312 THE SEA SHORE 



the same time for steering. Occasionally, too, the latter are both 

 spread out at right angles to the body when the fish desires to stop 

 suddenly, and are even employed at times in swimming backwards. 



When a fish wants to turn to cne side, it will be seen to give the 

 tail a sharp motion to the opposite side. The pectoral of the latter 

 side is also brought into play, while the other is kept close against 

 the body. 



If the pectoral or pelvic fin of one side is injured, the body of 

 the fish will incline to the opposite side ; and if all the paired fins 

 are functionless the fish swims with its head inclined downwards. 

 Observations of fishes in which the dorsal or ventral fins are 

 injured will also show that these organs are necessary to maintain a 

 steady motion in the water. 



In addition to the above facts, it may be mentioned here that the 

 paired fins are often modified into long finger-like processes that 

 serve as organs of touch, and even as means by which the fish can 

 creep along the bottom. This is notably the case with gurnards 

 and a few of the other fishes that spend their time almost exclusively 

 on the bed of the sea. 



Fishes are essentially gill-breathers, the gills being generally 

 fringe-like organs, supported on bony arches (the gill arches), num- 

 bering four on each side, the cavity containing them being covered 

 by a gill-cover (operculum) that opens behind. Water is taken in 

 at the mouth, whence it passes into the gill-chamber ; and after 

 passing between and around the gills, it escapes under the opercula. 

 The gills themselves are richly supplied with bloodvessels that are 

 distributed close to the surface, and an exchange of gases takes 

 place through their exceedingly thin walls, carbonic acid gas pass- 

 ing from the blood to the surrounding water, and oxygen, held in 

 solution in the water, passing from the water to the blood. 



When fishes are in foul water, containing but little oxygen in 

 solution, they rise to the surface in order to make up the deficiency 

 by taking oxygen direct from the air. This, however, is an unnatural 

 proceeding with the majority of fishes ; but there are some that are 

 provided with accessory breathing organs specially adapted to the 

 extraction of oxygen direct from the air, and these are so dependent 

 on the supply from this source that they are suffocated if prevented 

 from reaching the surface. 



In other fishes, such as the sharks and rays, the gills are of an 

 entirely different character from those described above, for they are 

 pouch-like and five in number on each side, each pouch communi- 



