SPONGES 29 



a new osculum; if it is down, the opposite end very gen- 

 erally regenerates the new organ. Thus in the regenera- 

 tion of the osculum, Stylotella shows some slight geo- 

 tropic activity, and while it must be admitted that the 

 common flesh of this sponge is contractile, this contrac- 

 tility does not seem to result in movements of the body 

 as a whole such as might be looked for in geotropic and 

 other like responses. It is possible that in this sponge 

 the skeleton, which is well developed, is too resistant to 

 allow the body as a whole to be bent, and that, therefore, 

 the contractility of the common flesh can make itself man- 

 ifest only in the local ways already mentioned. 



But, as has already been intimated, the chief activity 

 of sponges is not shown in the general movements of their 

 bodies, but in the currents of water that they produce. 

 These currents are due to the incessant activity of the 

 choanocytes in the passages within the body of the 

 sponge. Access to these passages is gained through the 

 innumerable pores on the general external surface of the 

 sponge and the exit from them is through the cloaca and 

 the relatively large terminal opening of the cloaca, the 

 osculum. The cessation or revival of these currents, as 

 seen in many sponges, is not due to changes in the activ- 

 ity of the choanocytes but is dependent upon the closing 

 or opening of the pores and of the osculum, whereby the 

 water current is checked or allowed to pass. 



The pores on the surface of Stylotella are of the kind 

 designated as .dermal pores, or ostia, in that instead of 

 leading directly to the inlet canals of the flagellated sys- 

 tem they admit to relatively large subdermal spaces which 

 in turn communicate through the incurrent canals proper 

 with the flagellated chambers (Fig. 5). These chambers 

 empty through the excurrent canals into the cloaca. 



