Marine gastropods of the genus Conus have been recorded for nearly a hundred years as having caused serious and even fatal accidents by stinging human beings, but have so far found no place among the venomous animals dealt with in textbooks of tropical medicine. Prior to 1932 all such accidents, of which five proved fatal, were confined to the Western Pacific. A fresh case of stinging by Conus geographus Linn., fortunately not fatal, is now reported from the Seychelles Islands and is the first case recorded from the Indian Ocean. The method by which the animal introduces its poison into the body of its victims has so far not been directly observed, but a definite apparatus which probably secretes the poison, and is adapted for its ejection, is revealed by anatomical dissection and histological study, and there is good circumstantial evidence to show that the poison is injected through a detached tooth from the radula used as a hypodermic needle. Histological study further reveals that the so-called “Gland of Leiblin” is not a gland at all and cannot be the source of the poison (except indirectly by acting as a reservoir for it and as a muscular propelling organ for its ejection). The source of the poison is almost certainly the coiled tube previously regarded as the duct of the “Gland of Leiblin”; being lined by tall, secreting epithelium, it is really equivalent to a tubular gland which can, however, at the same time, act as the poison duct. This is a new conception. Attempts to verify the effect of the poison on mice with extracts (in 30 per cent. alcohol) of the poison from the poison apparatus of Conus geographus Linn., and of Conus leopardus Röding (= millepunctatus Lamark) after prolonged storage in the tropics, failed either through inactivity of the extract or through refractoriness of the mice used. It is accordingly suggested that a fresh series of experiments might be undertaken by some worker in the tropics, where the poison could be obtained and used in the fresh condition. Since the effects of Conus poison appear to be opposite to those of the poison secreted by the gland at the base of the venomous spine of the dorsal fin of fishes of the genus Synanceia, investigations on the nature and effects of the former might profitably include attempts to confirm this notion. Observations might also be made on the nature of the stimuli able to produce the stinging reflex in Conus particularly when handled. Original work on the anatomy, macroscopic and microscopic, of the poison apparatus of Conus geographus and Conus leopardus, not previously attempted, is described and illustrated. As full extracts of reports on actual cases of cone stings and their sources of publication are given in the form of an Appendix, these have been omitted in the usual list of references at the end of the paper.