Reactions of Sponges 29 



been generally acknowledged since the time of Grant ('25, '26, 

 '27) to be uniform in their direction and to depend upon the action 

 of cilia-like organs. Some years ago Miklucho-Maclay ('68) and 

 Haeckel ('72) maintained that a reversal of the current could 

 occur, Dut more recent observers have not confirmed this state- 

 ment. In the thousands of living individuals of Stylotella that I 

 have examined I have never seen an exception to the rule that 

 water enters the ostia and sub-dermal cavities, when open, and 

 makes its exit through the osculum. Moreover I have never 

 found a living specimen of Stylotella in which currents could not 

 be demonstrated. Even in those in which the ostia and oscula 

 were closed and no external evidence of currents could be seen, 

 the cutting off of the oscular end and the consequent exposure 

 of the gastral and sub-dermal cavities always was followed by the 

 appearance of characteristic currents. It seems to me probable 

 that under nurmal conditions the choanocytes beat incessantly 

 in Stylotella. The currents produced by them would then be 

 controlled by the opening and closing of the ostia and the oscula. 

 It must be borne in mind, however, that a continuous current 

 does not necessarily mean that all flagellated chambers are con- 

 tmuouslv at work. Some may cease from time to time without 

 causing the general current to cease. All that the presence of 

 a continuous current really proves is that all flagellated chambers 

 are not inactive at once. It would not be surprising to me, how- 

 ever, to find, if evidence could be obtained, that the action of the 

 choanocytes is uninterrupted. The fact that a current could 

 always be demonstrated in all fingers of Stylotella by cutting off^ 

 the oscular ends leads to the conclusion that, aside from the ostia 

 and the oscula, there is no other complete check on the current 

 such as prosopylic or apopylic sphincters, etc. 



If the ostial and oscular sphincters are the organs of control for 

 the currents in a sponge, they must be strong enough to resist the 

 pressure produced by these currents, and when these apertures are 

 closed the tissues of the sponge must also withstand a certain 

 strain produced by the working of the choanocytes. Doubt has 

 been expressed by some writers as to the ability of the body of a 

 sponge to meet these mechanical requirements, but as no one, so 



