MOTOR REACTIONS OF THE FRESH-WATER SPONGE. 



155 



the body, is held up in tent-like elevations by the protruding 

 ends of the spicules. 



The oscular chimneys seem to be kept open by the pressure of 

 the currents of water passing out through them. They are 

 continuously changing their form. At times one will be long 

 and slender; again it will be shorter and much thicker or even 

 dome-like, the base being as much as three or four times the 

 height. In instances of the first type the mouth of the chimney 

 will probably be open almost to the width of the diameter, 

 although these mouths are frequently seen to be nearly closed. 

 In the type last mentioned they are usually small. 



D E F 



Fig. 1. Various forms in which the same oscular chimney was observed. 



These variations in shape lead to the belief that the entire 

 chimney contains sphincter-like bands. If all such bands were 

 about equally contracted the result would be a long slender 

 chimney, Fig. 1, A and B. If only those at the end contracted 

 and the others relaxed, the result would be the dome-like shape, 

 Fig. 1, C and D. Several instances of a contraction of the 

 terminal sphincter and also of the chimney near its base, with a 

 relaxation between, were noticed. The result was a sort of 

 globe-like chimney attached to the sponge body by a narrow 

 neck, Fig. 1, E. The chimneys also seem capable of a decided 

 shortening without much change in diameter. This may be 



