ADDITIONAL PACTS ABOUT THE FISH. 87 



it out over tlie gills and under the gill covers. If a 

 fish were to have its mouth propped open, it could 

 not produce this current, and so would very likely 

 suffocate. Since the current of water is backward 

 over the gills, it is easier for the fish to breathe when 

 it is fronting up stream. If it were held with the 

 head down stream in a swiftly flowing current, it 

 would have great difficulty in breathing. So, in 

 running water, fishes generally lie with their heads 

 up stream, and they swim up stream in preference to 

 swimming down. In this way fishes are distributed 

 to small streams, and at the time of a heavy rain, 

 when furrows and ditches are filled with water, they 

 may reach ponds that on ordinary occasions have no 

 outlet, or may even be stranded on places that are dry 

 when the rain is over. This is a sufficient explana- 

 tion for most of the stories that we hear of fishes 

 raining down. 



The blood is red. It consists of many red corpus- 

 cles and a smaller number of white corpuscles floating 

 in a liquid called the plasma. Each red corpuscle 

 has a small body in the center called the nucleus. 

 The blood of a fish has the same temperature as the 

 water in which it lives. As the water in which a fish 

 lives is generally colder than our bodies, and so feels 

 cold to our hands when we put them into it, we say 

 that the fish is cold blooded. 



The heart consists of one auricle and one ventricle. 

 Such a heart is said to be single. The blood is sent 

 forward to the gills. In consequence of the fact 

 that all the blood is sent from the heart to the breath- 

 ing organs, the heart is said to be a respiratory heart. 



When a fish is taken out of the water, it can live 

 only so long as the gills are kept moist. When the 

 gills dry out, as they do very soon, the membrane of 

 the gills cannot exchange the respiratory gases, and 

 the fish dies of suffocation. The gills of a fish are not 



