Although the Crustacea as a whole are predominantly active animals, many examples have already been mentioned of species which are more or less sluggish and sedentary in their habits. The extreme degree of passivity is reached by the Barnacles (Cirripedia), which differ from all other Crustacea (except some parasites) in being fixed to one spot, and quite without the power of locomotion in the adult state. Most of the Barnacles met with on the shore or in shallow water belong to the division of the Sessile Barnacles or Acorn-shells (Operculata). Every visitor to the seashore has noticed the little conical shells which cover exposed rocks as if with a coat of rough-cast. On the British coasts the commonest species is _Balanus balanoides_ (Plate III.), though other species closely resembling it are also common. They are to be found almost up to high-water mark in situations where they are left uncovered for many hours every day; but the valves which close the opening of the shell fit so tightly that a little sea-water is enclosed, and the animal is protected from drying up even when exposed to the heat of the sun. If a stone or a chip of rock, with a few of these animals on it, be placed in a jar of sea-water, their peculiar mode of obtaining food can easily be watched. The valves will presently be seen to open a little, and the curled cirri will be protruded, opened out like the fingers of a hand, and withdrawn again with a sort of grasping motion. These movements are continued without stopping while the animal is under water. If the cirri be examined with a pocket-lens or under a microscope, it will be seen that they are fringed with stiff bristles, so that, when they are opened out, the whole forms a kind of "casting-net." As it is swept through the water, this net entangles minute floating particles of animal or vegetable matter, and carries them into the shell, so that they can be seized by the jaws and swallowed. The cirri, as we have already seen, are really the modified thoracic limbs, so that, in Huxley's words, "A Barnacle may be said to be a Crustacean fixed by its head, and kicking the food into its mouth with its legs." A mode of obtaining food by "net-fishing," not unlike that employed by the Barnacles, is found in certain Crustacea belonging to a widely different group--the little "Porcelain Crabs" (Fig. 41) mentioned above. Mr. Gosse observed that the Broad-clawed Porcelain Crab (_Porcellana platycheles_) employed its third pair of maxillipeds, which are thickly fringed with long feathered hairs, in making alternate casting movements "exactly in the manner of the fringed hand of a Barnacle, of which both the organ and the action strongly reminded me." CHAPTER VI CRUSTACEA OF THE DEEP SEA