SPONGES 35 



son, Reniera and Lissodendoryx, not only may the pores 

 close by the formation of a pore membrane, but this 

 process may be supplemented by a closure of the pore 

 canal itself. This is probably due, according to Wilson, 

 to a contraction of the epithelial lining in the pore canal 

 acting after the fashion of a sphincter. Thus there ap- 

 pear to be two somewhat independent devices for the 

 closure of the pores, the pore membrane and the pore 



c 



FIG. 8. Three dermal pores in Stylotella showing steps in their closure by the pore mem- 

 brane; A, partly closed; B, more nearly closed; C, completely closed. (Modified from Wilson, 

 1910.) 



canal sphincter. All three sponges studied by Wilson 

 were found to close their pores by means of pore mem- 

 branes. Contraction of the pore canals was observed in 

 Reniera and in Lissodendoryx, but not in Stylotella, 

 though this sponge was not so favorable for such 

 observations. 



The closure of the pore canals, according to Wilson, is 

 quite obviously dependent upon the sphincter-like band 

 of cells on the wall of the canal. These cells are in every 

 way comparable to a primitive form of smooth muscle- 

 fiber. Their superficial position places them in contact 

 with the water passing through the canal and, as they 

 respond to the differences in this water, they are without 

 doubt capable of direct stimulation. The pore membrane 

 is less muscle'-like in its action than the wall of the pore 

 canal is and yet its movement is hardly to be described as 





