120 



OF RESPIRATION. 



Fig. 91. 



Fig. 92. 



complished by a different apparatus. The air is to be 



derived from the water, in which 

 more or less is always diffused. 

 The organs for this purpose are 

 called branchia or gills, and are 

 either delicate tufts or plumes floating outside of the body, 



as in some of the marine worms, 

 (Fig. 33,) and many mollusks, (Fig. 

 91, g ;) or they consist of deli- 

 cate combs and brushes, as in fishes, 

 (Fig. 92,) crabs, and most mollusks, 

 (Fig. 88, g.) These gills are al- 

 ways so situated that the water has 

 free access to them. In the lower aquatic animals, such as 

 the polypi, and some jelly-fishes and mollusks, respiration 

 takes place by the incessant motions of vibratory cilia, which 

 fringe both the outside and the cavities of the body ; the cur- 

 rents they produce bringing constantly fresh supplies of water, 

 containing air, into contact with the respiratory surface. 



250. Many animals living in water, however, rise to the 

 surface and breathe the atmosphere there, or are furnished 

 with the means of carrying away a temporary supply of air, 

 whilst others are furnished with reservoirs in which the blood 

 requiring oxygenation may be accumulated, and their stay 

 under water prolonged. This is the case with the seals, 

 whales, tortoises, frogs, many insects and mollusks, &c. 



251. The vivifying power of the air upon the blood is due 

 to its oxygen. If an animal be confined for a time in a 

 closed vessel, and the contained air be afterwards examined, 

 a considerable portion of its oxygen will have disappeared, 

 and another gas of a very different character, namely, car- 

 bonic acid gas, will have taken its place. The essential 

 office of respiration is to supply oxygen to the blood, at the 

 same time that carbon is removed from it. 





