armed. M. Jobert, in examining the breathing apparatus of land-crabs, has found that of Uca una to be the most complete. There is a regular movement of inspiration and exspiration to keep the air from stagnating in the breathing chamber, and between the third and fourth and the fourth and fifth limbs there are small supplemental inspiratory orifices coated externally with long hairs (see Sp. Bate, Brit. Assoc. Report,' 1880).

6

Family 2.-Ocypodidae.

The carapace is in general moderately convex, cancroid or trapezoidal, with the antero-lateral margins straight or arcuate, the branchial regions not greatly dilated. The front' is of moderate width or very narrow. The orbits and eye-stalks are of moderate size or greatly developed. The third maxillipeds have the fifth joint articulated at the front inner or rarely at the front outer angle. The chelipeds in the adult males are in general of moderate size, sometimes slender and elongate. The seventh joint in the walking legs is stiliform, without strong spines. The pleon does not always cover the whole width of the sternum between the last pair of legs.

The species are generally small, littoral, or inhabitants of shallow water, but are not unknown from considerable depths. There are nearly forty genera assigned to the various subdivisions of this family.

Ocypõde, Fabricius, 1798, has the orbits very large and open, extending all along the anterior margin on either side of the narrow and deflexed 'front.' The eye-stalks are large, with a short basal joint, the terminal part often prolonged distally as a spine or tubercle, the large corneæ covering much of the lower surface of this terminal joint. The chelipeds in the adult male are unequal and well developed, and usually the palm has a vertical series of short raised lines or tubercles on the inner surface, which form a stridulating ridge.

As the name swift-of-foot implies, these Crustacea are especially noted for their rapidity of movement. They are just the opposite of some of the strong-armed, thick

shelled, slow-moving Cancrida. On wind-swept stretches of sandy beach, and coloured like the sand, they sometimes seem rather to be borne on the wings of the wind than to run. Also with their compressed lancet-like fingers they are extremely dexterous in digging into the sand. They burrow holes an ell deep, generally perpendicular, and from these they wander far, when the tide is out, in search of food. Krauss observed in South Africa the species Ocypode ceratophthalmus (Pallas), and others. and he says that while they are busy hunting, every now and then they look carefully round, raising their stalkedeyes upright, and standing on tiptoe. At the slightest movement towards them they run with uncommon rapidity to the nearest hole, or, if the danger is too close, press themselves flat on the sand, till an attempt is made to seize them, and then off they dart. In running they carry their bodies high, doubling and dodging with such speed and cunning that it is a difficult matter to lay hold of them. When the tide comes up, they are enclosed in their flooded burrows, and as soon as the waves retreat, they are busily employed in clearing them, shovelling out the wet sand and heaping it at some little distance off. The American species, Ocypode arenaria (Catesby), is described by Professor S. I. Smith as having precisely similar habits. According to his observation it lives largely upon the Amphipods of the genus Talorchestia, known as 'beach-fleas,' which inhabit the same localities. 'It will lie in wait,' he says, 'and suddenly spring upon them, very much as a cat catches mice. It also feeds upon dead fishes and other animals that are thrown on the shore by the waves.'

It is of this species, under the synonym of Ocypoda rhombea, Fabricius, that Fritz Müller speaks in his memorable work For Darwin.' 'In the swift-footed Sand-crabs (Ocypoda),' he says, '-which are exclusively land animals, that can scarcely live in water for a single day, and which in far less time than that are reduced to a state of complete collapse in which all voluntary movements ceasethere has long been known a peculiar arrangement con

CROSS-EXAMINING A CRAB

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nected with the third and fourth pairs of legs, but that these had anything to do with the branchial cavity was not suspected. These two pairs are pressed more closely together than the rest. The opposed surfaces of their basal joints, that is, the hinder surface in the third, and the front surface in the fourth pair, are flat and smooth, and their margins are closely fringed with long, sheeny, peculiarly formed hairs. Milne-Edwards, who compares them to articular surfaces, as their appearance warrants, thinks that they serve to diminish the friction between the two legs. On this supposition the question arises why precisely in these crabs and only between these two pairs of legs such a provision for diminishing friction is necessary, not to mention that it leaves unexplained the singular hairs, which must augment instead of diminishing friction. While, then, I was bending to and fro in ever so many directions the legs of a large Sand-crab, in order to see in what movements of the animal friction occurred at the place in question, and whether perhaps these were movements often recurring and of special importance to it, I observed, when I had stretched the legs far apart, a round opening of considerable size between their bases, through which air could easily be blown into the branchial cavity or even a slender probe be introduced. The aperture opens into the branchial cavity behind a conical tubercle, which stands above the third foot at the place of a branchia which is wanting in Ocypoda. It is laterally bounded by ridges which rise above the articulation of the legs and to which the lower edge of the carapace is applied. Also outwardly it is overarched by these ridges with the exception of a narrow slit. Over this slit extends the carapace, which just at this point projects further downwards than elsewhere, and so a complete tube is formed. While Grapsus always admits water to its branchia only from in front, in Ocypoda I saw it also streaming in through the just described aperture.'

For its details about one particular crustacean such a passage is interesting, but it is far more important as a lesson in scientific observation. There are numbers of