FISHES. 209 



349. The Fish has not lungs, for these are organs which 

 are fitted to introduce air alone to the blood. But it has 

 gills, which are fitted to have the blood in them acted 

 upon by the air that is mingled with water. These gills 

 are fringes which are made up of very minute blood-ves- 

 sels. There are commonly four of them, fixed to some 

 arches of bone ; and they are covered on the outside by 

 a lid, called the operculum. In order to have the air in 

 the water act on the blood in them, the water is taken 

 into the mouth, and then passes out through these fringes. 

 If you watch fishes in an aquarium, you will see the mouth 

 constantly opened to take in the water, and the opercu- 

 lum as constantly raised to let it out. z' , ^ ; 



350. When a fish is taken out of the water it really 

 dies for want of air, although it is in the midst of a plenty 

 of it. The explanation is this : the fringes of the gills 

 are kept apart by the water while the fish is in its native 

 element ; but, when taken out of it, the fiinges fall to- 

 gether, and soon become dry. When they are in this 

 condition the blood will not circulate freely in them, and 

 what blood is there is not acted upon by the air. In 

 agreement with this explanation we find that those fishes 

 which live the shortest time out of the water have their 

 gills most exposed, while those that live a longer time 

 have their gill-openings narrow, thus tending to keep the 

 fringes moist. In some there is an especial .arrangement 

 for moistening them, and in such a case the fish can live 

 in air quite a long time. Dr. Carpenter states that some 

 fishes having this arrangement are accustomed to leave 

 the water and crawl about in the grass or on the ground. 



351. The plan of the circulation in the Fish is peculiar. 

 In the Mammals and Birds there is a double heart, as 

 illustrated in Fig. 155. In Reptiles the heart is double 

 only so far as the auricles are concerned, as illustrated in 

 Fig. 156. In Fishes the heart is. single, having but one 

 auricle and one ventricle. The blood passes from the 

 ventricle to the gills. Here it becomes arterial blood, as 



