2io COMPARATIVE ANATOMY AND PHYSIOLOGY. 



now see that Birds and Mammals have independently 

 acquired the hot-Ulooded condition. In connection 

 with this it is important to observe that they are the 

 only Vertebrates in which the arterial and venous 

 blood do not commingle at some point in the circulatory 

 system. The term cold-blooded is so far inaccurate, that 

 the blood is no colder than the surrounding medium, 

 whether that be air or water ; and, as its temperature 

 varies, it is better to use the term " poikilothermic." 



CHAPTER VI. 



ORGANS OF RESPIRATION. 



THE essential object of the process of respiration is 



the taking in of oxygen either from the atmospheric 

 air, which may be said to be oxygen dissolved in 

 nitrogen, or from water, which holds in solution a 

 larger quantity of oxygen than of nitrogen ; for, 

 whereas atmospheric air consists of twenty volumes of 

 oxygen to eighty of nitrogen, water contains about 33 

 per cent, of oxygen. In the simplest cases (that of naked 

 cells, such as Amcefoa) the oxygen passes directly 

 into the protoplasmic substance of the cell, and the 

 chief product of the oxydation of the tissue, carbonic 

 acid, passes freely out. In the Infusoria the oxygen 

 has to make its way through the cuticle, and some, no 

 doubt, enters with the drops of water that are taken 

 in by the mouth ; we are as yet altogether ignorant of 

 the conditions under which animal membranes allow of 

 the entrance of oxygen and the exit of carbonic acid. 

 We have already seen that the cilia are of use in driving 

 food particles towards the mouth, and it is clear that 

 they are also of assistance in the respiratory process by 

 producing currents around the cell, and thereby bring- 

 ing fresh supplies of oxygenated water to its surface. 



