An Introduction to the Invertebrates, Second Edition - tiera.ru

Chapter 5On being a wormAny soft-bodied legless animal whose length exceeds its width isliable to be described as a worm, and many invertebrates fit thisdescription. Four of the main worm-like phyla are discussedseparately in later chapters, but there are many other differentworms, belonging to phyla often castigated as ‘minor’, usuallybecause they have a small number of species or are very smallanimals. This chapter introduces the variety of worms, afterconsidering why worms should have evolved so many times andwhat muscular machinery is necessary for their locomotion.5.1 Why are there so many different kinds of worm?Mechanical facts about the molecules that make up animals meanthat worms are very easily produced. Cells secrete extracellularcompounds with charged molecular backbones: like charges repel,causing linear extension, and linkage between these large moleculesprovides orientation in a structure that will be anisotropic (i.e. havedifferent properties in different directions). If a blob of soft tissue hassuch orientated fibres, any event such as growth or motion orexternal pressure will automatically turn that blob into a cylinder.Orientated fibres will guide and limit the direction of growth, andenable it to change its shape. No further genetic instruction is neededto make a worm, in its simplest form.A worm, then, is easily produced: why should such a structurebe favoured by natural selection? Soft tissues are extraordinarilyresistant, and damage is readily repaired. There is a large range ofpossible sizes, from less than a millimetre (as parasites or in themarine ‘interstitial’ habitat in between sand grains) to over 30 metresin the sea. Movement in one direction will be favoured by worm-likeshape: an anterior end becomes established, usually with at least asimple ‘brain’ and sense organs, and the distinction between dorsal(top) and ventral (underside) surfaces confers bilateral rather thanradial symmetry. Metazoa with this construction are commonlygrouped as ‘Bilateria’.