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Annelida

The phylum comprising the multisegmented, invertebrate wormlike animals, of which the most numerous are the marine bristle worms and the most familiar the terrestrial earthworms. The Annelida (meaning little annuli or rings) include the Polychaeta (meaning many setae); the earthworms and fresh-water worms, or Oligochaeta (meaning few setae); the marine and fresh-water leeches or Hirudinea; and two other marine classes having affinities with the Polychaeta: the Archiannelida (meaning primitive annelids), small heteromorphic marine worms, and the Myzostomaria (meaning sucker mouths), parasites of crinoid echinoderms. These five groups share few common characters and little resemblance except that most have a wormlike body. Typically they are bilaterally symmetrical, lack a skeleton, and have a short to long linear body divided into rings or segments, which are separated from one another by transverse walls or septa. The mouth is an anteroventral or anterior vent at the forward end of the alimentary tract, and the anus posterodorsal or posterior at the hind end of the gut. Hirudinea Myzostomaria Oligochaeta Polychaeta

The linear series of segments, or metameres, from anterior to posterior ends constitute the annelid body. These segments may be similar throughout, resulting in an annulated cylinder, as in earthworms and Lumbrineris. More frequently the successive segments are dissimilar, resulting in regions modified for particular functions. Each segment may be simple (uniannular) corresponding to a metamere, or it may be divided (multiannulate). The total number of segments varies from five to several hundred. Segments may have lateral fleshy outgrowths called parapodia (meaning side feet), armed with special secreted bristles or rods, called setae and acicula; they provide protection and aid in locomotion. Setae are lacking in Hirudinea and some polychaetes. The body is covered by a thin to thick epithelium which is never shed.

Annelids have sense organs of many kinds. Most conspicuous are those on the anterior and on parapodia. Eyes which function as photoreceptors may be variously developed. Feelers or tactoreceptors are frequently on the prostomium (a lobe of skin projecting from the first body segment) as antennae; on anterior segments as long filiform or thick fleshy tentacles; and on parapodia as cirri, papillae, scales, or tactile hairs. Cilia in bands or clusters or in grooves occur in specific patterns. The most conspicuous receptors are the large nuchal organs of amphinomid polychaetes, fleshy, folded, paired organs surrounding the cephalic structures.

Setae detect changes in the environment through their basal attachments. Shallow-water species often have short, strong, resistant setae, whereas abyssal species have long, slender, simple setae. Each organ is unique and well adapted to its role in the development, growth, protection, and reproduction of the species involved.

Chromatophores are cell clusters which change their shape and size to conform to the shadows of the animal' s background, responding to changes in light intensities, and therefore are generally protective. They are well developed in translucent pelagic larvae which exist at the surface of the sea; they screen damaging intensities of light from delicate tissues. Oligochaetes and hirudineans, which generally lack eyes or special light receptors, are sensitive to light changes through the surface epithelium.

The alimentary tract of annelids is a straight or sinuous tube, consisting of mouth, pharynx, esophagus, stomach, gut, and pygidium or anus. The mouth may be a simple anterior (oligochaetes) or anteroventral (many polychaetes) pore provided with highly complex organs or accessory parts. Accessory organs include the grooved palpi of many polychaetes, which direct food to the mouth or also select and propel nutrients along ciliated tracts. The building organs and cementing glands of some tubicolous annelids are associated with the mouth; they select inert particles for shape and size and attach them in specific patterns to a basic secreted mucoid membrane, resulting in tubes which are highly characteristic.

The mouth is followed by the buccal cavity which may be modified as a phoboscis or saclike eversible pouch. The buccal cavity may be followed by a short to long muscularized eversible proboscis which captures and breaks up or compresses food particles. Its inner walls may be fortified by papillae or hard gnaths or jaws. A short esophagus leads to the muscular stomach or digestive region. Lateral ceca or pouches may be present, along esophageal and stomach portions, to increase the amount of surface for secretion and digestion, especially in short-bodied worms. Peristaltic (clasping and compressing) movements are rhythmic and result in the food bolus being digested, and wastes separated and pushed into the gut. In nonselective-feeding annelids the gut may be distended with great amounts of inert materials; in selective feeders there are few remains but those of living animals. The proctodaeum, or region preceding the anus, expels the wastes as fecal pellets of characteristic form.

The nervous system consists of a dorsal, bilaterally symmetrical, ganglionic mass or brain within or behind the preoral region. The brain is connected to the ventral cord through the circumesophageal connectives which extend about the oral cavity. The ventral cord may be single or paired, nearly smooth or nodular, or ganglionated according to the segmental pattern of the body. The brain sends out lateral branches to the eyes, palpi, antennae, or other structures; the ventral cord has lateral branches to all fleshy parts which receive stimuli. A giant axon is an enlarged part of the ventral cord, present in many long, muscular, or actively moving oligochaetes and polychaetes. It permits rapid transfer of stimuli and muscular response, resulting in abrupt response.

The circulatory system consists of dorsal and ventral longitudinal, median vessels located above and below the alimentary tract. Lateral branches extend to all parts of the body. Pulsating or propelling contractile portions, sometimes called hearts, are in the anterior dorsal vessel or also at intervals in the ventral vessels. In oligochaetes these are segmental vessels of varying number connecting the dorsal and ventral vessels and surrounding some portion of the alimentary tract. The contained blood may be red, through the presence of a hemoglobinlike substance, or green, through the presence of chlorocruorin. These colors when diluted are yellow or colorless, as in many small annelids.

Some annelids lack a closed circulatory system so that blood and coelomic fluids mix freely, resulting in a hemocoel; it may be partial or complete. Many annelids have a special organ called a cardiac or heart body surrounding the pulsating vessel and sometimes visible as a thick brown or red body of spongy tissue; its function is to dispose of circulatory wastes. The circulatory and coelomic fluids aid in maintaining turgidity of the annelid body, and with musculature control they act as a kind of skeleton.

Special organs for excretion are called nephridia. In their simplest form they are protonephridia, consisting of a strand of cells connecting the coelom to the body wall, usually a pair to a segment. More complex organs, or metanephridia, have a ciliated nephrostome or funnel opening into the coelom and continued to the surface as a complex organ. They function to transport wastes and at sexual maturity may serve to release gonadial products. Complex nephridia are present in all oligochaetes and many polychaetes.

The muscular system consists of an outer circular and an inner longitudinal system of muscles, each varying in extent and density according to species. In addition, an oblique series, between outer and inner layers, is well developed in annelids performing complex lateral movements. Long, very active burrowers or crawlers have an extensive musculature, whereas short, sluggish forms may have diminished musculature development. Movements are achieved mainly by coordinated muscular contractions and expansions of the laterally projecting parapodia or setae, resulting in an undulating or meandering movement. Swimming species may move from side to side or by successive forward darts and stops. Some annelids with reduced parapodia and a long proboscis use the latter in progression by extension and withdrawal of the eversible part of the alimentary tract.

The ability to replace lost parts is highly developed in annelids. Most frequent is the replacement of tail, parapodia, and setae. The anterior end may be replaced provided the break is postpharyngeal. The torn end is first covered over with scar tissue, then differentiated into epithelial cells and all other tissues characteristic of the whole animal.

Depending on the species, reproduction may be sexual, asexual, or both. Sexual reproduction may be dioecious, in which male and female are similar, rarely dissimilar. Individuals may be hermaphroditic, both male and female, but with cross fertilization. Some annelids are protandric hermaphrodites, in which the sexual stages alternate.

Fossil annelids, or segmented worms, are mostly soft-bodied, yet sufficiently common to indicate that this large and varied group of invertebrates has been abundant for more than 500 million years. The bulk of the annelidan fossil record is represented by the polychaetes. The earliest definite occurrences are from the Lower Cambrian of Greenland (Sirius Passet fauna).

The fossil record of oligochaetes is poor, in part because their predominantly terrestrial and fresh-water habitats have a relatively poor rock record. Nevertheless, the reproductive cocoons characteristic of the clitellates (oligochaetes and leeches) have been recognized as far back as the Triassic and may be more common as fossils than is generally realized.

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From McGraw-Hill Concise Encyclopedia of Environmental Science. The Content is a copyrighted work of McGraw-Hill and McGraw-Hill reserves all rights in and to the Content. The Work is © 2008 by The McGraw-Hill Companies, Inc.
 
 
 
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