The Larval Shell of Some Prosobranch Gastropods

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Abstract
Growth of shells of larvae of Nassarius reticulatus (L.) and Crepidula fornicata (L.) reared in the laboratory under constant temperature (14 °C) and illumination (155 lx) occurs by the addition of successive strips secreted at the mantle edge. Growth lines on the embryonic teloconch also indicate intermittent growth; the protoconch secreted by the shell gland has no such lines. There is evidence that growth periods are short and pauses separating them prolonged. The diameter of the older whorls of the shells of these and six other species examined increase with the volume of the viscera. This change in diameter is made possible by the flexibility of the organic matrix, the funda-mental constituent of the shell. The matrix, secreted as fibres at the mantle edge, forms sheets, and provides sites for the deposition of calcareous concretions; at metamorphosis the concretions apparently initiate the formation of crystals. The concretions also sculpture the surface of the shell producing a pattern sufficiently conspicuous to allow identification of some species with the aid of only the stereomicroscope. For example, large concretions form longitudinal rows over the protoconch and teloconch of Littorina neritoides (L.) and smaller ones combine to form a more decorative pattern in Rissoa sarsi Loven. In other species details of the surface design require a scanning electron micro-scope. Interspecific differences in the arrangement of concretions may be traceable to differences in the pallial glands which secrete the matrix.