XX GIANT FISHES, WHALES AND DOLPHINS 



of external gill-clefts, they all open into a common branchial 

 chamber on each side, with a single external gill-opening 

 at the back of the head. The outer wall of this chamber is 

 usually provided by a movable bony flap, the gill-cover or 

 operculum, and, if this is lifted up, the delicate red gill-plates, 

 attached in double rows to the outer edge of each of the hoop- 

 like bony gill-arches, may be observed crowded together in the 

 underlying cavity. 



In those fishes whose normal food consists of more or less 

 minute creatures swimming in the water, there is clearly a 

 danger of some of these escaping by way of the gill-clefts and 

 perhaps clogging or injuring the delicate gill-plates. To lessen 

 this danger special structures known as gill-rakers may be 

 present, which function in exactly the same way as the baleen 

 plates of the whalebone whales, and serve to strain the water, 

 which is to pass over the gills, and to prevent any solid particles 

 from passing with it. These gill-rakers take the form of a 

 double row of more or less stiff appendages on the inner edge 

 of each hoop-like gill-arch, which project across the inner 

 openings in the walls of the gullet. In fishes that feed upon 

 minute animals and plants the gill-rakers form long, slender 

 bristles, and are very numerous and set close together; in 

 those whose food consists mainly of other fishes the rakers 

 are few in number, stouter, and set wider apart, or they may 

 even be reduced to a few bony knobs. 



Mention may be made here of the organs known as spiracles, 

 which are found in many of the Selachians, but in only one 

 or two Bony Fishes. There is one spiracle on each side of the 

 head, and this takes the form of a (usually) small opening 

 situated close behind the eye. The spiracle actually represents 

 the vestige of what was once another external gill-cleft. 



The cetacean obtains its supplies of oxygen from the atmo- 

 spheric air by means of lungs like any other mammal, the air 

 being taken in by way of the blowhole. The channel from 

 nostrils to lungs is modified so that the windpipe extends to 

 fit round the hinder end of the nasal Canal. In this way a 

 continuous connection is made between blowhole and lungs, 

 so that no water can get into the latter by way of the mouth! 

 The blowhole is closed by an elaborate set of pocket valves 

 when the animal is submerged. 



