In a unicellular organism, the continuous streaming movements of the cytoplasm circulate materials within its one celled body. For example, in the Amoeba, which changes it s shapes as it moves, the cytoplasm from the rest front of the organism along the direction of its movement. In the paramecium which does not change its shape, the continuous circular movements of the cytoplasm bring about the distribution of materials within its body.
In a simple multicellular animal like the Hydra, the body is two layered and encloses a huge gut cavity. Movements of the gut wall draw water into the gut and cause the digested food and oxygen with in it to circulate. This enables the cells lining the gut to absorb the materials. some cells in the guy have flagella. The whipping movements of the flagella also help to circulate the materials in the gut.
In a slightly more complex animal like the flatworm, the body is thin and flat so that the S.A/V ratio is large. In addition, the gut branches extensively throughout the body. These features enable sufficient food and oxygen to diffuse into all the body cells. A branches excretory system is present to collect and get rid of waste movements of the body wall aid in the transport of waste materials out of its body.
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