Friday, January 13, 2012
The invertebrates are animals with no back bone. Some have no skeleton at all, but many have external skeletons or shells that give them a rigid shape and provide anchorage for their muscles. There are about thirty major groups or phyla of invertebrates, although the great majority of species belong to just two phyla- the Mollusca and the Arthropod. The later includes the insects, spiders, crustaceans and several other groups, all of which have segmented bodies and jointed legs. The majority of invertebrates are quite small, but examples of the largest the giant squid are known to have been as much as 15m long and possibly well cover a tone in weight, which is considerably more than most vertebrates.
Molluscs Molluscs are soft bodied animals with no sign of segmentation. Most have chalky shells. There are over 60 000 species, three quarters of which are slugs and snails. Bivalves, such as cockles and mussels, and the squids and their relatives make up most of the rest.
The crustaceans include animals as diverse as lobsters, barnacles and water fleas. All have hard outer skeletons impregnated with lime, and several pairs of legs. Most of the 30000 or so species are marine, but many live in freshwater and woodlice live on land.
The insects are the only type of invertebrates to have wings, although they are not all winged. Adults have three main body sections and three pairs of legs. Well over one million species are known, living in almost every habitat, although there are only a very few that live in the sea.
Spiders belong to the arthropods family. They have two main body section and four pair of legs. All are carnivorous and kill their prey usually insects with venom. Many species snare their prey with extremely intricate silken webs.