THE SHL'V.RK THAT HIBERNATES — MATTHEWS 417 



through the gill slits between the curved gill arches of bone or carti- 

 lage on which the gills are borne. The gills are complexly folded, 

 so that they have a very large surface area through which exchange of 

 gases between blood and water can take place; in a basking shark 7 

 meters long this area is approximately 270 square meters. In some 

 fishes, including the basking shark, the gill arches carry, in addition 

 to the gills, structures know^n as gill rakers. The rakers of the shark 

 resemble long bristles set along the edge of the gill arches with their 

 free ends pointing toward the mouth cavity ; there are 1,000 to 1,300 of 

 them on each side of all the 5 gill clefts. The rakers, which are highly 

 modified placoid scales, are flexible but stiff, and are calcified toward 

 their bases, and there is a mass of mucus glands at their attachment to 

 the arch. Muscle fibers connected to their bases can erect the rakers so 

 that they stand out and project across the space between adjacent 

 arches. Wlien the fish opens its mouth the rakers spring up and form 

 a fine sieve through wdiich the water has to pass before it flows over 

 the gills. 



When feeding near the surface the basking shark cruises slowly at 

 about 2 knots with the mouth widely open, filtering the plankton out 

 of the sea water and at the same time automatically respiring. The 

 gill slits are widely opened and the whole pharyngeal region is 

 greatly expanded and characteristically fanned out. The area of tlie 

 open mouth is at least % square meter so that, at 2 knots, 1,484 cubic 

 meters of water are filtered per hour — well over a thousand tons. 

 When the mouth is periodically shut the gill slits close and the 

 rakers fold down on to the arches so that the plankton entangled in the 

 mucus secreted over them passes into the mouth and is svvallowed. The 

 back of the mouth is covered with papillae which at the gullet are 

 complexly branched and knobbed and form a valve that closes the back 

 of the mouth — they look strangely like a huge cauliflower filling the 

 passage. The stomach is very large, and when the fish is feeding, it 

 contains about half a ton of tomato-red mush of plankton of which, 

 however, only about 30 percent by weight is organic matter, much of it 

 being mucus derived from the fish itself. 



The first stages of digestion take place in the stomach and the small 

 planktonic Crustacea are broken up into minute fragments ; they are 

 concentrated on passing into the narrow pyloric loop of the stomach 

 where great quantities of water are removed, for the content of this 

 organ is a firm paste resembling anchovy paste in color and consist- 

 ency — and, somewhat, in taste. On leaving the pylorus the gut con- 

 tents enter a small sac, the bursa entiana, in which the oily fraction is 

 separated. The sac is filled with bright red oil, the color being pro- 

 duced by the astacene derived from the Crustacea. It is difficult to 

 understand how the oil can be retained in the bursa while the solid 

 contents of the gut pass on, but so it is. The food from which the oil 



