36 GENERAL BIOLOGY 



branes, formed by the fusion of many cilia, that beat 

 with the motion of a fan and drive the food down 

 the gullet with force and precision. This function 

 is further subserved by undulating membranes 

 (see fig. 13) likewise formed of fused cilia. Each of 

 these types of locomotor organs has a special function 

 to perform. In some way there has come about a 

 division oj labor among the cilia in different parts of 

 the body, one group of cilia performing one function, 

 another group another; and in proportion to the 

 extension of this division of labor there has arisen 

 a corresponding efficiency of action of each part. 

 Along with this physiological specialization of 

 function there has developed the corresponding 

 modification of structure which we have called dif- 

 ferentiation. Physiological specialization and mor- 

 phological differentiation are thus two connected 

 consequences that result from a division of labor 

 among the parts of an organism. The fundamental 

 function involved in the examples just described 

 is, however, in each case the contractility of the 

 protoplasm. It is in the method or the physical 

 basis of utilizing this function that efficiency is 

 attained ; just as the same amount of current from 

 the same wire will produce a brighter light in a 

 tungsten filament than in a carbon incandescent 

 lamp. 



The limits of the specialization of contractile 

 organs are apparently soon reached. In some 

 Protista, however, there has been a differentiation 

 of contractile substance within the cell-body, a 



