CHARACTERISTICS AND HABITS OF FISHES 21 



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 con- 

 ditions 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 sur- 

 face 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 accomodate 

 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 



