PRIMITIVE METAZOA 



149 



whose motions are affected need not actually come into 

 contact with the object that causes the change in their 

 stroke. Usually it is the very tip of the Paramoecium's 

 body which makes the contact and the disturbance is 

 transmitted from this point through the body to the cilia. 

 Here we have in simplest form a mechanism for 

 adjustment. 



Fig. 39. — Diagram to illustrate how a sensitive cell, R, on the 

 surface, may lie in direct contact with a contractile cell, E, under- 

 neath, a disturbance of R being transmitted directly to E. 



Correlation in Primitive Metazoa (i.e., Many- 

 Celled Animals). — Among the very simplest of the 

 many-celled animals are to be found forms in which the 

 specialization of function has proceeded to such a point 

 that the cells which are sensitive to external influences 

 are not themselves capable of motion but lie in immedi- 

 ate contact with other cells which have the property of 

 motion highly developed and which represent the primi- 

 tive form of smooth muscle cells (see Chap. XIII). 

 The relation of the sensitive cells to those which can 

 achieve motion is illustrated in the diagram (Fig. 39). 



