Annelida

__**Annelida**__

The most complex worms are the segmented worms, called annelids. These worms have bodies with many segments running from the head to the tail. Most live in the ocean, some in fresh water, and of course the earthworm in the soil under our feet. They are more advanced than the other worms because they have a coelom. A coelom is a liquid-filled space or body cavity that holds the internal organs. On the outside of their bodies they have setae or bristles structures they use to burrow into the soil. They have a definite closed circulatory system, where the blood does not leave the hearts, veins or arteries. They have a complete digestive system, that starts at the mouth, moves through the pharynx, esophagus, crop, gizzard, intestine, and anus. they have a simple ganglian brain in the head that is attached to nerves that run down both the dorsal and ventral sides and control the muscles for movement. They have an excretory system that includes nephridia in each segment to rid the body of liquid wastes and the intestine to rid the worms of solid wastes. They do not have a respiratory system. They must remain moist so that they can absorb oxygen through the skin.

The Annelida are a medium sized phylum of more than 9,000 species of worms. Most species prefer aquatic environments, but there are also a number of well know terrestrial species. Only a few species of annelids are commonly known to human beings, these include the delightful Rain, Dew or Earthworms that work so hard to make our soils healthy, the Ragworms and Lugworms used by marine fishermen and the much smaller Tubifex or Red worms used by aquarists to feed their fish. In many countries people are still familiar with Medicinal leeches, and people who live closer to nature are naturally more familiar with a much wider range of Annelids than those who live in cities.

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The earthworms, of which there are many species, are exceedingly important in soil creation, particularly in temperate areas. Without them, agriculture and perhaps the whole of human society as we know it would never have evolved. Like so much of the unnoticed invertebrate world earthworms are essential to our very existence. In marine environments the numerous species of Polychaetes play a fundamentally important role in the maintenance of food chains and the whole ecological balance of the seas, thus supporting the seemingly endless stocks of fish we like to eat. Annelids range in size from the Giant Earthworms, of which //Michrochaetus rappi// (//Michrochaetus michrochaetus//) is the largest, this magnificent animal has an average length of 1.36 m (54 ins) and a record breaking specimen has been recorded that measured 6.7 meters (22 ft) in length, it was 2cm (0.8 ins) in diameter. Larger worms have been reported but not scientifically proven. The smallest Annelid known to science is //Chaetogaster annandalai// which is full grown at 0.5 mm (0.02 ins). Annelids have two main modes of existence, they either live rather quietly in holes or they live more active lives. The basic Annelid body plan is one of a head followed by a long thin body of numerous similar segments ending in a small tail. The head consists of a mouth (prostomium) and sometimes a peristomium., as it is not really a tail. Annelids are coelomate animals meaning they have a true coelom within their body. They have sets setae attached to each body segment, and these can be simple and small as in the Earthworms or complex and varied as in many Polychaetes. The head is often reduced and difficult to distinguish in the hole living species, but may be easily recognized, with eyes and other sensory devices in those species living a more active life.