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various contrivances are employed to adapt them to the specific gravity of the liquid. Among those not so adjusted are a large portion of the shell-covered Mollusca, which by reason of the weight of the shell are for the most part collected on the beds of lakes, rivers or the sea. Within reach of the tidal agitation, where the water is well charged with air, several races of Mollusca attach themselves-as the young oyster by its lower shell fixes itself permanently and furnishes support to others. Anomia is attached by a sort of plug, Pinna and the young Mytilus by a byssus spun through the agency of a muscular organ called the foot, which in all the races mentioned loses its usual function of locomotion.

The attachment of Actinia by its broad base, and of Patella by its circular mantle, is not of so permanent a character. Perhaps this is also the case with the suspensional ligament of Lingula and Terebratula. More curious examples are furnished by the cusps for adhesion which cover the arms of Cephalopoda1, both recent and fossil, and the sucking surfaces of Remora.

Flotation is accomplished in some of the marine races by very obvious contrivances. In the large Medusida whose figure is hemispherical, a steady position with the mouth downward is maintained by

1 The Polypus of Aristotle and Homer.

the help of fringes dependent from the edge; in Physalia and Velella, an air-bladder is raised above the general mass and catches the wind; in Nautilus the complicated air-chambers of the shell with a pipe through them all, appears to be another and more remarkable contrivance, which belonged to the group through all geological time; the air-bladder of fishes has been often celebrated in this respect; and we may add the thick oily integument of the Cetacea which at once balances their heavy bones, maintains the heat of their bodies in the polar oceans, and gives the boat-like form which fits them for motion in water.

Swimming, though chiefly exhibited by the Articulated and Vertebrated divisions of animals, is sometimes exercised by the other types. Some of the Infusoria swim by the aid of cilia on the periphery, or about the mouth, others by altering the form of the body. Some of the Cephalopoda employ the hinder expansion of the body as well as the arms and hydraulic funnel, for motion. But it is in the Articulata with jointed feet and among the Vertebrata, that swimming by special organs arrives at the most curious and diversified excellence.

We may remark in regard to nearly all of these special organs that the general idea carried out in them is that of striking the water forcibly with an

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expanded organ, which can again be drawn through the water more slowly and with less surface to be again expanded and presented for a fresh effort.

Thus whales and fishes, the Triton and the young Frog, all work the expanded tail after the manner which men use in sculling a boat; while the beautiful boat-beetle (Notonecta) floating on his back rows himself with long jointed, flattened oars, fringed with stiff bristles. When he strikes for motion, the oars extend themselves, and the bristles catch the water and widen the instrument; but in returning the oar is bent, and turned edgeways, and the bristles pass easily through the liquid—θαῦμα ιδέσθαι wonderful thing to behold.

The paddles of the Turtle and the pectoral fins of the Whale are instruments having the same general property of varying the extent of surface exposed to the water. A similar result is obtained by the very different contrivance of the webbed feet of the Swan and the Otter-and that remarkable organ for backward swimming or rather leaping in the water, the bending tail of the Lobster and Crawfish, which besides has its five plates endowed with lateral motion, so as to fold up into a small area.

It is interesting to observe this same structure in the Lobsters (Glyphia) of the ancient oolites, and to note in the Speeton clay and the London clay the

flexible tail of the Crayfish (Palinurus), distinct from that of the true Astacidæ in those remote ages as it is still found to be in the corresponding groups living together in the ocean. Equally curious to notice in the Ichthyosaurus and Teleosaurus of the Lias the same principles of watery propulsion, by the tail and lateral paddles, as in the Fish, Turtles and Alligators of the present period. One general idea runs through the whole series, the propulsive power is worked by a system of leverage placed more or less retrally, aided by a directing power placed forward. If Plesiosaurus appear to be an exception, the anomaly may be regarded as compensated by the extension of the neck, for thus in fact the dynamic centre is really retral, as to the general figure, while the approximation of the paddles gives remarkable power of quick turning, suitable to its way of life.

Diving. The same backward position of the propulsive organs is even more remarkable in the case of the diving birds, such as Colymbus, Uria, Aptenodytes, whose general form resembles that given by mechanicians to ships constructed for swift motion, and suggests to every observer the idea of the solid of least resistance in water. The same peculiarities appear in a different manner in the very different group of diving insects, such as Dytiscus, Colymbetes, and Hydrophilus, and may be found

again in some of the ciliated Infusoria like Trichoda anas. But nowhere does it reach so high a degree of perfection in air-breathing animals, as in the waterbirds alluded to; in which if we examine the form of the head, the texture and arrangement of the feathers, and the elegant bony apparatus of the chest, admitting of much expansion and much contraction, we see how well, in every part, the whole living machine is fitted for its peculiar purpose, and calculated to follow with success even the fishes which glide so easily through their native element.

Boring into solid substances, by which, in contrast with all these cases, some marine tribes excavate for themselves a dwelling, which they can never leave, is accomplished by several quite different contrivances. Digging into sand is effected by some bivalve Mollusca, as Lutraria and Mya, by the action of the same muscular mass called a foot, which enables the Cockle to spring some distance, and the Unio to cut its way through the slime. Pholades bore into chalk and much harder limestone by turning round their body and its sharply serrated shelly covering on the pivot of the foot; Teredines in a similar manner destroy the substance of ships, and Limnoria terebrans, a Crustacean, eats its way into the wooden piles of harbours. So in early geological times, perforating Modiola and Pholades are traced by their

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