Once past this hurdle, all sorts of specialisation amongst the cells that comprise an organism become possible. Everything more advanced than a flatworm has discovered the way around this is to have a circulatory system to reach the parts that diffusion cannot reach. Any arrangement other than being flat puts the innermost cells to far away from the outside to survive. One of the main disadvantages of being a flatworm is that you have to be flat to allow oxygen to diffuse into the innermost layers of cells (and also to let CO 2 diffuse out). Homeostasis may now occur at both the level of the single cell and at the level of the whole organism. In short the outer layer of the flatworm creates a comfortable internal environment for all the other cells to live in. Nevertheless, the cells on the outside of the cylinder are specialised and act to hold the animal together, to protect the inner layers and to absorb food from the environment. Flatworms do not have digestive or circulatory systems most of the animal is a flattened cylinder of cells. A flatworm is a good example of the next stage of development. However, the real advantages to being multicellular aren't apparent until component cells start to show specialisation. It is possible to disperse a sponge through a sieve, if you leave the pieces in a bucket for a few days they will reform back into a colony with apparently no ill effects. A sponge is a collection of identical cells that exist as a colony. The simplest possible multicellular organism is a sponge. Although it might be argued that bacteria represent the most successful cellular lifeform, because there are more of them then anything else, being a multicellular organism does seem to have some things going for it. Body Fluid compartments Body Fluid Compartments From single cell to multicellular organismīeing a cell, as opposed to being a free-floating self-replicating molecule would appear to offer survival advantages, after all there are lots of cells around now and very few non-cellular life forms (even if you count viruses as non-cellular life forms they don't make up much of the total biomass).
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