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Circulation in Animals Every structural unit of the biological machinery requires continuous supply of both food and oxygen to sustain life.To ensure constant supply of food and oxygen in different cells and to remove metabolic wastes,a system known as circulatory or vascular system has been devised. Types of Blood Vascular Systems The blood vascular system is mainly of two types: (1)Open Circulatory System-It is found in invertebrates such as annelids(e.g. Leech),arthropods(e.g.Prawn),and molluscs(e.g.Snail).He re,the haemolymph does not remain confined to blood vessels but flows through open spaces and channels called sinuses or haemocoel in the tissues.In sinuses,the blood is in direct with the tissue cells.The flow of blood through open sinuses is very slow. (2)Closed circulatory system-It is found in all vertebrates.Here,blood runs in closed vessels and never comes in direct contact with the tissue cells.Blood vascular system is a network of arteries and veins centrally connected...

Theories of Evolution Various theories have been postulated by different evolutionists. Most important ones are; 1-Lamarcks theory of inheritance of acquired characters(Lamarckism) 2-Darwin theory of natural selection (Darwinism) LAMARCKISM (Theory of inheritance of Acquired Characters) This theory was proposed by French biologist Lamarck in 1809.It is popularly known as ’The inheritance of Acquired Characters in Organisms’ and comprises of four assumptions- (1)Living organisms and their parts tend to increase in size continuously due to internal forces of life(Origin of need). (2)Formation of new organ in the body of an organisms is the result of a new need and new movement which this need initiates and maintains in the body(Effect of need). (3)If an organ is used continuously and constantly it will tend to become highly developed, whereas disuse results in its degeneration (Use and Disuse). (4)Modifications acquired during the life time of an individual are inherited by its offspri...