It might be thought that in animals like the Lobster, enclosed in a hard shelly covering, the sense of _touch_ must be very dull, if not altogether absent. This, however, is not the case. What is probably a very delicate tactile sense is provided for by the numerous hairs which are found, of many sorts and sizes, all over the body and limbs. Each of these hairs is really a hollow outgrowth of the chitinous covering, containing a delicate prolongation of the soft tissues underneath, and also supplied, in many if not in all cases, with a nerve-fibre, so that the slightest movement of the hair caused by contact with a solid body is perceived by the animal. Many of these hairs are themselves beset with delicate secondary hairs, arranged so that the whole looks like a feather or like a bottle-brush. These hairs are adapted for detecting slight movements or vibrations in the surrounding water. Whether Crustacea living in water can _hear_, in the sense in which the word is used of animals living in air, is doubtful; but it is certain that they are extremely sensitive to vibrations only a little coarser, so to speak, than those we know as sound. The Lobster, and many other Crustacea, do indeed possess a structure which was long supposed to be an organ of hearing, and may possibly in part fulfil that function, although it is now known that that is not its only or even its chief use. It consists of a small cavity in the basal segment of the stalk of the antennule, opening to the outside by a narrow slit on the upper surface of the segment. The cavity is lined by a delicate continuation of the chitinous covering of the body, and has on its inner surface a series of feathered hairs of the kind described above, which are richly supplied with nerve-fibres from a large nerve entering the base of the antennule. Within the cavity, and for the most part entangled among these hairs, are a number of grains of sand. When the Lobster moults, the lining membrane of this cavity is thrown off like the rest of the exoskeleton, and with it the contained sand-grains. While the shell is still soft after moulting, and the lips of the slit are not rigid, as they afterwards become, fresh sand-grains find their way into the cavity to take the place of those which have been cast off. Perhaps, like some other Crustacea, the Lobster buries its head in the sand to insure that some grains may find their way in; for its pincers are too clumsy for it to pick up sand-grains and to place them in the cavity, as some Prawns have been seen to do. At all events, if a freshly moulted Prawn be placed in a vessel of sea-water, and supplied, instead of sand, with powdered glass or metal filings, particles of glass or metal will after a short time be found in its antennular cavities. This habit has been utilized in a very ingenious experiment by which the function of these organs was demonstrated. A Prawn had been induced in this way to place