1. Macrura (Great-tailed), 2. Anomura (Peculiar-tailed),
3. Brachyura (Short-tailed);
the first containing lobsters, shrimps, &c.; the third the typical crabs, such as the shore crab and the edible crab; while in the second are placed those species of crabs which have been regarded as intermediate in character. Thus, in the Anomura we find decapods in which the abdomen, though not so well developed as in the Macrura, is either permanently extended or is capable of being extended and used for swimming as occasion requires. The hindmost legs, also, are not well developed and adapted for walking, but are employed only as organs of prehension; and, as is the case with the first sub-order, there are often two pairs of well-developed antennæ.
In this sub-order of ‘queer tails’ we find the Soldier or Hermit Crabs, and those flat-bodied crabs that live almost exclusively on the surface of stones, and are hence known as Stone Crabs; but as opinion now seems inclined against the formation of a special suborder for these creatures, we shall briefly deal with them as a first section of the Brachyura.
The Stone Crabs are extremely interesting creatures, and the observation of their habits, both in and out of the water, is particularly entertaining and instructive. One species—the Broad-Clawed Porcelain Crab (Porcellana platycheles), shown on [Plate VI.]—is very abundant on all our rocky coasts, and may be found in immense numbers near low-water mark.
Turn over some of the large encrusted stones that strew the beach among the rocks, and you are almost sure to find numbers of these little crabs clinging to the freshly exposed surface. A few of them may remain perfectly still, and exhibit no sign of surprise on their untimely exposure to the light; and these, on account of their small size, the closeness with which they apply their flattened bodies to the encrusted stone, and more than all to the protective colouring of their dingy bodies, which so closely resembles that of the surface to which they cling, may well be overlooked by the inexperienced collector. But the majority of them will immediately scamper away in their own peculiar fashion towards the edge of the stone, and rapidly make their way to what is now the under side. As they progress with a hasty, sliding movement they never for one moment loosen their firm hold on the rough surface of the stone, but keep both body and limbs in close contact with it, clinging hard by means of their pointed claws as well as by the numerous hairs and bristles with which their appendages are liberally fringed.
Attempt to pull one from its hold, or even take other than the gentlest means to arrest its progress, and you will probably find that it suddenly parts company with one of its broad claws in its endeavour to escape; and, unless some special precautions be taken to remove these crabs, it is possible that quite half the specimens taken will have been damaged in this way during their struggles to escape. If, however, you gently thrust the point of a penknife beneath the body, and then apply the thumb above, you may lift them from a stone without injury. Another plan is to press a frond of smooth sea weed as closely as possible to the surface of the stone in the front of the crabs, and then allow them to crawl on to it, or cause them to do so if necessary. The piece of weed, with crab or crabs attached, may then be bagged for future examination.
On turning over the Broad-clawed Crab its under surface will be seen to be perfectly smooth, with an appearance closely resembling that of white porcelain. Its foot-jaws, also, are proportionately large, and closely fringed with hairs; and the last pair of legs, which are very slender in build, are folded closely beneath the body. Further, the abdomen is wide, composed of six distinct movable segments, and terminating in a tail-fin composed of five fringed plates.
Drop the crab into water, and it will immediately extend its abdomen, which it will flap sharply under its body somewhat after the manner of lobsters and shrimps, and thus swim backward by a series of jerks as it sinks to the bottom. On reaching the bottom it instantly grasps the solid material, applies itself closely to the surface, and glides away into the nearest chink it can find.
As one observes the nature and movements of these interesting little crabs one cannot fail to see how beautifully their form and structure are adapted to their habits. They are peculiarly constructed for abode in narrow chinks and crannies, and for feeding on the small forms of life that inhabit such sheltered places. Their legs move in the plane of their flattened bodies, and as they glide among the confervæ and other low forms of life that encrust the stones of the beach they feel their way by, and are possibly also guided by the sense of smell located in, their long outer antennæ, while the close fringes of their claws and foot-jaws form admirable sweep-nets by means of which the little animals that form their food are swept towards the mouth.