Among the lower hletazoa, 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 o\Tercome these limitations and once achieved they ha\.e resulted in major phyletic radiations. Small organisms up to 1-2mm 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 hletazoa exceeded this limit and formed the basis of an important radiation of surface creeping forms including Turbellaria, Nemertea and hlollusca, 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 suficiently powerful forces to be generated for burrowing into the substratum. The evolution of the coelom allowed the colonizat:on 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 ineficient in worms with an undivided coelom. Cornpartmentation 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 coclom 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.
