ceaseless movements similar to those of the wings of the butterfly. These fins are rowed with remarkable ease and quickness, and according to the direction of the motion, the creatures advance horizontally, rise or descend, while the shell is kept in a vertical position or slightly inclined. At times the Pteropod will turn itself without altering its place, or poise itself still and motionless at the same depth; but this immobility is to be observed only in a few species, while all make habitually the papilionaceous movements. If, during their course, any strange body comes in contact with them, or a sudden shock agitates the vessel in which they are kept, they fearfully fold their fins upon themselves, or, in some species, withdraw them in the shell, and allow the body to fall to the bottom of the vessel. It is probable that on a similar alarm in a state of liberty, they would unfold anew their fins, and stop their downward fall, when the body had sunk to a depth which placed it beyond the reach of danger. Rang has asserted that some of the Creseis do occasionally cluster on the gulf-weed, where they rest themselves by embracing the leaves and stalks with their fins, but D'Orbigny has never been witness of such an occurrence, which is probably accidental, for the species of Pteropods of which it is related become, he says, rare in the gulf-weed fields; and as the fins are not calculated for prehension, they might too easily be driven amid the crowded weeds where their thin brittle shells could hardly resist the shocks to which they would be exposed.†

The Pteropods are most numerous, both in species and individuals, under the torrid zone, but a few swarm populous" in the arctic seas, whose prolific waters are in strong contrast with the sterility of their shores and barren lands. The Clio borealis, in especial, fills them in some seasons with such teemful fecundity, that the whale cannot open his mouth without necessarily engulphing myriads of them. The Limacina, (Fig. 13) is not less profuse in the same seas, of whose habits the worthy Otho Fabricius has given a lively account, which loses, however, much of its spirit in my translation. "The shell is its boat, which the snail rows admirably through the water by the regularlytimed strokes of the raised fins. In this act the open extremity of the shell is the prow, the opposite end occupies the place of a poop, and the margin of the body-whorl resembles and performs the office of the keel. I have often seen it with admiration and pleasure," quod sæpius * Manual, 21.

Ann. des. Sc. Nat. Zoologic, n. s. iv. 189-192.

mirans delectansque adspexi." It can move in a retrograde manner. When weary with rowing, or when touched, the little boatman contracts its oary fins, and drawing itself within the shell, sinks to the bottom, where it rests a short space either upon the keel, or the prow, or the vertex, but never on the umbilicus. Then again it rises upwards, rowing obliquely until the surface has been gained, where its course is held in a straight line* o'er the trackless surge."

The small Gasteropod order which Cuvier has called Heteropods, are fully as pelagic as the Pteropods, and, like them, have no other way of changing their place than by natation. The foot, instead of forming a flat horizontal sole, you will remember, has a vertical direction, and assumes the figure of a compressed semicircular fin, which being moved by its own muscles from right to left, propels the animal forwards,like a sculler who works his boat with a single oar. The fin is ventral, but, on a hasty glance, you might mistake it for a dorsal crest; for the Heteropods-as indeed all pelagian mollusca do-swim in a reversed posture, the foot to the surface and the back looking downwards. In the Carinariabeautiful creatures! clear as crystal and painted with the liveliest colours,-and in the Firola, this ventral fin is aided in its office by some subsidiary membranes situated upon the neck or near the tail, but whose powers of propulsion are inferior to its own. Combined, however, they give to these genera a velocity superior to what has been noticed in any other tribes of mollusks; being, indeed, very remarkable for the quickness of their movements, propelling themselves in a forward or a backward course, in a straight or a curved line, with equal facility, and without any retardation of their pace. The Atlantes, which are destitute of these secondary fins, and whose body, compressed in the small space of their spiral shell, presents less resistance to the circumfluent medium, are slower of foot; and instead of moving in an even line, they advance after the manner of the Hyales, with alternate periods of activity and rest.

For the Pteropods, as we have seen, there is no organ of rest; they float or swim unfixedly in the abyss of waters, realising in their persons the fable of the paradise-birds, which ever hovered in the heavens, too aerial and spiritual to require the support of our gross earth; but the Heteropods need occasional repose and a cessation from activity,and how admirably is the foreseen want provided against! Where are they to rest-where fix their anchor in the world

* Fauna Groenlandica, 388.

of unstable water around them? They are created to live, and are born amidst the fields of sea-weed, which voyagers describe with amazement, as covering leagues of sea within the Tropics; and to enable them to attach themselves to the narrow leaves of the sargassum, they are furnished by their Creator with a small sucker, which, like a cupping-glass applied against the surface of the leaf, suspends them there without exertion. If such wonderful adaptations—such alliances between things which seem most remote-such design in such apparent chance, do not warm you with a conviction of the presence of an Omniscience, whose eye is over all his works, you are made of an earthier soul than I am well persuaded you are, and are most unfit for a naturalist. The little sucking pouch is situated on the superior and posterior margin of the ventral fin, and is formed by a kind of duplicature of the membrane which covers that organ.

*

The presence of a sucker in these molluscans reminds me of the cotyligerous Cephalopods, of which it has been asserted that the species, having two long arms in addition to the shorter feet (Decapods), anchor themselves to the submarine rocks when they cannot otherwise withstand the agitation of a stormy sea;t although, in general, the suckers are rather to them organs of prehension to arrest their prey than organs of rest. All the Cephalopods are good swimmers; but, in their mode of natation, they are as peculiar as they are in appearance and character, for their movements are retrograde, while the head is directed downwards and backwards, and the body held nearly in a perpendicular position. The majority of the Decapods have a muscular fin on each side, by whose aid they accomplish their movements in this apparently inconvenient posture, moving with great vivacity by sudden and irregular jerks. Pliny says, that the Loligines can fly, ‡ and the term is in truth as applicable to

* Rang. Man. pp. 26-28, and 120.

+ Rondel. Hist. des Poiss. i. 366.

Holland's Pliny, i. 250. Dr. Gould says of Loligo illecebrosa,"So swift and straight is their progress that they look like arrows shooting through the water."-Invert. Massachus. 318.

"The action of the powerful muscles in the terminal fins of the Calamaries must be aided in its effect upon the body by the elasticity of the internal pen or gladius. By these means they are enabled not only to propel themselves forward in the sea, but they can strike the surface of the water with such force as to raise themselves above it, and dart like the flying-fish for a short distance through the air. This is the highest act of locomotion, the nearest approach to flight, which any of the molluscous animals have presented."-OWEN's Lect. Invert. Anim. 348.

them as it is to the flying-fish,-being, by the vigour of their strokes, sometimes raised some feet above the level of the water. Thus Colonel Sykes mentions, that several specimens of Loligo sagittata leaped on board the vessel in which he was returning from India, while the wind was light and the sea calm.* In some species their motions are greatly assisted by the broad membranes that fringe the feet and connect them together at their base. Such membranes are found on the two inferior pairs of feet in the Loligopsis veranii, a species to which I shall call your attention more particularly hereafter. The Sepia aid and regulate their motions by the power they have of introducing air at option into the numerous cells of the backbone, and thus at will varying their proportionable weight to the sea in which they live. In the finless Octopods the feet, which are all winged with a membrane, become the sole organs of natation; for though Lamarck has chosen to maintain that this family can only trail themselves along the bottom of the shore they inhabit by means of their arms, we know very well that they are excellent swimmers, § propelling themselves by repeated strokes of their members, used much in the fashion that a frog uses its legs. Thus Professor Grant, when describing the Octopus ventricosus, says, "The animal swam several times hurriedly across the basin, always with its posterior extremity forward, by repeatedly striking forward the whole of its webbed arms at the same instant." || Mr. Cranch likewise informs us, that the finless Ocythoes. swim freely when out of their shell; having, as he adds, all the actions of the common Octopus of our seas. Octopods, however, do walk with equal ease, dragging their body, which is round and proportionably small, along the ground at the rate, it has been ascertained, of not less than seven feet in a minute. Should they wish to accelerate their pace, they inflate their body until it resembles a distended bladder; when, leaving go all hold and casting themselves forward, they roll over and over with great velocity, and often effect an escape which would otherwise have been impossible.¶

*Proc. Zool. Soc. 1833, pt. i. p. 90. + Good's Study of Medicine, iv. 424. Anim. s. Vert. vii. 583, 656.

$ Cuvier, Mém. i. 3.

The

|| Edinb. Phil. Journal, xvi. 313 See also Darwin's Journal, iii. 6. Yet Mr. Couch says of Octopus vulgaris, "It is scarcely capable of swimming; but it is a common amusement of boys to cause it to climb up the ascent of a pole or mast."-Cornish Fauna, 82.

¶ Blainville Man. de Malacologie, 149. The Naturalist, i. 190.

The testaceous Cephalopods, of which I have already told you there are only a few living representatives, appear to reside habitually at great depths in the ocean, whence they have the capability of ascending from time to time to the surface; yet their navigation there, so far from being a natural portraiture of a ship driven with sails and oars, is in all probability of a passive kind, or influenced only by the reaction of the respiratory currents when expelled by the funnel upon the surrounding medium. The specimen of Nautilus pompilius, brought home to this country by Mr. Bennet, and which has afforded Mr. Owen the means of preparing one of the best and most beautiful monographs in comparative anatomy, was taken on the coast of the New Hebrides floating on the surface, but just in the act of again sinking to the bottom,*-where lies its proper scene of action, for the chief locomotive organ is a flattened muscular disk that surmounts the head, analogous to the plane foot of the Gasteropods, which the Nautilus must also resemble in its manner of creeping. The description of Rumphius is very graphic. "When he thus floats on the water he puts out his head and all his barbs (tentacles), and spreads them upon the water, with the poop (of the shell) above water; but at the bottom he creeps in the reverse position, with his boat above him, and with his head and barbs upon the ground, making a tolerably quick progress. He keeps himself chiefly upon the ground, creeping sometimes also into the nets of the fishermen; but after a storm, as the weather becomes calm, they are seen in troops floating on the water, being driven up by the agitation of the waves. Whence one may infer, that they congregate in troops at the bottom. This sailing, however, is not of long continuance; for having taken in all their tentacles they upset their boat, and so return to the bottom."+-By what mechanism the Nautilus effects his ascent and descent is still conjectural. Dr. Hook supposed that it had the power of generating air into, and expelling it from, the deserted chambers, thus regulating its specific gravity in the same manner as fish by means of their air-bladders. Mr. Parkinson, in adopting this theory, assumes that the seat of the accumulation of the gaseous fluid is the membranous tube which runs through the siphuncular apertures of the septa and traverses all the chambers; and he believes that this tube has a corresponding power of dila

*Mr. Bennet says that the Nautilus, when at rest, either afloat or on the ground, covers its shell with the mantle, which is like that of the Cypræa. -New South Wales, ii. 409.

+ Owen's Memoir, 53.