STUDIES IN AJSriMAL LIFE. 25 



criminately. Here is our Opalina, for example, 

 without mouth, or stomach, or any other organ. It 

 is an assimilating surface in every part; in every 

 part a breathing, sensitive surface. Living on liquid 

 food, it does not need a mouth to seize, or a stomach 

 to digest such food. The liquid, or gas, passes 

 through the Opalina's delicate skin by a process 

 which is called endosmosis ; it there serves as food ; 

 and the refuse passes out again by a similar process, 

 called exosmosis. This is the way in which many 

 animals and all plants are nourished. The cell at 

 the end of a rootlet, which the plant sends burrow- 

 ing through the earth, has no mouth to seize, no 

 open pores to admit the liquid that it needs ; never- 

 theless, the liquid passes into the cell through its 

 delicate cell-wall, and passes from this cell to other 

 cells upward from the rootlet to the bud. It is in 

 this way, also, that the Opalina feeds: it is all- 

 mouth, no-mouth ; all-stomach, no-stomach. Every 

 part of its body performs the functions which in 

 more complex animals are performed by organs 

 specially set apart. It feeds without mouth, breathes 

 without lungs, and moves without muscles. 



The Opalina, as I said, is a parasite. It may be 

 found in lyarious animals, and almost always in the 

 frog. You will perhaps ask why it should be con- 

 sidered a parasite ? why may it not have been swal- 

 lowed by the frog in a gulp of water ? Certainly 

 nothing would have been easier. But, to remove 

 your doubts, I open the skull of this frog, and care- 

 B 



