THE UNITY OF LIFE 



209 



In the ameba and other one-celled organisms the single cell 

 carries on all the life functions— feeding and assimilation, 

 breathing and oxidation, movement, excretion, sensation, repro- 

 duction. Here we can say that the protoplasm is alive. But 

 in a many-celled 

 plant or animal we 

 are not impressed 

 by the similarity 

 of the protoplasm 

 in all species or in 

 all parts of one 

 organism ; we are 

 impressed rather 

 by the differences 

 between the bone 

 cell and the gland 

 cell and between 

 the skin cell and 

 the muscle cell. 

 The ameba does 

 with the whole 

 body, so to speak, 

 everything neces- 

 sary to keep alive. 

 A lobster or a fish 

 does one kind of 

 work with one or- 

 gan and another 

 kind of necessary 

 work with another 



000(30 



o 



o 



Fig. 105. Simple tissues in a simple animal 



The hydra is among the simplest of many-celled animals, 

 consisting of a hollow bag whose wall is made up of two 

 layers of cells. There are many outgrowths around the 

 open end. There is a division of labor between the inner 

 layer of digesting cells and the outer layer of protecting 

 cells. In a section of the wall we may see that the outer 

 cells, a, have elongations, b, at their bases, which are 

 highly contractile, and that interspersed among these cells 

 are smaller ones, c, which are highly sensitive and extended 

 into delicate threads and expansions, d, which may be 

 considered to correspond to nerves. (Microphotographs lent 

 by J. R. Bray Productions, Inc.) 



This fact 



of having special organs for special functions has been called 

 the division of labor (see section 26). The division of labor 

 (or the physiological division of labor, as it is sometimes called) 

 in plants and animals began very early in the history of living 

 things ; but it must have begun after cells began to cling to- 



organ 



