The length of the shell varies between 25 m mand 40 mm. [4]
The shell of these animals is very thin and delicate, and internal, completely covered by the mantle (which has fused lobes) so the appearance of these animals more closely resembles that of dorid sea slugs rather than sea snails, but anatomically quite different. They are readily distinguished by the anterior siphon and typically snail-like, tentacle-bearing head beneath the
anterior mantle. There are no rhinophores (chemosensory tentacles) or dorsal circlet of gills.
The shell is present but is completely internal. It is covered by a fleshy mantle. The ventral surface features a distinct foot and a head bearing tentacles with basal eyes. The anterior of the dorsal surface (notum) is indented along the midline, forming a short siphon. The mantle is relatively firm but has a somewhat gelatinous texture, appearing mostly smooth. The internal shell resembles an ear, being thin and fragile. [4]
The colour is translucent, greyish-white to pinkish or yellow with black/brown spots and blotches. This colouration is variable, resembling that of the ascidian prey on which they live and feed and thus providing camouflage. [4]
^Philippe Bouchet, Rocroi J.P., Hausdorf B., Kaim A., Kano Y., Nützel A., Parkhaev P., Schrödl M. & Strong E.E. (2017). Revised classification, nomenclator and typification of gastropod and monoplacophoran families. Malacologia. 61(1-2): 1-526.
Philippe Bouchet, Rocroi J.P., Hausdorf B., Kaim A., Kano Y., Nützel A., Parkhaev P., Schrödl M. & Strong E.E. (2017). Revised classification, nomenclator and typification of gastropod and monoplacophoran families. Malacologia. 61(1-2): 1-526.
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