Introduction to Animal Morphology. 33 



The exposure to air takes place in the respiratory 

 organs, of which there are four types ist. In aquatic 

 animals, the blood, contained in organs called branchiae 

 (gills), is exposed to water containing oxygen in 

 solution; 2nd. Also in water dwellers, a system of 

 tubes communicating with the external water and 

 traversing the animal's body, admits this fluid so 

 as (through their walls) to aerate the blood in the 

 perivisceral space (water vascular system) ; 3rd. A 

 system of tubes (tracheae) containing air, ramifying 

 through the body, whereby the blood may be directly 

 aerated ; or 4th. A series of sacs (pulmonary sacs, 

 lungs) set apart for the reception of air, to which the 

 impure blood is brought, and exposed in capillary 

 vessels.* 



The glandular depurants remove from the blood 

 the waste of nitrogenized material. They are the 

 dermal glandsf (the only ones in the lowest forms), 

 and kidneys.J 



* In the respirator}- organs, water, carbonic acid, and traces of other 

 substances are got rid of and oxygen taken in. As this oxygen enters into 

 chemical combination with the protoplasm in the tissues in the life 

 -scs, in all living forms heat is evolved during such chemical 

 action, and the amount thereof varies with the activity of the com- 

 binations. 



t In the perspiration, or excretion of the dermal glands is contained 

 water, sodic chloride, ammonic lactatc, butyratc, and acetate, some nitro- 



1, and an odorous principle. 



; itcs from the blood, urea, uric acid, often combined 

 ..rnmonia, krcatin and kreatinin, sometimes hippuric and hen 

 xanthin and hypoxantliin, >imilar to those in blood 



scrum. 



u 



