Of these different groups or species, the marine fishes are industrially by far the most important, for at least two-thirds of all the fish in the world live in the sea, and the capture of these sea-fish in enormous quantities constitutes the fishing industry, with which we are concerned. The different species of marine fishes can be divided into three well-marked groups, according to their habits and habitats. [Illustration: FIG. 2 COD (_Gadus morrhua_) Length up to 5 ft.; usually caught at about 3 ft. _Food._—Small crustaceans, molluscs, and young fish. _Range._—North of Norway and Iceland to the Bay of Biscay, and from Greenland to New York.] (1) There are the true deep-sea fishes that live at the bottom of the sea, for example cod, haddock, plaice, sole. These are called “demersal” fish. Fish, like birds, inhabit a medium that is continuous throughout the world. A glance at the map of the world will show that the three great oceans—Atlantic, Indian and Pacific—are united in the southern hemisphere. In Tertiary times, it is practically certain that the Pacific and the Atlantic oceans were also united at Darien, and that the Mediterranean was united with the Red Sea. Apart, therefore, from differences in local conditions, for example of temperature and food supply, there is practically no obstacle to the world-wide distribution of any particular species of fish. At the bottom of the sea, the temperature, the food supply, and the general conditions of life are singularly uniform all over the world, consequently there are no barriers at all to the dispersion of demersal fish, and we find various species widely distributed in all seas. Demersal fish, on the whole, are more primitive in type than those that live nearer the surface. They have well-developed senses of touch and smell by means of which they hunt for their food. They differ markedly in structure and shape from surface or shallow-water fish, their bodies being designed to resist the greater pressure of deep water. The body is generally lean and is enclosed by a wall of muscular fibre. Shallow-water fish, if introduced into deep water, would be crushed inward by the pressure. Similarly, the deep-living, demersal fish are unable to accommodate themselves to shallow water and, if placed in it, soon become unhealthy. A cod floats helplessly on its side when placed in shallow water, owing to the dilatation of its swimming bladder. If the bladder is pricked it collapses, and the fish is able to regain an upright position. This is done when cod and other similar demersal fish are kept alive in sea-water tanks on board ship, to be delivered to the markets alive. In Denmark, fish are delivered alive to the shops. When fishes from great depths are brought to the surface, their bodies break into pieces owing to the reduced external pressure, the scales start from their skin and the eyes from their sockets.