SPONGES. 139 



course. I continued to watch the same orifice, at 

 short intervals, for five hours, sometimes observing 

 it for a quarter of an hour at a time, but still the 

 stream rolled on with a constant and equal 

 velocity. About the end of this time, however, 

 the current became languid, and, in the course of 

 another hour, it ceased entirely." Similar currents 

 were afterwards observed by Dr. Grant in a great 

 variety of species. They take place only from 

 those parts which are under water, and immedi- 

 ately cease when the same parts are uncovered, or 

 when the animal dies. 



It thus appears that the round apertures in the 

 surface of a living sponge are destined for the dis- 

 charge of a constant stream of water from the 

 interior of the body ; carrying away particles which 

 separate from the sides of the canals, and which are 

 not only seen, under the microscope, constantly issu- 

 ing from these orifices, but may even be perceived 

 by the naked eye, propelled occasionally in larger 

 masses.* Hence they are termed foecal orifices. 



For the supply of these constant streams, it is 

 evident that a large quantity of water must be con- 

 tinually 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 



* 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 tlie surface, the motions of 

 which will render the currents very sensible to the eye. Fig. 53 

 exhibits these phenomena. 



