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Zoology involves the study of animals. Not just any study, but how to classify animals, their history, their anatomy and physiology, their development, and so much more.

Zoology is a very broad field; zoologists study subjects ranging from single-celled organisms to the behavior of groups of animals. Some zoologists study the biology of a particular animal, others study the structure and function of the bodies of animals.

Annelida (L., annellus = little ring or F .anneler = to arrange in rings) are triploblastic, bilaterally symmetrical, coelomate and segmented Metazoa. Body is covered by a thin cuticle having chitinous setae in most. Body wall is covered with glandular epidermis, below which are muscles forming an outer layer of circular muscles and an inner layer of longitudinal muscles.

The body is divided for the first time in the animal series into metamerically arranged segments or metameres arranged in a linear series, but the segments are integrated into a single functional unit. Perivisceral cavity is a schizocoelic coelom between two layers of mesoderm. The coelom contains a coelomic fluid which is incompressible, consequently it acts as a hydraulic skeleton.

There is a single pre-oral segment called prostomium and a similar post-segmental region posteriorly known as a pygidium. The nervous system has a pair of pre-oral ganglia or brain, and paired ventral nerve cords ganglionated in each segment. There is a closed circulatory system.

The digestive tract is more or less straight but differentiated into well-defined regions; digestion is entirely extracellular. Organs of excretion are metameric ectodermal nephridia, besides which there are tubular mesodermal coelomoducts used for passage of reproductive cells. The larva, if present, is a trochosphere.

The members of the phylum are modified for sedentary, active, or ectoparasitic life, they occur on land, in freshwater, or in the sea.

The phylum contains over 8,600 known species and is divided into four classes called Polychaeta, Oligochaeta, Himdinea, and Archiannelida. But in some systems of classification, Echiuroidea, Sipunculoidea, Priapulida, and Myzostomaria have also been included as classes, while in other systems these classes have been considered as appendix to phylum Annelida or to some of its classes.

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