Phylogenetic affinities of Acanthocephala: an assessment of ultrastructural evidence

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Abstract
The phylogenetic relationships of the Acanthocephala are reviewed in the light of recent ultrastructural evidence. Comparisons of spermatozoan ultrastructure suggest that the Acanthocephala cannot be closely related to the Platyhelminthes and that the similarities which exist between cestodes and acanthocephalans are the result of convergence. Similarities in the spermatozoan ultrastructure of acanthocephalans, rotifers, nematomorphs and nematodes probably have a phylogenetic significance and suggest that the acanthocephalans have a close common ancestor with some at least of the Aschelminthes. The case for this affinity is strengthened by ultrastructural evidence concerning the body walls and muscles of acanthocephalans, nematodes and rotifers. I am indebted to Professor D. R. Arthur and Professor F. R. Whatley at King's College, London, for the provision of facilities. The work reported in this paper was undertaken during the tenure of a Research Fellowship from the Nuffield Foundation.