388 EVENINGS AT THE MICROSCOPE. 



are formed by the manner in which these are grouped ; 

 and this is seen much more clearly in the case of the three- 

 rayed needles of lime in the Grantia. Mr. Bowerbank has 

 shown that in G. compressa the substance is divided into 

 very regular chambers in a double series, divided by a 

 diaphragm, whose axis is at right angles to the axis of the 

 sponge ; and that these chambers are defined by walls 

 made up of the three-rayed needles in their mutual inter- 

 lacement. 



