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Invertebrates are so named because they lack a proper vertebral column as in 'in' - 'vertebrates', where 'in' refers to 'absence' of a vertebral column. Whenever we talk about skeletons, the first thing that comes to our mind is a set of bones. Hydrostatics and the skeleton formed by it in the body of an organism is a remarkable character chosen by natural selection, which is well exhibited in case of annelids. The system of tubes made of specialised tissues which connect to form a ladder like system containing fluid within it is briefly termed as the Hydrostatic Skeleton.
Evolution of the Hydrostatic Skeleton edit
The differentiation of muscular tissue was one of the consequences of the establishment of metazoan organization, but it was not sufficient in itself to bring about successful movement in the organism. For this, jointed skeletons had evolved. These muscles are attached to the movable parts of the skeleton. In muscles, contraction is an active process but relaxation is not. Relaxation is brought about by an external force. The muscles in these higher groups of animals are so attached, that when one contracts, the other one relaxes, i.e. they act as antagonists. These muscles antagonize each other, and make it possible to have successful movement. This is definitely not the only role played by the jointed skeleton. The other significant functions of the skeleton are, by providing a system of levers it secures the economical application of energy, while it also contributes to the general support and protection of the body. Rigid skeletons made a comparatively late appearance in the history of animal evolution, and even today there are many metazoan animals that do not possess a rigid, jointed skeleton. There is, however, a remarkable absence of skeletal structures in the pre-cambrian strata. They seem to have evolved relatively suddenly at about the onset of Cambrian times.
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