130 OF THE SECRETIONS. 



all the recesses of the rays ; and, in color and structure, re- 

 sembles the liver of mollusks. Even in polyps, we find pe- 

 culiar brown cells lining the digestive cavity, which, proba- 

 bly, perform functions similar to those of the liver in the 

 higher animals. 



270. The great importance of the respiratory organs in 

 discharging carbon from the blood has already been spoken 

 of, (245, 251.) The substances removed by the liver and 

 the lungs are of the same class, being those which are desti- 

 tute of nitrogen. These organs seem, in some sense, sub- 

 sidiary to each other ; and hence, in those animals where 

 the respiratory organs are largely developed, the biliary 

 organs are comparatively small, and vice versa. Another 

 and opposite class of impurities, and no less pernicious if 

 retained in the blood, is removed by the KIDNEYS ; and, 

 consequently, organs answering to the kidneys are found 

 very far down in the series of animals. Most of the peculiar 

 ingredients of the urine are capable of assuming solid, crys- 

 talline forms ; and, in some animals, as in reptiles and 

 birds, the whole secretion of the kidneys is solid. In most 

 cases, however, the urinary salts are largely diluted with 

 water ; arid, as the lungs and liver are supplementary to 

 each other in the removal of carbon, so the lungs, the kid- 

 neys, and the skin mutually relieve each other in the removal 

 of the watery portions of the blood. 



