Crustaceans

HABITAT:

The group crustaceans are the sub group of the arthopods and it means ' hard shell'. They have many joined legs. They have no bones and a shell covers it's body ( exoskeleton ). Some live on land, some at sea, some both. They include crabs, lobsters, barnicles, shrimp. There are around 40,000 thousand groups of crustaceans. The largest in the giant spider grab of Japan, measuring upto 3.5 metres with outstretched claws. The smallest of crustaceans may be less than 1 of a cm

DESCRIPTION:

They generally feed on tiny aquatic organisms ( at least the small ones do ) and they in turn are eaten by fish for food. Thus they are the link between the small food producing microorganisms and the larger aquatic creatures Some of them ( especially crabs and lobsters ) are eaten by people and some like barnicles and wood worms are harmful to man .

Barnicles attach themselves to a staceans has three parts - the head, thorax, the abdomen. Each part has segments. These crustaceans have antennae, that rise from the head and the head has a mouth, six jaws ( three pairs ) and eyes. The eyes may be either on the head, or on stalks. Each segement of the thorax has two legs or one pair. Most crustaceans have around 12 to 28 pairs of legs ( give or take a few ). They generally use their legs to swim or walk , while some of the legs are to hunt or fight. The abdomen generally lack legs, though lobsters and shrimp do have legs there. They are used generally to carry eggs. The external skeleton is to protect the internal organs. These animals may not have heart, and body movement promotes circulation of blood These animals have small brains . A nerve cord extends on to the body, and nerve cells perform operations. Most of them breathe through gills, or sometimes the skin. Most have compound eyes that can easily sense any movment. They sometimes have simple eyes. They may have tiny hairs that are sensitive to touch, smell, taste.

They generally deposit sperms on the females shell for reproduction. The sperm then fertilizes and the eggs are carried with the female till hatching. Upon hatching they are in larvael form, and go through several changes before looking like their parents. Since their skeletons do not expand, they are shed and regrown. This is moulting. Before this a soft new shell forms under the old one and the old shell is broken out of. The animals swells up before the new shell hardens. Each moulting adds a new segment, thereby new legs and becomes one step closer to an adult. In some, the whole process is over in one stop, while in others it is not so. If certain parts are damaged, they can be regrown