120 NATURE STUDY 



united in one. The arm and leg fins do the swimming, 

 while the steering is done by the muscular tail and the 

 tail fin. 



To understand clearly the breathing of fish, it is neces- 

 sary for the teacher to understand the breathing of plants. 

 This function is properly called respiration in plants as 

 well as in animals, because in both the outward visible 

 sign is the same, viz., the giving off of carbon dioxide. 

 Respiration in the case of plants is continually taking 

 place, but under certain conditions more than enough 

 oxygen is given off to mask completely the breathing. 

 These conditions are the assimilation of the food which 

 is brought about by the presence of some carbon dioxide 

 and strong light. The carbon dioxide is breathed out by 

 both fish and plants. 



Now it follows that an aquarium with plants, light, 

 and animals can be made self-supporting, since in the 

 light the plants yield what the fish must have in abun- 

 dance for their respiration, while the fish give in return 

 the carbon dioxide necessary to the assimilation of the 

 plants. In breathing, the fish swallows the water oxy- 

 genated by the plants. As it is washed past its four 

 pairs of red gills, an osmosis of the two gases — the car- 

 bon dioxide of the blood and the free oxygen in the water 

 — takes place. The blood is thiis purified, and the water 

 charged with carbon dioxide passes out under the gill 

 covers, carrying to the plant what it needs for its food, 

 and receiving from it again the oxygen necessary to all 

 the animals in the aquarium. 



The nostrils of the gold fish are well developed and 

 easily seen. In them is located the sense of smell. They 

 take no part in the work of breathing. 



The eyes are movable, but unprotected by lids. 



