Janthina janthina is a small snail. It's shell can be up to 4 cm high and 3 cm broad. The shell is globular, fragile and thin with a dark blueish-purple colouration which is darker at the base. The surface of the shell apears smooth but may possess regular growth lines and irregular spiral grooves, especially at the edge of the last whorl. The shell can be high or low with the last whorl constituting 75-80% of the the height in high shells, and aproximately 90% in low shells (Graham, 1988). The aperture would be 45% and 70% respectively (Graham, 1988). The protoconch (whorls at shell apex formed by the larval snail) axis is slanted. The large aperture is angular with a thin lip and small umbilicus, which is where the outer lip and columella meet. 

The head of the violet snail has a long cylindrical snout with a terminal mouth. The tentacles are forked, have no eyes and project half way along its side's. The foot has a broad anterior and tapers to a posterior point. There is a broad depression or funnel at the anterior part of the sole, where the float is secreted. The posterior of the sole has longitudinal grooves. The flesh of this species is deep violet or black and in adults the operculum is absent.

Janithina janthina is the commonest member of the Janthinidae family. This species is ovoviparous where the eggs develop within the females genital tract and later shed as veliger larvae from the left side of the mantle cavity (Graham, 1988). The males are aphallic (no penis) and produce spermatozeugmata that swim to the female and enter the genital tract. Therefore fertilization occurs before the eggs have left the ovary, suggesting the species is a protandrous consecutive hermaphrodite.