94 OF RESPIRATION. 



complished by a different mechanism. The air is to be 



derived from the water, in which 

 more or less is always diffused. 

 The organs for this purpose are 

 91- called branchi<z 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 folds, as in fishes (Fig. 92), 

 crabs and most mollusks (Fig. 88, g}. 

 These gills are always so situated 

 that the water has free access to Fig. 92. 



them. In the lower aquatic animals, such as the polypi, 

 jelly-fishes and some mollusks, respiration is facilitated 

 by the incessant motions of vibratory cilia, which line 

 the respiratory organs as well as other portions of the sur- 

 face of the body ; the currents they produce bringing 

 constantly fresh supplies of water containing air in contact 

 with the respiratory organs. 



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. 

 This is the case with the whale tribe, many insects and 

 mollusks. 



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 ex- 

 amined, a considerable portion of its oxygen will have 

 disappeared, and another gas of a very different character, 

 namely, carbonic acid gas, will have taken its place. 

 The essential office of respiration is to supply oxygen to 

 the blood, whereby also carbon is removed from it. 



252. An immediately obvious effect of respiration in the 



