The Phylum Annelida

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PHYLUM ANNELIDA The name Annelida was first used by Larmarck in 1809 for the higher segmented worms. Annelida is derived from the word annelus which means little ring or to arrange in rings. The body is divided for the first time in the animal kingdom into a linear series of metamerically arranged segments or metameres but all segments are integrated into a single functional unit. All organ systems are present and well developed. Characteristics • They are triploblastic, bilaterally symmetrical and metamerically segmented coelomates. • Mostly aquatic, marine or freshwater, some terrestrial, burrowing or living in tubes, sedentary or freeliving. Few are parasitic. • Organ – System grade of body organization. • Segmentally arranged paired setae or chatae, composed of chitin are typically present and aid in locomotion. • Excretory system consists of segmentally arranged paired nephridia, which open into the coelom and extract nitrogenous waste from it. • Central nervous system consists of solid, ventral, double, ganglionated nerve cord and paired cerebral ganglia, the brain. • Alimentary system is tube-like, complete, extending from mouth to anus. • Blood vascular system is the closed type (i.e. blood confined to the blood vessels) • Sexes usually separate though hermaphroditic forms are also present. Development is direct in the former and indirect in the latter. • Free-swimming trocophore larval stage is characteristic of indirect development, while in others, this stage is passed during development. • Asexual reproduction by budding in some. Annelida holds vital position in the animal kingdom because important biological phenomena, which open ways for wide complexity and variability in form and structure in animals, made their first appearance in the group. These biological phenomena include:- 1. Metamerism (Body Segmentation): This is the greatest

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