SEGMENTED WORMS
Segmented WormsSegmented worms (phylum Annelida) are so named because of their elongated, more or less cylindrical bodies divided by grooves into a series of ringlike segments. Typically, the external grooves correspond to internal partitions called septa, which divide the internal body space into a series of compartments. Perhaps the most familiar examples of segmented worms are the common earthworms or night crawlers, and the freshwater leeches. Actually, the more numerous and typical members of the phylum are marine, crawling or hiding under rocks, or living in burrows, or in tubes, or in the sediment.There are approximately 15,000 living species of annelids, placed in three major classes: the Polychaeta (mostly marine), the Oligochaeta (mostly terrestrial), and the Hirudinea (mostly freshwater).
Polychaetes are either "errant"—moving and feeding actively, or "sedentary"—with a passive lifestyle. The basic body plan of an errant form is illustrated by the sandworm Nereis. The anterior end of Nereis is specialized to form a "head," possessing two pairs of eyes and several pairs of sensory appendages. The remainder of the body consists of a large number (100 or more) of similar segments, each with a pair of distinct lateral appendages called parapodia. The parapodium is muscular, highly mobile, and divided into two lobes, an upper, or dorsal, "notopodium," and a lower, or ventral "neuropodium." Each lobe bears a bundle of bristles, or setae. The setae, made of a substance called chitin, are used in crawling or in swimming. Nereis is a carnivore. Its food consists of small live organisms, or fragments of dead organisms, which it grasps by means of a pair of powerful jaws located at the tip of an eversible muscular pharynx. The food is ground up and digested as it passes through successive parts of the straight, tubular gut. The undigested residue is discarded through the anus located at the posterior end.
Most other body systems are arranged on a "segmental plan," which means that structures performing a particular body function are repeated in each segment. Thus, for excretion each segment contains a pair of coiled, ciliated tubes called nephridia. At one end the nephridial tube opens into the spacious cavity (called coelom) between the body wall and the gut; at the other end it opens to the outside. There is a well developed circulatory system. The blood, which is red in color due to the presence of hemoglobin, circulates in blood vessels. Gas exchange occurs between blood and sea water across the thin, leaf-like lobes of the parapodia.
Each body segment also has a pair of nerve ganglia and three or four pairs of nerves for receiving sensory input and coordinating muscular activity. Ganglia in successive segments are connected by means of a pair of longitudinal nerve cords, so that nerve impulses can be transmitted back and forth between each segment and the "cerebral ganglion" or "brain" located in the head. Sexes are separate, although no external characteristics distinguish males from females.
Read more: Segmented Worms - Body, Leeches, Polychaetes, and Blood - JRank Articles http://science.jrank.org/pages/6062/Segmented-Worms.html#ixzz1pnIAahYW
Segmented WormsSegmented worms (phylum Annelida) are so named because of their elongated, more or less cylindrical bodies divided by grooves into a series of ringlike segments. Typically, the external grooves correspond to internal partitions called septa, which divide the internal body space into a series of compartments. Perhaps the most familiar examples of segmented worms are the common earthworms or night crawlers, and the freshwater leeches. Actually, the more numerous and typical members of the phylum are marine, crawling or hiding under rocks, or living in burrows, or in tubes, or in the sediment.There are approximately 15,000 living species of annelids, placed in three major classes: the Polychaeta (mostly marine), the Oligochaeta (mostly terrestrial), and the Hirudinea (mostly freshwater).
Polychaetes are either "errant"—moving and feeding actively, or "sedentary"—with a passive lifestyle. The basic body plan of an errant form is illustrated by the sandworm Nereis. The anterior end of Nereis is specialized to form a "head," possessing two pairs of eyes and several pairs of sensory appendages. The remainder of the body consists of a large number (100 or more) of similar segments, each with a pair of distinct lateral appendages called parapodia. The parapodium is muscular, highly mobile, and divided into two lobes, an upper, or dorsal, "notopodium," and a lower, or ventral "neuropodium." Each lobe bears a bundle of bristles, or setae. The setae, made of a substance called chitin, are used in crawling or in swimming. Nereis is a carnivore. Its food consists of small live organisms, or fragments of dead organisms, which it grasps by means of a pair of powerful jaws located at the tip of an eversible muscular pharynx. The food is ground up and digested as it passes through successive parts of the straight, tubular gut. The undigested residue is discarded through the anus located at the posterior end.
Most other body systems are arranged on a "segmental plan," which means that structures performing a particular body function are repeated in each segment. Thus, for excretion each segment contains a pair of coiled, ciliated tubes called nephridia. At one end the nephridial tube opens into the spacious cavity (called coelom) between the body wall and the gut; at the other end it opens to the outside. There is a well developed circulatory system. The blood, which is red in color due to the presence of hemoglobin, circulates in blood vessels. Gas exchange occurs between blood and sea water across the thin, leaf-like lobes of the parapodia.
Each body segment also has a pair of nerve ganglia and three or four pairs of nerves for receiving sensory input and coordinating muscular activity. Ganglia in successive segments are connected by means of a pair of longitudinal nerve cords, so that nerve impulses can be transmitted back and forth between each segment and the "cerebral ganglion" or "brain" located in the head. Sexes are separate, although no external characteristics distinguish males from females.
Read more: Segmented Worms - Body, Leeches, Polychaetes, and Blood - JRank Articles http://science.jrank.org/pages/6062/Segmented-Worms.html#ixzz1pnIAahYW
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