22 SALT-WATER FISHES 



erect and ready ror use, while those situated further back lie 

 flat and point towards the throat. 



The gills, which may be regarded as dividing the head from 

 the body, are the means by which the fish oxygenates its blood, 

 taking in the water at the mouth and passing it 

 * through the gills. In some sharks and in all the 

 rays, which have the gill-slits external and separate, not 

 protected by covers as in bony fishes, the water for this 

 purpose is also taken in through small holes situated behind 

 the eye and called " spiracles." These must be particularly 

 helpful to the rays, which pass most of their time lying flat in 

 the sand with their mouth more or less buried. Most sharks 

 (and all the rays) have five of these external gill-slits ; but one 

 of the former, a rare visitor to our seas, has six, and a relative 

 occurring in the Mediterranean has seven. Bony fishes have 

 no breathing spiracle, and the gills, usually four in number, 

 instead of opening out on the surface as distinct giU-slits, all 

 lead into what is called a branchial chamber, strongly protected 

 by a hinged shield in several sections known as the gill-covers, 

 one portion of which, the " preopercle," is often furnished 

 with a toothed or spiked edge. The teeth in the gill-covers 

 are clearly designed to arrest foreign matter that might, without 

 their intervention, clog the gills. Strictly speaking, they are not 

 teeth at all, but may be regarded rather as tooth-like growths. 

 The bars of the gills are edged with hair-like growths, which 

 in fishes of very different groups, such as the herring [Clupea) 

 and basking shark {Selache), lie so close as to form a very 

 beautiful sieve arrangement, which, like the baleen of whale- 

 bone whales, retains even the most minute organisms from the 

 expelled water. The so-called false gills {^Pseudobranchia) are 

 the remains of a fifth gill, the use of which is restricted to 

 the embryonic stage of the fish. Those fishes {e.g. herring) 

 in which the gill-openings are largest soon die out of water ; 

 others {e.g. eel) in which the gill-openings are smaller can sur- 

 vive a period of removal from the water without inconvenience. 



