In 1884, Philip Henry Gosse reported observations by "Professor Grant" (possibly Robert Edmond Grant) that two species of nudibranchs emit sounds that are audible to humans. Two very elegant species of Sea-slug, viz., "Eolis punctata" [i.e. "Facelina annulicornis"], and "Tritonia arborescens" [i.e. "Dendronotus frondosus"], certainly produce audible sounds. Professor Grant, who first observed the interesting fact in some specimens of the latter which he was keeping in an aquarium, says of the sounds, that 'they resemble very much the clink of a steel wire on the side of the jar, one stroke only been given at a time, and repeated at intervals of a minute or two; when placed in a large basin of water the sound is much obscured, and is like that of a watch, one stroke being repeated, as before, at intervals. The sound is longest and most often repeated when the "Tritonia" are lively and moving about, and is not heard when they are cold and without any motion; in the dark I have not observed any light emitted at the time of the stroke; no globule of air escapes to the surface of the water, nor is any ripple produced on the surface at the instant of the stroke; the sound, when in a glass vessel, is mellow and distinct.' The Professor has kept these "Tritonia" alive in his room for a month, and during the whole period of their confinement they have continued to produce the sounds with very little diminution of their original intensity. In a small apartment they are audible at the distance of twelve feet. The sounds obviously proceed from the mouth of the animal; and at the instant of the stroke, we observe the lips suddenly separate, as if to allow the water to rush into a small vacuum formed within. As these animals are hermaphrodites, requiring mutual impregnation, the sounds may possibly be a means of communication between them, or, if they are of an electric nature, they may be the means of defending from foreign enemies one of the most delicate, defenceless, and beautiful Gasteropods that inhabit the deep. Nudibranchs are hermaphroditic, thus have a set of reproductive organs for both sexes, but they cannot fertilize themselves. Mating usually takes a few minutes, and involves a dance-like courtship. Nudibranchs typically deposit their eggs within a gelatinous spiral, which is often described as looking like a ribbon.