PROTOZOANS, SPONGES, AND CCELENTEEATES 341 



a few of these systems in the animals lower than man and 

 see in what they resemble his or differ from his. 



Sources of oxygen. — All animal forms ^ obtain their 

 oxygen from one of two sources, — from air or from water. 



Water has the power to dissolve certain gases with \vhich 

 it is in contact, and thus water which is exposed to air 

 for any length of time is bound to contain more or less 

 oxygen in solution. Animals which have the power to 

 extract this dissolved oxygen from water for use by their 

 cells, are often called water-breathing, to distinguish them 

 from those forms which take their oxygen from the air 

 or air-breathing forms. In both air and water breathers 

 the process is the same, namely, the extraction of the 

 oxygen for the use of the tissue cells. 



Types of Respiration in Invertebrates. 



Protozoans, sponges, and ccelenterates. — All protozoans 

 (one-celled animals), in their active state, live in water, or 

 fluids containing water, and extract their oxygen from 

 this water. With them, there is no need for blood or other 

 respiratory apparatus, since each cell is in contact Avith 

 the water, and can take in oxygen and give off carbon 

 dioxide as needed. 



The sponges and ccelenterates, such as the jellyfishes, 

 anemones, etc., are built up of many cells, but through 

 the use of cilia or other means the currents of water which 

 bring food to the internal cells are so directed as to bathe 



' Certain plant forms are known, called the An^robia and limited 

 mainly to certain groups of bacteria, that are able to live in media that 

 contains no oxygen. This does not mean that these organisms are able 

 to live without oxygen, but it is supposed that they have the power to 

 extract oxygen from the food which they receive. 



