i66 ELEMENTS OF BIOLOGY 



duct. Some mammals, for example the horse, do not have a gall 

 bladder. 



Fundamentals of a Respiratory System. The importance 

 of oxidations within the cell as the energy-releasing reaction in vital 

 phenomena has already been emphasized (p. 58). For all aerobic 

 animals some sort of mechanism provides for the reception of oxy- 

 gen from the air, either directly or as oxygen from the air dissolved 

 in liquids. In the single-cell animals and in the more simple Meta- 

 zoa in which all cells are in direct contact with the water contain- 

 ing oxygen, the entire surface, wherever cells are in contact with 

 the water, serves to receive oxygen and to dispel carbon dioxide. In 

 more complex forms various types of respiratory mechanisms are to 

 be found. All, however, have three characteristics in common: (i) a 

 membrane freely permeable to oxygen and to carbon dioxide; (2) a 

 relatively large area offered by such a membrane; (3) in many ani- 

 mals, some sort of mechanism for renewing the supply of air or 

 water that is in contact with this membrane. The presence of a res- 

 piratory membrane is also associated with some means of transport- 

 ing the absorbed oxygen to deep tissues. 



Types of Respiratory Systems. It is possible here to men- 

 tion only briefly how these provisions are made in various animals. 

 In the Porifera, Ccelenterata, and Platyhelminthes no special respira- 

 tory apparatus is developed, the body walls serving for the inter- 

 change of gases. In Annelida with appendages, for example, Nereis, 

 the sandworm (Fig. 84), one region of the flattened, oar-hke ap- 

 pendages on each segment is specialized for the respiratory function, 

 although the integument of the body wall also serves. In the earth- 

 worm (Fig. 83) the entire body surface constitutes a respiratory 

 membrane, under which is the blood that receives the oxygen and 

 gives off the carbon dioxide. Among the Arthropoda three types of 

 respiratory systems occur. In members of the class Crustacea, the 

 lobster (Fig. 109) and its allies, a set of feathery gills is attached to 

 each of the walking legs. These gills are housed in a fold on either 



