Crustaceans

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series of tubes joined by hinges. Crustacean limbs usually terminate in a two-way branch, which in many cases you could call a claw. The head is segmented too although, as with the vertebrate head, the segmental pattern is more disguised here than in the rest of the body. There are five pairs of limbs lurking in the head, although it might sound a bit strange to call them limbs since they are modified to become antennae or components of the jaw apparatus. They are therefore usually called appendages rather than limbs. More or less invariably, the five segmental appendages of the head, reading from the front, consist of first antennae (or antennules), second antennae (often just called antennae), mandibles, first maxillae (or maxillules) and second maxillae. The antennules and antennae are mostly engaged in sensing things. The mandibles and maxillae are concerned with chewing, milling or otherwise processing food. As we proceed back along the body, the segmental appendages or limbs are pretty variable, the middle ones often consisting of walking legs, while those sprouting from the rearmost segments are often pressed into service doing other things such as swimming.

In a lobster or a prawn, after the usual five head segment appendages, the first body segment appendages are the claws. The next four pairs are walking legs. The segments bearing claws and walking legs are bunched together as the thorax. The rest of the body is called the abdomen. Its segments, at least until you reach the tip of the tail, are the 'swimmerets', feathery appendages that help with swimming, quite importantly so in some delicately graceful prawns. In crabs, the head and thorax have merged into a single large unit, to which all the first ten pairs of limbs are attached. The abdomen is doubled back under the head/thorax so that you can't see it at all from above. But if you turn a crab over, you can clearly see the abdomen's segmental pattern. The picture below shows the typical narrow abdomen of a male crab. The female abdomen is wider and resembles an apron, which it is indeed called. Hermit crabs are unusual in that the abdomen is asymmetrical (to fit into the empty mollusc shell which is its house), and soft and unarmoured (because the mollusc shell provides protection).

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