Simnia patula is a small species of up to 2 cm long and 0.8 cm wide. It has a thin glossy shell that is oval in shape. In adults, its shell is convoluted with a spire enclosed by the body whorl. However, in young individuals the spire is exposed, with three swollen whorls. The aperture is elongate and oval, with flared lips that are longer than the other parts of the shell. The lips extend at each end into a canal. The basal inhalant canal is much broader and less distinct from the aperture than the apical exhalent canal. Simnia patula has no operculum and the extremities of the shell may be ornamented with growth rings. The colour of the shell can be white, yellow, pink or dark red with a yellow body having brown streaks and spots primarily on the mantle. In this species the mantle edge actually forms an inhalant siphon. The mantle also has 2 large lateral flaps that cover the shell when the animal is mobile. Simnua patula has a large foot that is ridged longitudinally and pink at the front. The head bears two long tentacles that are white on the upper surface with brown tips. Each tentacle has a basal eye that may be visible with a hand lens.Simnua patula feeds on Alcyonium digitatum, Alcyonium glomeratum, Eunicella verrucosa and the hydroid Tubularia indivisa. It lays its eggs in egg capsules in an irregular roundish mass spread in a layer over an inch thick on Alcyonium digitatum colonies. Each egg capsule is ca 0.35 cm in diameter and round to polygonal in shape and stick together in a tough, slightly yellowish - colourless layer (Lebour, 1932). All of the eggs develop into free-swimming larvae which can be the predominant larval species in the plankton during the summer months, especially August. Lebour (1932) noted that Simnua patula appear to have a long larval stage as the late larvae lived for several weeks without losing the velum.
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