PEOTOZOA AND SPONGES. 443 



this slice. These take a curious form, exactly that of 

 the pins which we use on our dressing tables ; each 

 consisting of a cylindrical slender rod, pointed at one 

 end, and at the other surmounted by a globular head, 

 the whole formed of glass— ;^m^ glass literally. You 

 see them bristling all round the edge of the section, 

 being stuck into the surface of the sponge, exactly as 

 pins are loosely stuck into a pin-cushion. The heads 

 and points, too, project into the cavities ; more, how- 

 ever, than they did during life, for you must make al- 

 lowance for the shrinking of the soft parts ; and thus 

 you perceive how the whole structure is permeated by 

 these glassy pins, which seem to be entangled together 

 quite at random without rule or arrangement. And yet 

 there is an arrangement discernible here ; for the canals 

 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 Grantim. Mr. Bow- 

 erbank has shown that in G. compressa the substance ia 

 divided into very regular chambers in a double series, 

 separated by a diaphragm, whose axis is at right an- 

 gles to the axis of the sponge ; and that these chambers 

 are defined by walls made up of the threg-rayed needles 

 in their mutual interlacement. 



