Aspects of the biology of Hyolitha (Mollusca)

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Abstract
Hyolitha constitute an extinct group of class rank assigned to Mollusca. Two orders, Orthothecida and Hyolithida, are well established and most knowledge of morphologic detail is derived from the latter. Both orders had an operculum not hinged to the conch. Hyolithida also had paired, curved, whiskerlike appendages, requiring complex musculature to move them and the operculum. The Hyolithida were probably deposit feeders living in shallow water, and accordingly were tentaculate. A reconstruction of the soft parts of this sedentary organism is given-a shallow mantle cavity on the dorsal side, anterior tentacles, a long intestine, and a reduced ventral foot. Except for the more complex musculature associated with an elaborate operculum, Orthothecida are judged to have had a similar anatomy.