions of the foot, puts the shell on the point or summit, as if aware that this is the position the most favourable of any to avoid the resistance which the sand opposes to the motion. It then stretches out the leg as far as possible, makes it embrace a portion of the shell, and, by a sudden movement, similar to that of a spring let loose, it strikes the earth with its leg, and effects the leap.† This, in some species, is considerable, for Mr. Sutchbury told the Rev. Mr. Kirby that Trigoniæ of New Holland would leap over the gunwale of a boat, to the height of above four inches. Some Clams or Pectens, and the closely allied Limæ, are also salient; but their leaps are the result of a sudden strong effort to close the valves, after they have been opened to the utmost, and which the remarkable size of the adductor Mém. de l'Acad. Roy. des Sc. an. 1710, p. 581. + Smellie's Phil. Nat. Hist. i. 138; but more particularly Reaumur's memoir just quoted, p. 600. raro. Bridgew. Treat. i. 264. Of Pecten islandicus Fabricius says, "Editur quidem ab incolis, sed |