Typical adult M. perspicua are over 10 mm long, have a tuberculate dorsal surface and come in a wide range of colours and patterns, often resembling sessile animals in the vicinity. Smooth, whitish, juvenile M. perspicua like this, with yellow marks near the edge of the mantle (1) and a fine black stipple with greyish discs mimicking the ascidian Tridemnum, occasionally occur and can be easily mistaken for adult L. latens which typically has this combination of features. In such cases, examination of the shell is necessary to see if the spire (2) protrudes beyond the outline of the shell (3) (M. perspicua) or not (L. latens). The shape of the shell and its protruding spire (4) can be detected on this live specimen by the yellowish viscera covered by it.
For defence against predators, M. perspicua relies on its cryptic colouring and on the ability of the mantle, when irritated, to secrete highly distasteful acid from white glands in the mantle skirt near the edge of the internal shell (Usually hidden by foot on living specimens; see image 17.1 for dissection). Body length 11 mm, shell length 6.6 mm, Dorset, England. April 2012.