178 PR ACTIO AX, ZOOLOGY. 



and general appearance, the back of a kid glove, vary- 

 ing from dark reddish-brown to almost black. The 

 consistency of the living sponge is about the same as 

 that of beef liver. If one of these live sponges be 

 watched, a current of water is found to come out of 

 the larger holes at the top, and currents pass in 

 through the numerous smaller holes on the exterior. 



If the sponge be handled, many of the smaller holes 

 close and entirely disappear. 



In order to understand a little more clearly the 

 structure of the common sponge, and to see how the 

 currents of water are maintained, an examination of a 

 simple sponge will be useful. Our simplest sponges 

 have no elastic skeleton composed of horny fibres like 

 those of the commercial sponge, but have little needle- 

 shaped and three-pronged spicules of limy matter. 



One form common on the northern Atlantic coast 

 is a simple or branched white tube, an inch or so in 

 height and sometimes as thick as a pigeon's quill. 

 These are in clusters, attached by one end and open at 

 the other. Imbedded in the wall of each tube are the 

 spicules above mentioned, projecting both on the out- 

 side and on the inside. The inside of the tube is lined 

 with cells bearing cilia which, by their vibration, drive 

 the contained water out of the mouth of the tube ; to 

 replace which, water enters through many holes which 

 pierce the wall of the tube. In sponges a little more 

 complicated, the cilia, instead of lining the main tube, 

 are limited to small pouches, or lateral branches of the 

 main tube, extending into the body wall and communi- 

 cating with the exterior through small pores. In others 

 the cilia are found only in certain enlarged portions 



