Evolution of Zooparasitic Groups in the Phylum Nematoda, with Special Reference to Host-Distribution

Abstract

In the NEMATODA the biologist is confronted with a well-defined group of organisms, relatively small as regards different structural types, but exceedingly numerous as individuals and successful in the most varied habitats. Traditionally in general zoology and parasitology texts-in the English language, at least-the group is considered an order or class, which, with orders or classes GORDIACEA (or NEMATOMORPHA) and ACANTHOCEPHALA, constitutes a phylum NEMATHELMINTHES. However, as the leading nematologist, B. G. Chitwood (1950c), has pointed out in a review of the historical placement of the NEMATODA in the animal kingdom, such a grouping has never had the support of serious students of invertebrate evolution. H-e has marshaled evidence in a succinct manner for the view, recognized by a growing number of zoologists, that the nematodes constitute the largest of a group of minor phyla--or classes within a rather broadly defined phylum, to which Hyman (1940, 1951a, b [in press]) has followed earlier German workers in applying the name ASCHELMINTHES Grobben, 1908. Whether one wishes to consider the NEMATODA an independent phylum or an aschelminth class is of relatively minor importance (although it may be remarked that there is probably more justification in combining the ANNELIDA, ONYCHOPHORA, and ARTHROPODA into a single phylum than the so-called aschelminth classes). For present purposes I am considering the NEMATODA a phylum. What is actually important is recognition of the interesting evolutionary status of the NEMATODA. They represent one of a group of Nature's minor evolutionary experiments in metazoan organization and have exploited various possibilities of non-segmented, pseudocoelomatous, vermiform structure. They have never attained the remarkable diversity of the major animal phyla-in particular the MOLLUSCA, ARTHROPODA, and CHORDATA-presumably because evolutionary processes have been able to do only so