Locomotion and the phylogeny of the Metazoa

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Abstract
Among the lower Metazoa, the basic structure of the animals sets limits to the locomotory techniques available to them and to the habitats they can occupy. Small but critical structural changes overcome these limitations and once achieved they have resulted in major phyletic radiations. Small organisms up to 1–2 mm in length can, by ciliary action, swim, creep on the substratum or creep in it as a meiofauna. Once this size is much exceeded only creeping on the substratum is possible. The largest of the earliest Metazoa exceeded this limit and formed the basis of an important radiation of surface creeping forms including Turbellaria, Nemertea and Mollusca, the larger members of which supplement or replace ciliary locomotion by muscular activity. A cellular hydrostatic skeleton permits reversible changes of shape produced by contraction of the body-wall muscles in small organisms, but with increasing size, particularly as structural elements are developed to conserve the shape of the animal, such a hydrostatic skeleton offers too great a resistance for rapid or powerful shape changes. Only a liquid-filled compartment in the body (usually the coelom) permits sufficiently powerful forces to be generated for burrowing into the substratum. The evolution of the coelom allowed the colonization of this habitat and introduced the radiation of coelomate animals. Burrowing by retrograde peristaltic movements requires circular and longitudinal muscles in any region of the body-wall to act as antagonists to one-another. This is possible but mechanically inefficient in worms with an undivided coelom. Compartmentation of the coelom by septa provides hydrostatic isolation of different regions of the body in oligochaetes and they are vagrant burrowers. Coelomate worms without a segmented coelom usually employ direct peristaltic locomotion which does not require the isolation of parts of the body, but these worms are sedentary burrowers or live in secreted tubes. The evolution of lateral (polychaetes) or ventral (arthropods) appendages in segmented worms introduced fundamental structural change in the organization of the segment, related to the development of new locomotory techniques and the colonization of new habitats: the surface of the substratum (arthropods) and probably a surficial coze (polychaetes). Although these structural changes, related to locomotory techniques, were important developments in the evolution of the lower Metazoa, the resolution of problems in the phylogeny of existing phyla must depend on comparative anatomical and embrylogical evidence, and, where available, palaeontological evidence. Solution of these problems must, however, be consistent with what is known about the mechanical and locomotory capabilities or organisms of particular grades of construction.