THE ELEMENTARY NERVOUS SYSTEM 



ligature they at once recommenced. From these obser- 

 vations it is quite clear that the osculum controls in a 

 purely mechanical way the current within the sponge. 

 When the osculum is open, this current may run ; when it 

 is closed, the current ceases even though the dermal pores 

 are open and the choanocytes continue to beat. In view 

 of these facts the oscular ends of the fingers of Stylotella 

 were regularly removed. 



Although the presence of a pore current is conclusive 

 evidence of the open condition of the pores, its absence 



FIG. 6. Diagram of a finger of Stylotella from which the tip has been cut showing cur- 

 rents entering the subdermal spaces and emerging from the cloaca. 



is not proof that the pores are closed even supposing that 

 the oscular end is cut off, for it is conceivable that the 

 choanocytes may cease to beat, in which case the cessa- 

 tion of the currents would be misleading as to the condi- 

 tion of the pores. To meet this difficulty a simple pro- 

 cedure was adopted. If the oscular end of a finger of 

 Stylotella is cut off at some distance from the osculum 

 itself, the cut face includes not only the cloaca and some 

 of the flagellated chambers, but also the subdermal spaces 

 (Fig. 6). An examination of the currents from such a 

 cut end will show a large, slow, central current emerging 

 from the cloaca, and a considerable number of smaller 

 more rapid currents entering the surrounding subdermal 



