Hydrostatic skeletons and muscular hydrostats

Abstract

Abstract A hydrostatic skeleton is a fluid mechanism that provides a means by which contractile elements may be antagonized (1). Hydrostatic skeletons occur in a remarkable variety of organisms with examples not only from invertebrates but also from vertebrates. A hydrostatic skeleton is typically considered to include a liquid-filled cavity surrounded by a muscular wall reinforced with connective tissue fibres. This form of hydrostatic skeleton is seen, for example, in cnidarian polyps, holothuroid echinoderms, echinoderm water vascular systems, many molluscan bodies and organs and annelid, sipunculid, ncmcrtean, and nematode worms (for reviews see refs. 1-7). Comparison of a variety of hydrostatic skeletal support systems shows that the extent and volume of the liquid-filled cavity is variable. In particular, recent work bas identified a number of hydrostatic skeletons, termed muscular hydroslah, that consist of a tightly packed three-dimensional array for muscle fibres (8, 9). Examples of muscular hydrostats include the arms and tentacles, fins, suckers and mantles of cephalopod molluscs, a variety of molluscan structures, the tongues of many mammals and lizards. and the trunk of the elephant.