The major parasitic platyhelminth classes — progressive or regressive evolution

Abstract

The evolution of certain ontogenetic processes appears to be of special importance to subsequent evolution of a phyletic line, and their occurrence suggests there are various ‘evolutionary capacity levels’ occupied by major taxa. Two groups that are at the same evolutionary capacity level probably originated in common within that level, which in turn is often shared with several other groups. Morphogenesis by invagination is characteristic of an early evolutionary capacity level common to poriferans, cnidarians, deuterostomes, and platyhelminths, inter alia. In the early ontogeny of the former three groups, this process produces the blasto- pore and archenteron, but in platyhelminths, e.g. rhabdocoels, two different processes form the digestive system: the mouth forms by invagination and the intestine by a split in the mesenchyme/parenchyma. I have selected some characters and processes to illustrate evolutionary capacity levels and to describe the possible origin of a number of invertebrate groups from various levels and their inter-relationships. My analysis on the basis of evolutionary capacity levels suggests that the deuterostomes originated early (very likely before mollusks and annelids), that tegumental absorption of nutriments is the original mode of nutriment uptake and so predates intestinal ingestion in Platyhelminthes, that ontogenetic characters show that the presumed origin of cercomeromorphaeans from rhabdocoels is improbable, and that features of ontogeny and life cycles indicate that the major parasitic platyhelminth classes arose through progressive rather than regressive evolution. Thus the non-parasitic and the parasitic platyhelminth classes very likely belong to separate evolutionary lines, all originating from the same evolutionary capacity level.