Sponges are almost exclusively marine, the Spongillae alone being inhabitants of fresh water; and they are of almost universal occurrence. The sponges of commerce are mostly obtained from the Grecian Archipelago and the Bahama Islands. The common marine sponges are mostly found attached to some solid object between tide-marks or in deep water. One genus (Cliona) inhabits branching cavities in shells, which the sponge excavates for itself, apparently by means of its siliceous spicula; and fossil shells mined by a boring-sponge, allied to the recent Clionae, are found from the Silurian rocks upwards. The siliceous sponges appear to be exclusively inhabitants of the deeper parts of the ocean, and our knowledge of these beautiful forms has been enormously increased during late years by the researches into the fauna of the deep sea, which have been carried out by Sir Wyville Thomson, Carpenter, Sars, and other well-known observers. Much also has been added to our knowledge of the Calcispongiae by the elaborate investigations of Haeckel, Oscar Schmidt, Metschnikoff, and others. The calcareous sponges are all marine, and all inhabitants of shallow water, and the living forms are all of small size.