Classification of Animals

The fundamental characters examined so far — the level of organisation of the cells, the body symmetry, the number of germ layers, the nature of the coelom, the presence of segmentation, and the presence or absence of a notochord — are used together to sort the kingdom Animalia into its broad divisions. No single feature works alone; each phylum is characterised by a particular combination of these features, and it is that combination that fixes where a group belongs.

Using these shared fundamental features, the whole of Animalia is arranged into a broad classification, which the textbook sets out as a chart. Working down this scheme, the various phyla are separated from one another according to how they stand on each character — for instance, whether their cells reach only the cellular level or the full organ-system level, whether they are radially or bilaterally symmetrical, whether they are diploblastic or triploblastic, whether they are acoelomate, pseudocoelomate or coelomate, whether the body is segmented, and whether a notochord is present. (One point noted in the chart is that echinoderms ma …