44 THE ELEMENTARY NERVOUS SYSTEM 



10 



the osculuin is accomplished by one set of contractile cells, 

 in the nature of primitive muscle, working against an- 

 other set of contractile cells or even against simple tis- 

 sue elasticity. In this respect the motor mechanism of 



the osculum is more uniform 

 than that of the dermal 

 pores, for in the osculum 

 there is no pore membrane. 

 In the osculum as in the der- 

 mal pore, the active mechan- 

 ism is apparently directly 

 stimulated and exhibits, 

 therefore, another instance 

 of independent effectors 

 normally called into action 

 by direct stimulation. 



The water currents pro- 

 duced by sponges are depen- 

 dent upon the activity of 

 their flagellate cells or cho- 

 anocytes, and these cells ap- 

 parently beat on unremit- 

 tingly and incessantly. There 

 is no reason to suppose that 

 they ever stand still or re- 

 verse their direction of 

 stroke. The only control 

 over these currents seems to 

 be the opening and closing mechanisms of the dermal 

 pores, oscula, and other like devices. It has, how r ever, 

 been suggested that the currents are too considerable to 

 be checked thus and that other means of control must be 

 present. The strength of the water current from Stylo- 



Fio. 11. Diagram of the apparatus for 

 measuring the strength of a current of sea- 

 water produced by Stylotella. A finger of 

 this sponge, whose currents were in full 

 activity, was tied off at the base and firmly 

 attached at the tip to a vertical glass tube 

 into which it discharged seawater. The 

 seawater in the tube rose to a higher level 

 than that of the outside water in conse- 

 quence of the strength of current produced 

 by the sponge. This strength of current 

 could be measured by an attached scale. 



