Crab_1

Crab. The Crab is the best known member of the short-tailed ten-footed Crustacea, i.e. the group Brachyura of the order Decapoda. In the main points of its anatomy it agrees with the Crayfish, and as this latter is simpler and more convenient for dissection, it is taken as the type of the order. The crab that is mostly used as food in England is known as Cancer pagurus; the commoner Shore Crab (Carcinus maenas) is also largely used. If a specimen of either be compared with a Crayfish, it will be seen that the larger anterior plate (cephalothorax) covers the whole of the upper or dorsal surface of the body, whereas in the Crayfish it occupies only the front half, while behind it occurs a long jointed abdomen or "tail." The plate, moreover, is broad in the Crab, and narrow and long in the Crayfish. The eyes are placed on stalks (peduncles) on the anterior margin, and look upward and forward instead of sideways. The abdomen in the Crab is much reduced in size, and is bent up under the main mass of the body (or thorax). The appendages are constructed on much the same plan as in the Crayfish, but those on the abdomen are rudimentary. The general anatomy of the soft parts agrees with that of the Crayfish, though in the Crab they are more closely packed together owing to its broader and shorter shape. This is notably the case with the nervous system in which the ganglia, except the first two pairs, are closely collected together into one group, instead of extending as a long chain of ganglia as in the Crayfish. The hard outer skin or shell of the crabs is not capable of growth, and is therefore periodically thrown off to allow of the expansion of the animal; this is known as "ecdysis," and before the new coat has again become hard the crabs are known as "soft crabs." The crabs are mainly inhabitants of the sea, and breathe by branchiae or gills; but there are several genera of Land Crabs, such as Gecarcinus or the Calling Crabs (Gelasimus), in which air is admitted directly into the branchial chamber where it aerates the branchiae. The development of the crabs shows that they are a more specialised group than the Long-tailed Decapoda (Inacrura), such as the Lobster; for in most cases the young pass through a "Zooea" stage in which they have a long, narrow six-jointed abdomen resembling that of the Lobsters, etc. In some of the Land Crabs, such as Gecarcinus, the development is direct, i.e. there is no metamorphosis. As regards geological distribution, the crabs make their first appearance in England in the Oolite rocks with the Palaeinachus longipes (H. Woodw.) of the Forest Marble. They are fairly abundant in the Tertiary rocks, and considerable numbers may be found in the London Clay of Sheppey; these belong especially to the genera Xanthopsis, Plagionotus, etc. Of recent crabs the small Pea Crabs (Pinotheres), the large Inachus of Japan which measures ten feet between the tips of the great claws, and the burrowing Racer Crab of Ceylon (Ocypoda) are among the more interesting forms. The "Hermit Crabs" belong to the Inacrura (sub-order Anomura), and are not, therefore, included here.