GENERAL ORGANOLOGY. 



1U7 



though mot so great, are the differences between carnivorous and 

 plant-eating beetles. 



II. Respiratory Organs. 



Sources of the Oxygen used in Breathing. — The oxygen which 

 each animal must obtain in exchange for the carbon dioxide formed 

 in the tissues is derived either from the air or from tlie water, 

 according as the animal is terrestrial or aquatic. Less frequently 

 it is the case that water-dwellers breathe air, and hence are com- 

 pelled, from time to time, to rise to the surface of the water for a 



Fig 61 —Left second foot of a crayfish with attached gill ('ir). (After Huxley. > (xp, 

 cox'oDodite- dp, basipodite; ip, isohiopodite; mp, meropodite; cp, carpopodite; pi>, 

 propodite; dp, dactylopodite; cx.s, bristles of the coxopodite; c, lamina of the gill. 



supply of air; this is true for the great marine mammals, and for 

 many insects, spiders, and snails which are found in fresh water. 

 Air- and water-breathing takes place exclusively through the skin, 

 so lonty as this is delicate and readily permeable, and so long as 

 no hio-her development of organization necessitates a more active 

 intercliange of material. If, on the other hand, the demand for 

 oxvo-en be greater, other more special breathing-organs are found 

 Vfpg for water-breathing, lungs and trachea for air-breathing, in 



