Summary

The taxonomy of the New Zealand vermetids of the genus Serpulorbis is discussed. The shell, animal and dentition of S. zelandicus (Q. & G.) are described, and a new species, S. aotearoicus, is proposed. Novastoa zelandica becomes Novastoa lamellosa (Hutton). A full description is given of the external features, pallial organs and mode of ciliary feeding. Resemblances to “Vermetus” novae-hollandiae are indicated. In addition, S. zelandicus is shown to possess a large pedal mucous gland, and to feed to some extent by the supplementary mode of forming mucus strings. The species is regarded as illustrating the transition from exclusive ciliary feeding in “Vermetus” novae hollandiae to the complete development of mucus trap feeding in Serpulorbis gigas. The morphology and histology of the alimentary canal of Serpulorbis zelandicus is described, with an account of the ciliary currents and mode of function of the stomach and crystalline style caecum. The female reproductive system shows adaptations for current fertilization and for incubation of the eggs within the shell of the female. A comparative discussion follows, on the position of Serpulorbis zelandicus with relation to other members of the Vermetidae. Yonge's view that the ciliary feeders and the mucus trap feeders form two distinct groups is not upheld. A continuous evolutionary trend is shown from the more primitive ciliary feeding condition to mucus feeding, with reduction of the ctenidium and enlargement of the pedal gland. The genera Bivonia, Aletes, Petaloconchus, and Spiroglyphus are associated with the present series.

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