SPONGES. 117 



immediately cease when the same parts are uncovered, or 

 when the anima^^dies. 



It thus appears that the round apertures in 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 particles, which separate from the sides of the canals, 

 and which are not only seen, under the microscope, con- 

 stantly issuing from these orifices, but may even be per- 

 ceived 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 i-eceived inta 

 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 nu- 

 merous channels of communication which pervade the sub- 

 stance of the body, it is collected into wider passages, ter- 

 minating 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 movements; 

 and the analogy of other zoophytes would lead us to ascribe 

 them to the action of fibrils, or cilia, as they are termed, pro- 

 jecting from the sides of the canals through which the streams 

 pass; but these cilia have hitherto eluded observation, even 

 with the highest powers of the microscope. 



The organization of sponges is as regular and determinate 

 as that of any other animal structure, and presents as syste- 

 matic an arrangement of parts. In some species, such as the 

 common sponge, the basis is horny and elastic, and composed 

 of cylindric tubes, which open into each other, and thus form 

 continuous canals throughout the whole mass. 



• 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. 



