CHAR A CTERISTJCS. 



5*7 



careful inspection, groups of minute pores will be seen perforating the meshes. 



The large holes at the summits of the craters are termed oscules, and the small 



ones on the general surface, pores. Ellis, who put some specimens in a glass 



vessel of sea water, wrote that " We could plainly observe) those little tubes to 



receive and pass the water to and fro " ; and further, " The sponge is an animal 



whose mouths are so many holes or ends of branched tubes opening on its 



surface; with these it receives its nourishment, and by these it discharges like 



the polyps its excrement." Ellis's observations were erroneous in one important 



point. The water always passes 



out of the large orifices, and is 



not passed in by them. It is 



true that while the torrent is 



gushing out of the centre of an 



oscule, there is a slight passive 



return current at the margin. 



Ellis attributed the current to 



the contraction of the walls of 



the canals. He found that the 



current continued in the absence 



of any worms or crustaceans in 



the body of the sponges. 



Our knowledge of sponges 

 really begins in 1825 with the 

 observations of Grant, who ex- 

 amined a fragment of a living 

 branch of a branching -sponge. 

 On bringing one of the large 

 apertures on the side of the 

 branch fully into view, he 

 beheld this living fountain 

 vomiting forth from a circular 

 cavity a torrent of liquid matter, 

 and hurling along in rapid 

 succession opaque masses, which 

 it strewed everywhere around. 

 After many experiments, Grant convinced himself that a current flowed out of 

 all the large orifices, and not into one and out of another. He also rubbed 

 powdered chalk on the surface of a bread-crumb sponge, and saw particles which 

 clogged the margins of the minute pores on the surface driven into the interior ; 

 and thereby demonstrated the passage of currents into the interior through 

 the pores. The origin of the sponge-fountains was now traced. In all sponges 

 currents of water pass into the body through pores, and out again by one or 

 more ways different from those by which they entered. To ascertain the cause 

 of the currents, it is necessary to examine the anatomy of the sponge. A thin 

 skin, which can be peeled off, is separated from the body by numerous minute 

 supporting pillars. On cutting into the sponge, large canals are seen passing down 



«, FLAGELLATED CHAMBER OF BREAD-CRUMB SPONGE, SHOWING COLLAR- 

 CELLS ; b, FLAGELLATED CHAMBER OF FRESH-WATER SPONGE. 



(Both figures 1600 diameters. ) — After Vosmaer. 



