Hydrostatic skeleton

The Hydro skeleton is the phylogenetically oldest form of the musculoskeletal system. A part of the muscle works against an incompressible fluid cushion, but this must be as plump filling to effectively pass on mechanical forces. The operation is comparable to a hydraulic engineering system.

Hydro skeletons can be found for example in nematodes in the form of a single fluid space, as well as in annelids, in which the fluid spaces are allocated to the individual body segments and adjacent to the cross - a longitudinal subdivision have (reduction phenomena by dissolving the separated tissue walls but are very common ). These fluid-filled cavities are formed voluminous and therefore clearly visible: When the nematodes as so-called Pseudocoel, in annelids than coelom. Especially the Coelomräume are a common feature of many animal blueprints. About their Homologisierbarkeit but there is disagreement among many groups: you could have a common origin, but also arisen independently several times.

In many arthropods flexor muscles of the leg joints work against an inner liquid filling, resulting in effective ( energy-saving ) running movements result, or even long jumps ( this is often by abruptly increasing pressure ratios). Liquid extends the leg, whereas the flexor presses them out of the leg in the body, causing the flexion of the joint. In the arthropods so the phylogenetically earlier trained hydro skeleton is used in a significantly trained exoskeleton still. Especially during the molting process, these hydraulic mechanisms are vital to draw out the body with all body appendages from the old exoskeleton covering.

As bulky hydro skeleton and the Gastralraum or the Gastrovaskularsystem of cnidarians may be considered. The flatworms, however, it is formed by fluid-filled crevices in the parenchyma, and the parenchymal cells themselves represent small hydraulic units

Also, the strength of herbaceous, nichtverholzter plants based on a hydraulic skeleton. This is based on the turgor pressure, the osmotic pressure of water-filled vacuoles, which have up to 95 % of the cell volume and press against the sturdy wall of cellulose.

Due to the wide spread in the animal kingdom has been trying to trace the evolution of various animal plans gradually on the basis of expression of the hydraulic skeleton. The idea is that the different fluid cushion is always associated with the animals in a certain way with the musculoskeletal system, ie particularly the muscles, but also various connective tissue structures, for example, may be associated with the muscles. Since the musculoskeletal system in evolution can not be rebuilt by leaps and bounds and in any manner, hydro skeletal systems provide a way to gradually reconstruct functional alterations in those groups in which one due to the purely weichkörprigen construction has little hope of extensive fossil series. This applies to the majority of animal phyla, namely on most invertebrates, too.