1 64 Elementary Zoology. 



When a definite heart is established, it is customary to 

 speak of arteries and veins. To those trunks which convey 

 blood away from the heart the term " artery " is applied ; veins 

 are the trunks which convey blood to the heart. Both vessels 

 communicate peripherally, usually by means of finer tubes (the 

 capillaries), in which region the distinction between artery and 

 vein is extinguished. 



Respiratory Organs. 



In animals with a delicate body-wall it frequently happens 

 that there are no special organs of respiration at all. The 

 thinness of the body-wall permits an seration of the blood 

 capillaries which it contains. This is the case with the earth- 

 worm, where the entire surface of the body performs the office 

 of a lung, or branchia. In animals with a thicker body- wall, 

 where such an aeration cannot take place, special organs occur 

 which are devoted to respiration. 



While there are certain grounds for believing that the 

 organs for the secretion of nitrogenous waste are homologous 

 throughout the animal series, this is by no means the case with 

 the organs of respiration. In the animals whose anatomy has 

 been described in the foregoing pages four types of respiratory 

 organs are met. 



1. Branchia. — These are outgrowths of the epidermis, 

 with some of ihe underlying mesoblastic structures included. 

 In the latter course the blood-vessels, which come to be 

 separated from the oxygen-containing medium by a thin 

 epidermis only. Such branchire, or gills, are commonly 

 arborescent, in order to increase as much as possible, without 

 an undue increase of the length of the organ, the respiratory 

 surface. The branchiae of the Crayfish and of anodon, and the 

 external gills of the young tadpole, belong to this category.^ 



2. TrachccB. — The trachefe of the cockroach are respiratory 



' A variation of this form of respiratory organ is found in tlie ecliino- 

 derms, and even in a few annelids (Branchiiira), in which the branchia 

 contains a prolongation of the coelom. 



