Basis of Classification — Levels of Organisation and Symmetry

Animals show an extraordinary diversity of body forms, yet classifying this diversity in a scientifically useful way requires looking past superficial resemblance and habitat to a set of shared, fundamental features of body organisation. Two animals that look nothing alike in size or colour can share a common architectural plan, while two animals of similar size and habitat can be built on completely different plans. Six such basis-of-classification features run through the whole of this chapter — the level of organisation of the body, the type of symmetry, whether the body is diploblastic or triploblastic, the nature of the coelom (body cavity), the presence or absence of segmentation, and the presence or absence of a notochord — and a WBCHSE Class XI student should be able to state, for any phylum studied, where it stands on each of these six criteria.

Levels of organisation. Cells that work more or less independently, without being organised into tissues, are said to show the cellular level of organisation; sponges (phylum Porifera) are the clearest example, since even though a sponge's body is unmistakably multicellular, its component cells still show a striking degree of individual, division-of-labour-free activity. Where cells performing the same function are grouped together into a tissue, but the tissues themselves are not further built up into organs, the animal shows the tissue level of organisation — the coelenterates (Cnidaria) are the standard example, with a tissue layer for digestion, another for the nerve net, and so on, but no true digestive, nervous or reproductive organs as such. Where tissues are further organised into distinct organs, each with a specific job, the body shows the organ level of organisation — flatworms (Platyhelminthes) are the entry point for this grade, having, for instance, a distinct pharynx and reproductive organs. Finally, where organs cooperate as complete organ systems, with each system handling one major physiological function and the systems interacting with one another, the body shows the organ-system level of organisation; this is the plan followed by every phylum from the roundworms (Aschelminthes) onward, right through to the vertebrates, and it is the most complex of the four grades.

Symmetry. An animal's body may show no symmetry at all — an irregular form, as in most sponges — which is described as asymmetrical. Where the body can be divided by a plane passing through the centre, along the main axis, into two mirror-image halves in more than one plane (cut it through the centre in any direction and the two halves match), the animal shows radial symmetry — the coelenterates, ctenophores and the adults of the echinoderms (though not their larvae) are built this way, an arrangement well suited to an animal that must respond to stimuli from every direction equally, as a sessile or slow-drifting organism does. Where only a single plane, running down the long axis, divides the body into equivalent right and left halves, the animal is said to be bilaterally symmetrical — this plan appears from the flatworms onward and dominates the rest of the animal kingdom, and it is closely tied to active, directional movement, since a bilaterally symmetrical animal has a clear head end (cephalisation), a clear tail end, an upper (dorsal) surface and a lower (ventral) surface.

These two criteria — level of organisation and symmetry — are the first two entries on the six-point checklist that the rest of this chapter applies, phylum by phylum and class by class, to every group of animals studied.