Search over 40,000 articles from the original, classic Encyclopedia Britannica, 11th Edition.
|
See also:COYSEVOX, ,See also: See See also: broad and flattened third pair of foot-jaws. The abdomen is usually narrow and triangular in the See also:males, but in the See also:females it is broad and rounded and bears appendages to which the eggs are attached after spawning (fig. I). As in most Crustacea, the See also:young of nearly all crabs, when newly hatched, are very different from their parents. The first larval See also:stage is known as a Zoea, this name having been given to it when it was believed by naturalists to be a distinct and inde- pendent See also:species of See also:animal. The Zoea is a See also:minute transparent organism, swimming at the See also:surface of the See also:sea. It has a rounded body, armed with long spines, and a long segmented tail. The eyes are large but not set on stalks, the legs are not yet See also:developed, and the foot-jaws form swimming paddles. After casting its skin several times as it grows in size, the young crab passes into a stage known as the Megalopa (fig. 2), also formerly regarded as an See also:independent animal, in which the body and limbs are more crab-like, but the abdomen is large and not filled up. After a further See also:moult the animal assumes a form very similar to that of the adult. There are a few crabs, living on See also:land or in fresh See also:water, which do not pass through a See also:metamorphosis but leave the See also:egg as See also:miniature adults. Most crabs live in the sea, and even the land-crabs, which are abundant in tropical countries, nearly all visit the sea occasionally and pass through their See also:early stages in it. Many See also:shore-crabs living between See also:tide-marks are more or less amphibious, and the See also:river-crab of See also:southern See also:Europe or Lenten crab (Potamon edule, better known as Thelphusa fluviatilis) is an example of the fresh-water crabs which are abundant in most of the warmer regions of the See also:world. As a See also:rule, crabs breathe by gills, which are lodged in a pair of cavities at the sides of the carapace, but in the true land-crabs the cavities become enlarged and modified so as to See also:act as lungs for breathing See also:air. Walking or crawling is the usual mode of locomotion, and the See also:peculiar sidelong gait See also:familiar to most See also:people in the See also:common shore-crab, is characteristic of most members of the See also:group. The crabs of the family Portunidae, and some others, swim with great dexterity by means of their flattened See also:paddle-shaped feet. Like many other Crustacea, crabs are often omnivorous and act as the scavengers of the sea, but many are predatory in their habits and some are content with a See also:vegetable See also:diet. Though no crab, perhaps, is truly parasitic, some live in relations of " commensalism " with other animals. The best known examples of this are the little " See also:mussel-crabs " (Pinnotheridae) which live within the shells of mussels and other bivalve See also:mollusca and probably See also:share the See also:food of their hosts. Some crabs live among See also:corals, and one species at least gives rise to hollow swellings on the branches of a See also:coral like the " See also:galls" which are formed on See also:plants by certain See also:insects. Another crab (Melia tesselata) carries in each of its claws a living sea-See also:anemone which it uses as an animated weapon of See also:defence and an See also:implement for the See also:capture of See also:prey. Many of the sluggish spider-crabs (Maiidae) have their shells covered by a See also:forest of growing sea-weeds, zoophytes and See also:sponges, which are " planted " there by the crab itself, and which afford it a very effective disguise. Many of the larger crabs are sought for as food by See also:man. The most important and valuable are the edible crab of See also:British and See also:European coasts (See also:Cancer pagurus) and the See also:blue crab of the See also:Atlantic See also:coast of the See also:United States (Callinectes sapidus). Among the Anomura, the best known are the See also:hermit-crabs, which live in the empty shells of Gasteropod Mollusca, which they carry about 'with them as portable dwellings. In these, the abdomen is soft-skinned and spirally See also:twisted so as to See also:fit into the shells which they inhabit. The common hermit-crab of the British coasts (Pagurus or Eupagurus Bernhardus) is sometimes called the soldier-crab from its pugnacity. Small specimens are found between tide-marks inhabiting the shells of periwinkles and other small molluscs, but the full-grown specimens live in deeper water and are usually found in the shell of the whelk (Buccinum). As the crab grows it changes its dwelling from See also:time to time, often having to fight with its See also:fellows for the See also:possession of an empty shell. Sometimes an annelid See also:worm lives inside the shell along with the hermit and often the outside is covered with zoophytes. In some species, as in the British Eupagurus prideauxi, a sea-anemone is constantly found attached to the shell, profiting by the active locomotion of the crab and probably sharing the crumbs of its food, while it affords its See also:host See also:protection by its stinging See also:powers. In tropical countries the hermit-crabs of the family Coenobitidae live on land, often at considerable distances from the sea, to which, however, they return for the purpose of hatching out their spawn. The large robber-crab or See also:cocoa-See also:nut crab of the Indo-Pacific islands (Birgus latro), which belongs to this family, has given up the See also:habit of carrying a portable dwelling, and the upper surface of its abdomen has become covered by shelly plates. The stories of its climbing See also:palm-trees to get the See also:fruit were long doubted, but it has been seen, and even photo-graphed in the act. (W. T. CA.) Fm. 2.-Zoea of Common Shore-Crab in its second stage. r, Rostral spine; s, Dorsal spine; m, Maxillipeds; t, Buds of thoracic feet; a, Abdomen. Additional information and CommentsThere are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click, and select "copy." Then paste it into your website, email, or other HTML. Site content, images, and layout Copyright © 2006 - Net Industries, worldwide. |
|
|
[back] COYPU |
[next] CRABBE, GEORGE (1754-1832) |