SPONGES. 153 



the surface of a living sponge are destined for 

 the discharge of a constant stream of water from 

 the interior of the body ; carrying away par- 

 ticles, which separate from the sides of the 

 canals, and which are not only seen, under the 

 microscope, constantly issuing from these orifices, 

 but may even be perceived by the naked eye, 

 propelled occasionally in larger masses.* 



For the supply of these constant streams, it is 

 evident that a large quantity of water must be 

 continually received into the body of the sponge. 

 It is by the myriads of minute pores, which exist 

 in every part of the surface, that this water 

 enters, conveying with it the materials necessary 

 for the subsistence of the animal. These pores 

 conduct the fluid into the interior, where, after 

 percolating through the numerous channels of 

 communication which pervade the substance of 

 the body, it is collected into wider passages, 

 terminating in the fecal orifices above described, 

 and is finally discharged. The mechanism by 

 which these currents are produced is involved 

 in much obscurity. There can be no doubt 

 that they are occasioned by some internal move- 

 ments ; and the analogy of other zoophytes 



* The currents issuing from the larger orifices are best seen 

 by placing the living animal in a shallow vessel of sea water, 

 and strewing a little powdered chalk on the surface, the motions 

 of which will render the currents very sensible to the eye. Fig. 

 53 exhibits these phenomena. 



