Though all the members of animalia are multicelluler,all of them do not exhibit the same pattern of organisation of cells. for example, in sponges,the cells are arranged as loose cell aggregates,i.e.,they exhibit cellular level of organisation.some division of labour occurs among the cells. In coelentraterates,ghe arrangement of the cells is more complex.Here the cells performing the same function are arranged into tissues,hence is called tissue level of organisation. A still higher level of organisation,ie.,organ level is exhibited by members of platyhelminthes and other higher phyla where tissues are grouped together to form organs,each specialised for a particular function. In animals like Annelidas,Arthropods,Mollusces,Echinoderms and chordates,organs have associated to form functional sysyems,each system concerned with a specific physiological function.This pattern is called organ system level of organisation. Organ system in different groups of animals exhibits various patterns of complexities. For example,the digestive system has two openings,mouth and anus.Similaraly,the circulatory system may be of two types.
1... open type in which the blood is pumped out of the heart and the cells and tissues are directly bathed in it and.
2...closed type in which the blood is circulated through a series of vesseles of varying diameters(arteries,veins and capillaries).
1... open type in which the blood is pumped out of the heart and the cells and tissues are directly bathed in it and.
2...closed type in which the blood is circulated through a series of vesseles of varying diameters(arteries,veins and capillaries).
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