Possible Boring Structures of Sipunculids

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Abstract
At the present time there is no experimental evidence which links the supposed boring activities of sipunculids to a specific organ or structure. Structures which have been speculatively associated in the literature with boring are: hooks and spines of the introvert, cuticular papillae with associated epidermal glands, anterior and posterior horny shields, and anterior calcareous shields. In this review these structures are described as they occur in five representative species of sipunculids collected by the author from calcareous rock in the Indian Ocean or the Caribbean Sea. The five species are: Pliascolosoma antillarum Grube and Oersted, Phascolosoma dentigerum (Selenka and de Man) , Paraspidosiphon steenstrupi (Diesing), Lithacrosiphon gurjanovae Murina, and Cloeosiphon aspergillum (Quatrefages). Localities of collections are cited, habitats and burrows are described, and the behavior of the animals as observed in the field and laboratory is noted. In view of the morphology of the possible boring structures and in light of observations on habitats and behavior, the possible roles of the structures in boring activities are discussed. Highly organized horny shields are present at the anterior and posterior extremities of thetrunk or Paraspidosiphon steenstrupi, whereas anterior calcareous shields are characteristic of Cloeosiphon aspergillum and Lithacrosiphon gurjanovae. Papillae and epidermal glands are present in all five of the species but these are most highly developed in Phascolosoma dentigerum and P. antillarum. Of the species considered, only P. antillarum lacks hooks on the introvert. Because of the position of the animal within the rock with anterior end directed toward themouth of the burrow, it is assumed that the anterior shields and the hooks of the introvert play no significant role in the formation of the burrow. However, the rigid papillae of the trunk and the thickened posterior shield, if rubbed against the wall of the burrow, presumably could be utilized in the mechanical attrition of the more friable rock, whereas the secretory products of the numerous epidermal glands might be implicated in the chemical dissolution of the harder substrates.

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