CLASE POLYCHAETA PDF

Common, projecting from crevices in rocks; also on wharf pilings and marina floats. In smaller individuals, end of tube flattens and curls up when worm withdraws. Common; in sandy substrate adjacent to kelp forests in Monterey region, usually solitary, but associated with beds of Diopatra ornata. Range: Alaska to central California Synonyms: Similar species: Eudistylia polymorpha is very similar, but usually occurs in rocky areas and its branchial plume is uniform orange, tan, or maroon.

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Although most are 5 to 10 cm long, some are less than 1 mm, and others may be as long as 3 m. Some are brightly colored in reds and greens; others are dull or iridescent. Figure Tube-dwelling sedentary polychaetes. A, One of the featherduster worms called a Christmas-tree worm , Spirobranchus giganteus, has a double crown of radioles and lives in a calcareous tube.

B, Sabellid polychaetes, Bispira brunnea, live in leathery tubes. Polychaetes differ from other annelids in having a well-differentiated head with specialized sense organs; paired appendages, called parapodia, on most segments; and no clitellum Figure As their name implies, they have many setae, usually arranged in bundles on the parapodia. They show a pronounced differentiation of some body somites and a specialization of sensory organs practically unknown among clitellates.

Figure Nereis virens, an errant polychaete. A, Anterior end, with pharynx everted. B, External structure. C, Posterior end. D, Generalized transverse section through region of the intestine. Many polychaetes are euryhaline can tolerate a wide range of environmental salinity and occur in brackish water. The freshwater polychaete fauna is more diversified in warmer regions than in the temperate zones. Figure Amphitrite, which builds its tubes in mud or sand, extends long grooved tentacles out over the mud to pick up bits of organic matter.

The smallest particles are moved along food grooves by cilia, larger particles by peristaltic movement. Its plumelike gills are blood red. A, Section through exploratory end of tentacle. B, Section through tentacle in area adhering to substratum. C, Section showing ciliary groove. D, Particle being carried toward mouth. Polychaetes live under rocks, in coral crevices, or in abandoned shells, or they burrow into mud or sand; some build their own tubes on submerged objects or in bottom material; some adopt the tubes or homes of other animals; some are pelagic, making up a part of the planktonic population.

They are extremely abundant in some areas; for example, a square meter of mudflat may contain thousands of polychaetes. They play a significant part in marine food chains because they are eaten by fish, crustaceans, hydroids, and many others. They are often divided for convenience into two groups formerly the basis of subclasses : sedentary polychaetes, and errant or free-moving polychaetes. Sedentary polychaetes are mainly tubicolous, spending all or much of their time in tubes or permanent burrows.

Many of them, especially those that live in tubes, have elaborate devices for feeding and respiration. Errant polychaetes L. Most of these, like clam worms Nereis Gr. They have a muscular eversible pharynx armed with teeth that can be thrust out with surprising speed and dexterity for capturing prey. Form and Function The polychaete typically has a head, or prostomium, which may or may not be retractile and which often bears eyes, tentacles, and sensory palps Figures and The first segment peristomium surrounds the mouth and may bear setae, palps, or, in predatory forms, chitinous jaws.

Ciliary feeders may bear a tentacular crown that can be opened like a fan or withdrawn into the tube. Figure Arenicola, the lugworm, lives in an L-shaped burrow in intertidal mudflats. It burrows by successive eversions and retractions of its proboscis. By peristaltic movements it keeps water filtering through the sand. The worm then ingests the food-laden sand. The trunk is segmented, and most segments bear parapodia, which may have lobes, cirri, setae, and other parts on them see Figure Parapodia are used in crawling, swimming, or anchoring in tubes.

They usually serve as the chief respiratory organs, although some polychaetes also have gills. Amphitrite, for example, has three pairs of branched gills and long extensible tentacles Figure Arenicola, the lugworm Figure , which burrows through sand leaving characteristic castings at the entrance to its burrow, has paired gills on certain somites. Nutrition The polychaete digestive system consists of a foregut, midgut, and hindgut. The foregut includes a stomodeum, pharynx, and anterior esophagus.

It is lined with cuticle, and the jaws, where present, are constructed of cuticular protein. The midgut is derived from endoderm. More anterior portions secrete digestive enzymes, and absorption takes place toward the posterior. The short, ectodermally-derived hindgut connects the midgut to the exterior via the anus, which is on the pygidium. Errant polychaetes are mostly predators and scavengers.

Sedentary polychaetes feed on suspended particles, or they are deposit feeders, consuming particles on or in the sediment. We discuss food habits of some specific polychaetes in the following text. Circulation and Respiration Polychaetes show considerable diversity in both circulatory and respiratory structure and function. As mentioned before, parapodia and gills serve for gaseous exchange in various species.

In some polychaetes there are no special organs for respiration, and gaseous exchange takes place across the body surface. The circulatory pattern varies greatly. In Nereis a dorsal longitudinal vessel carries blood anteriorly, and a ventral longitudinal vessel conducts it posteriorly Figure C. Blood flows between these two vessels via segmental networks in the parapodia, septa, and around the intestine.

In Glycera the circulatory system is reduced and joins with the coelom. Septa are incomplete, and thus the coelomic fluid assumes the function of circulation. Many polychaetes have respiratory pigments such as hemoglobin, chlorocruorin, or hemerythrin. Excretion Although there is some variety in excretory organs, including possession of protonephridia and mixed proto- and metanephridia in some, most polychaetes have metanephridia Figure There is one pair per metamere, with the inner end of each nephrostome opening into a coelomic compartment.

Coelomic fluid passes into the nephrostome, and selective resorption occurs along the nephridial duct, as in oligochaetes see Figure Nervous System and Sense Organs Organization of the central nervous system in polychaetes follows the basic annelid plan see Figure Dorsal cerebral ganglia connect with a subpharyngeal ganglion via a circumpharyngeal commissure.

A double ventral nerve cord courses the length of the worm, with metamerically arranged ganglia. Sense organs are more highly developed in polychaetes than in oligochaetes and include eyes, nuchal organs, and statocysts. Eyes, when present, may range from simple eyespots to well-developed organs. Eyes are most conspicuous in errant worms. Usually the eyes are retinal cups, with rodlike photoreceptor cells lining the cup wall and directed toward the lumen of the cup. The highest degree of development occurs in the family Alciopidae, which has large, imageresolving eyes similar in structure to those of some cephalopod molluscs Figure , with cornea, lens, retina, and retinal pigment.

Alciopid eyes also have accessory retinas, a characteristic shared by deepsea fishes and some deep-sea cephalopods. Different wavelengths of light penetrate to different depths in water, and the accessory retinas of alciopids are sensitive to different wavelengths. The eyes of these pelagic animals may be well adapted to function as the light varies with depth. Studies with electroencephalograms show that they are sensitive to dim light of the deep sea.

Nuchal organs are ciliated sensory pits or slits that appear to be chemoreceptive, an important factor in food gathering. Some burrowing and tubebuilding polychaetes have statocysts that function in body orientation. Figure

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Polychaete

Although most are 5 to 10 cm long, some are less than 1 mm, and others may be as long as 3 m. Some are brightly colored in reds and greens; others are dull or iridescent. Figure Tube-dwelling sedentary polychaetes. A, One of the featherduster worms called a Christmas-tree worm , Spirobranchus giganteus, has a double crown of radioles and lives in a calcareous tube.

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Class Polychaeta (Poly, many + chaise, hair)

They can sometimes be brightly coloured, and may be iridescent or even luminescent. Bundles of bristles, called chaetae , project from the parapodia. The most generalised polychaetes are those that crawl along the bottom, but others have adapted to many different ecological niches , including burrowing, swimming, pelagic life, tube-dwelling or boring, commensalism , and parasitism , requiring various modifications to their body structures. The head, or prostomium , is relatively well developed, compared with other annelids. The head normally includes two to four pair of eyes, although some species are blind. These are typically fairly simple structures, capable of distinguishing only light and dark, although some species have large eyes with lenses that may be capable of more sophisticated vision.

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