24 BIOLOGY 



The simplest forms of living things are single-celled, 

 and in these single cells all the daily and periodic func- 

 tions, such as digestion, growth, response to stimuli, 

 excretion, and reproduction occur, and are carried on 

 just as efficiently as in the more highly complex, and 

 that, too, without any special organs for carrying out 

 these functions. 



From these single-celled forms we pass to others 

 where we have the first trace of differentiation, the outer 

 layer of the body developed into a special layer or wall. 

 In such forms we may have special organs of loco- 

 motion developed either as hair-like or whip-like out- 

 growths. We find also that certain spots on the cell are 

 set apart for special purposes, for whereas in amoeba the 

 food could enter at any point, in paramoecium it can 

 enter at one spot only. A skeleton begins to make its 

 appearance and reaches a high degree of complexity 

 in such forms as the Radiolaria, Foraminifera, and 

 Diatoms. 



We pass from these single-celled forms to a stage 

 where we have a large number of similar forms living 

 together in a group, yet preserving their individual 

 identity. This is seen, for example, in Microgromia, in 

 Epistylis, in Carchesium, and in Spirogyra, but it must 

 be remembered that from these groups any member 

 may break away and start an independent life. A 

 further complication ensues in such forms as Volvox, 

 where the cells are incapable of breaking away ; that is, 

 they are mutually dependent on one another ; and this 

 is still further complicated by the fact that this mutual 

 dependence has, as a result, the setting aside of certain 

 of the cells of the group for special duties. In Volvox 



