PHYLOGENETIC INFLUENCES

There is widespread acceptance that the phylum Acanthocephala can be divided into three major classes (Amin, 1985a), and some authorities also recognise a fourth, very small, class. The characterisation of these taxa is based on a number of structural features such as body size, number of cement glands and trunk spination and also on a number of biological features including habitat and identity of intermediate and definitive hosts.

The most important of these biological features in relation to the three main classes are summarised in Table 3.1. The Archiacanthocephala are truly terrestrial: they use terrestrial insects and myriapods as intermediate hosts and predatory birds and mammals as definitive hosts, and many, though not all, species also have the ability to use paratenic hosts (Schmidt, 1985), particularly reptiles and amphibians. By contrast the Palaeacanthocephala are mostly aquatic and use aquatic arthropods as intermediate hosts. They show the greatest diversity of all three classes in their use of definitive hosts. Many species use teleost fish, but others use amphibians, birds and mammals. These birds and mammals generally show strong aquatic links and species such as ducks, seals and whales serve as hosts. The use of paratenic hosts is not very common, but when they are employed it is often to bridge levels in food chains, for example in the use of fish to bridge the levels of crustacean intermediate hosts and piscivorous seals as definitive hosts.