418 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1962 



has been extracted, passes on as a brownisli-yellow chyme to the duo- 

 denum and spiral valve, where digestion is completed under the influ- 

 ence of the bile and pancreatic secretions. The oil in the bursa is taken 

 up by the cells of its lining and finally transferred to the liver after 

 undergoing metabolic change. 



The liver is comparatively enormous, its two huge lobes weighing 

 together about a ton, no less than a quarter of the total weight of the 

 fish, by far the greater part of its weight being due to the oil. In the 

 economy of the fish the liver acts as a hydrostatic organ so that the 

 total density of the fish, which possess no air bladder, approximates 

 to that of the sea water. In the economy of man the liver is the fish's 

 undoing, for the oil is easily extracted and has a commercial value — 

 without this wealth to be had for the taking, the basking shark would 

 be of no interest to the fisherman. On the west coasts of Ireland and 

 Scotland the basking shark had been for centuries the subject of a 

 cottage industry until the introduction of paraffin about the middle 

 of the last century. A moderate number of them were taken annually 

 with primitive gear worked from small boats in order to obtain illumi- 

 nating oil for winter nights. Petroleum killed this fishery and for 

 long the sharks were seldom molested but in recent years commercial 

 fishing on a larger scale has been developed. 



Basking sharks appear off the British coasts in some abundance 

 during the spring and depart in the early autumn — whence they come 

 and whither they go is unknown. But even in the winter occasional 

 fish are seen and sometimes caught or stranded on the shore. There 

 is a very peculiar thing about these winter strays — they have no gill 

 rakers; in every way the fish are normal except in the complete ab- 

 sence of rakers from the gill arches. Careful dissection, however, 

 reveals that under the skin of the gill arches a set of rakers is in course 

 of being developed. As the rakers are homologous with scales and 

 teeth which are replaced as they are worn out, there is no reason why 

 they too should not be replaced when necessary. When fully devel- 

 oped they erupt through the skin of the arches much as we cut our 

 second set of teeth. 



Why should this replacement be necessary? And how does the 

 shark feed when it has no rakers? During the warmer months the 

 fish feed luxuriously on the rich summer plankton, gulping it down 

 by the ton ; but in winter the pastures of the seas are bare and there is 

 little nourishment to be fomid. The power needed to propel a feeding 

 shark of average length at its feeding speed of 2 knots is about 0.33 

 h.p. ; the heat equivalent of this is 212 calories per hour. Allowing 

 40 percent efficiency to the shark's muscles in doing work, and 80 per- 

 cent to its tail as a propeller — a very generous allowance — a shark 7 

 meters long would need to take each hour food with a calorific value 



