GENERAL ORGANOLOQY. 107 



though -not 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 the 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 o a crayfish with attached gill (br). (After Huxley.) czp, 

 coxopodite; bp, oasipodite; ip, ischiopodite; mp, meropodite; cp, carpopodite; pp, 

 propodite; dp, dactylopodite; cxs, bristles of the coxopodite; e, 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 long as this is delicate and readily permeable, and so long as 

 no higher development of organization necessitates a more active 

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

 oxygen be greater, other more special breathing-organs are found 

 gills for water-breathing, lungs and tracheae for air-breathing, in 



