Ascaridoid nematodes of Sirenians—the Heterocheilinae redefined

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Abstract
The two ascaridoids of sirenians, Heterocheilus tunicatus Diesing, 1839, and Paradujardinia halicoris (Owen, 1833), are redescribed. A basic affinity between the two species is indicated in the bilateral structure of the excretory system, the globular ventriculus, long intestinal caecum, and the form of the male tail. The subfamily Heterocheilinae is redefined so as to contain these two genera. It is proposed that the subfamily comprises a natural suprageneric group within the Ascarididae sensu Hartwich, 1974 containing, in addition to Heterocheilus and Paradujardinia, the following genera occurring in freshwater, estuarine and coastal vertebrates: Multicaecum Baylis, 1923; Dujardinascaris Baylis, 1947; Gedoelstascaris Sprent, 1978; Ortleppascaris Sprent, 1978; Brevimulticaecum Mozgovoy sensu Sprent, 1979; Krefftascaris Sprent, 1980. The suggestion is made that the ascaridoids of sirenians originated from forms in fluviatile lower vertebrates.