416 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1962 



downward by the action of the tail fin. Tlie paired pelvic fins are 

 much smaller and lie well back toward the tail. The tip of the snout 

 is bluntly pointed, and the front of the mouth lies about 18 inches be- 

 hind it ; at this point the tubular nostrils lie on each side of the mid- 

 line, and a little behind and above them the comparatively small eyes. 

 The mouth is enormous and measures about 3 feet across when open. 

 The most conspicuous feature of the head region, however, is the 

 series of gill openings, five on each side. As in all sharks the spaces 

 between the gill arches open separately on the surface and are not 

 concealed by a gill cover as in the bony fishes. In most sharks the 

 spaces appear as slits at the side of the "neck," but in the basking 

 shark they are so large and extend so far around the sides of the 

 "neck" that those of opposite sides almost meet on the upper and 

 lower surfaces — it is surprising that so little tissue is sufficient to 

 prevent the head falling off. 



The color of the basking shark is dark gray, almost black, with an 

 inconstant amount of light gray or white along the center line of the 

 belly. There are darker patches on the background color, and when 

 there are many they are arranged as indefinite longitudinal streaks. 

 The skin is entirely covered with small placoid scales, or denticles, 

 shaped like minute thorns about a millimeter long with the points 

 facing backward. The skin feels smooth to a hand passed over it from 

 before backward, but exceedingly rough to one passed in the reverse 

 direction. It is also plentifully supplied with mucus glands, so that 

 during life a film of slime covers the entire surface to the summits of 

 the denticles. The skin covering the body is continuous with that 

 lining the mouth (on which, however, the denticles are very much 

 smaller), but where the skin passes over the jaws the denticles are 

 enlarged and form the teeth. The teeth are unlike those one expects 

 to see in the mouth of a shark — there is no fearsome array of pointed 

 blades, sharp as razors — for they are no more than about 5 millimeters 

 long and are arranged in four to seven rows so that they look like a 

 rather coarse rasp. 



The teeth alone show that the basking shark is no fierce predator, 

 and indeed all its anatomy and habits are highly adapted to its very 

 specialized method of feeding. Like the great whalebone whales this 

 shark is a plankton feeder, and although it is one of the biggest fishes 

 in the sea its food consists of minute creatures barely visible to the 

 naked eye. Like the plankton-feeding whales, too, the basking shark 

 is provided with a special apparatus for straining its food in bulk 

 from the sea water. Fishes breathe by pumping a stream of water 

 over the gills, where some of the oxygen dissolved in the water passes 

 through the exceedingly thin covering membrane into the blood in the 

 underlying capillaries, and the carbon dioxide in the blood passes out 

 into the water. The stream enters the fish's mouth and passes out 



