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Wednesday, October 12, 2011

Annelids

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Phylum of soft-bodied, bilaterally symmetrical, segmented worms are known as Annelids. The name Annelida is derived from the Latin word annellus, which means little ring, and refers to the ringlike appearance of the body segments. Annelids range in size from tiny aquatic worms, less then half a millimeter long to giant earthworms more than meters long. They also vary greatly in color, patterning, number of bristles, and other superficial features. However, most annelids are quite wormlike in appearance. Annelids are found throughout the world, from deep ocean bottoms to high mountain glaciers. They live in protected habitats such as mud, sand, rock crevices and tubes that they secrete themselves. They also live in and among other invertebrate animals, such as sponges. Annelids include earthworms, polychaete worms, leeches, and some other 1,000 wormlike invertebrate animals with well-developed segmentation. These species are grouped into three main classes the Polychaeta (clamworm; mostly marine), the Oligochaeta (earthworm; mostly terrestrial), and the Hirudinea (leech; mostly freshwater).


A fundamental characteristic of the phylum annelida is the division of the body into a linear series of cylindrical segments or metameres. Each metamere consists of a cross section of the body wall and a compartment of the body cavity with its internal organs. The external divisions, which may be seen in the common earthworm, correspond to the internal divisions. Metamerism increases the efficiency of body movement by allowing the effect of muscle contraction to be extremely localized, and makes it possible for the development of greater complexity in general body organization. Components of all the major body systems, excretory, circulatory, reproductive, and nervous are repeated in each segment. The annelid body consists of a head region; a trunk, made up of metameres; and an unsegmented terminal region called the pygidium. Besides being segmented, the body wall of annelids is characterized by being made up of both circular and longitudinal muscle fibers surrounded by a moist, cellular cuticle that is secreted by an epidermal epithelium. The body wall is covered with epidermis overlaid with a thin, pliant cuticle secreted by the epidermal cells. The body wall consists of well developed, segmentally arranged muscles used for crawling and swimming movements. Most annelids possess short external bristles called setae, or chaete, composed of chitin (tough, protective substance forming portions of the cell wall). Setae are used to grip the soil, to hold the animal in a tube, or to increase the surface areas of appendages for swimming.


Characteristics of the circulatory system vary within the phylum. The blood usually contains hemoglobin, a red oxygen-carrying pigment; some annelids have a green oxygen-carrying pigment, and others have unpigmented blood. The circulatory system is closed and confined within well-developed blood vessels. Blood flows towards the head through a contractile vessel above the gut and returns to the terminal region through vessels below the gut; it is distributed to each body compartment by lateral vessels.


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The digestive tract is divided into several well-developed organs pharynx, esophagus, crop, gizzard, and intestine. The system consists of an unsegmented gut that runs through the middle of the body from the mouth, to the anus. The gut is separated from the coelom (cavity within the body of primitive worms, formed by the splitting of the embryonic mesoderm into two layers). The segmented compartments of the coelom are usually separated from by thin sheets of tissue, called septa, which are perforated by the gut and by blood vessels. Annelids have evolved structures and behaviors that allow them to eat a variety of foods. One feeding organ that has evolved many different forms in different groups of annelids is the pharynx, or the muscular front end of the digestive tube. If the worm is punctured, it loses its ability to move properly, since functioning of body muscles is dependent on the maintenance of the fluid volume in the coelom.


Some aquatic annelids have thin walled, feathery gills through which gasses are exchanged between the blood and the environment. Most annelids have no special organs for gas exchange, and respiration occurs directly through the body wall. Many annelids take in oxygen and give off carbon dioxide through their skin.


The nervous system typically consists of a primitive brain, located in the head region and a pair of cephalic ganglia (specific group of nerve cells), attached to double nerve cords that run the length of the animal along the ventral body wall, with ganglia and branches in each segment. Sense organs are best developed in free- living marine species of annelids. Annelids have a combination of tactile organs, chemoreceptors, balance receptors, and photoreceptors. A few forms have fairly well developed eyes, including lenses.


Reproduction is either sexual or asexual. Asexual reproduction is carried out by fragmentation, budding or fission. Among sexually reproducing annelids hermaphrodites (earthworms having male and female reproductive organs), are common, but most species have separate sexes. Fertilized eggs of marine annelids usually develop into free swimming larvae. Eggs of terrestrial forms are inclosed in cocoons and hatch as miniature versions of the adult. The ability to regenerate lost body parts is highly developed in many annelids.


Annelids have two major groups of muscles in their body walls. One group called longitudinal muscles run from the front of the worm to the rear. When these muscles contract they make the worm shorter. Another group of muscles runs in circles around the body of the worm. When these muscles contract they make the worm skinnier. Burrowing annelids use their muscles to force their way through heavy sediment.


Like other animals, annelids produce two kinds of waste. Solid wastes pass out through the anus. Wastes resulting from cellular metabolism are eliminated by nephridia (simple tube-shaped excretory organs). A pair of nephridia in each body segment removes waste products from the bodies fluids and carries them outside.


Most segmented worms found in marine environments represent a major evolutionary branch of annelids, Class Polychaeta. This name means “many bristles.” They are abundant from the interidial zone to depths of over 16,450 feet. They range in length from less than 1/8 inch to more than feet, but most are from to 4 inches. Their colors are often brilliant, and some species are iridescent. The class has usually been divided on the basis of mode of existence into two groups, the errantia and the sedentaria. Errant polychaetes include actively crawling or swimming forms, which spend time in burrows or crevices, or under rocks on the seashore. Sedentary polychaetes are usually adapted to living permanently in tubes or burrows; many such as the lugworm attach themselves to rocks or piers. Species living in rocky environments have leg-like parapods (bristles), used in walking. In burrowing species, parapods function as digging paddles. In some worms the parapods are short and combine with peristaltic contractions to move the worm through the mud. Polychaete feeding styles vary from eating organic material that settles on the surface of the muddy substrate, to filtering plankton and detritus from the water using feathery tentacles, to eating their neighbors. On hard substrates, such as rocks or corals, species of polychaete worms build temporary or permanent tubes, where they lead stationary lives by filtering the water for suspended food.


The second class of annelids is most common in fresh water or on land. The oligochaetes, members of the Class Oligochaeta, usually have few setae on each segment. The name means “few bristles.” These species completely lack parapods. The members of this class range in length from about 1/ inches to 10 feet. Oligochaetes occur in a variety of habitats throughout the world. Most are burrowers in the soil, but the class also includes worms that inhabit wells, marshes, and swamps. Other species live under rocks on the seashore, in the leaves of the tropical trees and vines, on the surface of glaciers, or on the gills of freshwater crayfish. Like polychaete, oligochaetes have bodies divided into segments. These worms are hermaphroditic, in which the sex is indiscriminate.


This class includes the 500 species of leeches, flattened, predacious, or parasitic annelids equipped with suckers used for creeping. The Class Hirudinea is thought to be a main branch of the oligochaete line of annelids. These range in length from about ½ inch to 8 inches. They are commonly black, brown, green, red, and may have stripes or spots. They are primarily freshwater annelids, but some live in the ocean and some in moist soil or vegetation. The majority are predators on small invertebrates; most swallow prey whole, but some suck the soft parts from their victims. These lack bristles and their external rings do not correspond to the internal segmentation. They are also hermaphroditic and carry their developing young on their bodies.


Annelids are of great ecological significance in marine and terrestrial habitats. Polychaetes and especially their larvae constitute important links in food chains in the ocean. Oligochaetes perform an essential task in conditioning soil. Hirudinea are used in bloodletting. They are also important in scientific research, especially in trying to understand the complexities of the nervous system.





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