Vastness and Diversity OF Animals If one is asked to tell names of the animals found in the world,he may tell names of a large numbers of animals. But this large number constitutes only a very small fraction of vast assemblage of animals forms that exists and existed in the world.It is estimated that 1,90,000 animals species have been described in the literature of zoology, and that each year about 10,000 new ones are identified and added. these animals exhibit avery profound diversity among themselves; these differs from one another in shpe ,colour,body organization,ways of life and behavior. The diversity of shape among different animals is related to the arrangement of body parts and way of life. The orderly arrangements of parts in the body is called symmetry. This provides definite shape to the body. The radial symmetry in which simila5r body parts, the antimeres, radiates from the central axis as in hydra is best suite to sessile[attached or atleast immobile] animals. The similarity of the antimeres enables these animals to obtain food repel enemies from all sides. some simple sponges, the majority of coelenterates and adult echinoderms are radially symmetrical; their body can be cut into two equal halves along any plane that passes through centre of the body. The biradical symmetry is found in some coelenterates of the class Anthozoa.Organisms,spherical in shape ,have spherical symmetry,as found in volvox, is advantageous to floatin forms, since these undergo an indefinite type of locomotion. The bilateral symmetry, evidenced by arangement of the main organs or structures in pairs on either sides of median longitudinal axis, is characteristic of most animals living today[all vertebrates and most invertebrates]. Animals with such symmetry move forward and have a differentiation of head end. Besides shape and symmetry, animals also exhibit diversity in the numbers of cells composing their body;protozoans' body is just the cell but the body of all other animals is multicellular.The multicellular body of animals also ehibit diversity in the number of cells . The multicellularsbody is differen tiated inmto layers ,tissues ,organs and systems.Theporiferens and coelenterates are diploblastic or two layerd matazons , but all other metazoans[multi cellular body of animals is differentiated into three layers .Inspite of their multicellularity , the poriferans have a cell-lavel organization of the body due to lack of tissue , or a very poor distinction of the tissue ,.In contrast with them all others multicellular animals exihibit tissue -level of the body organization . These,excepts coelenterates,exhibit organ-system level of the organization.Some of theser lack a body cavity such as flat worms ; others have a false coelum , the pseudocoel as a round or thread but many have coelum [a cavity surrounded by the mesoderm on all sides ];the triploblastic animals except flat and round worms , have extensive or reduced coelum .The coelum doesnot usually contain blood but in molluscs , arthropods , onychphorun and other animals ,it conntains blood or the blood flows around through it hence it is the haemocoel .The coelum of earthworms and most of the annelids divide into the small coelomic com partment or chambers by the septa .Somwtimes ,the ver4tebrates coelum is divided into thoracic abdominal cavities by the daph gram ; this condition of the coelum is found only in animals . Many animals of course , bilaterally symmetrical animals , ahve a district anterior end and a posterior end and ar e said to be polar .In these anima, a concentration of nervous tissue is found within head at the anterior end and a posterior end and are said to be polar . In these animals a concentration of nervous tissue is found with in head at the exterior end ; this morphological state is called cephalization ends ; others do not .besides ,animals , is composed of al inear series of similar parts ; this morphological state is called segmentation appears only externally , probably in body wall alone ; such segmentation segmentation .Many animals ,including all vertebrates , have internal or mata meric segmentation .In vertebrates ,the vertebrae , ribs and nerves are meta merically arranged .The earth worms have both super ficial and meta meric segmentations Most animals live free but certain animals live in association of other as parasite or commensal .The para site s lives on or within the body of animal .In order to fit well in the mode of their life , the parasites have undergone degeneration and specialization and hence appear much different from thier free living kins.
this is all about classification of animals
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