Classification should at least to some extent show phylogenetic relationships. It is therefore reasonable to base any classification on a discussion of phylogeny and analysis of phylogenetically relevant features. Adaptational features likely to evolve many times must be sorted out in the discussion of relationships, but may be useful to describe the evolution inside phyletic lineages defined by other means. The Proetida, as defined by ForteyOwens 1975, are taken as an example of a heterogeneous taxon based on purely morphological features. A phylogenetic approach to ontogenetical, morphological and functional criteria seems to indicate that the Proetida fall into at least three groups without close relationships.
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