GLASSY GROUP. 537 



size of the thimble-shaped flagellated chambers, which attain an average length 

 o f -1^ of an inch in Euplectella. Leaving out of consideration the skeleton, the 

 soft tissues typically form a tubular sac open at the top, and, with the- walls, formed 

 of five layers, an outer dermal and an inner gastral membrane with a layer of 

 flagellated chambers suspended between and supported by subderrnal and sub- 

 gastral networks of fibres ; the direction of the water current being always from the 

 dermal to the gastral surface. The six-rayed spicule is the form best adapted to 



SILICEOUS SPICULES OF SIX-RAYED, OR GLASS-SPONGES ; IN THE CENTRE AN EIGHT-SIDED INTERSECTION 

 NODE OF A FOSSIL VENTRICULITE SPONGE. 



support a soft walled sac of this description, one axis being vertical to the walls, 

 and the other two tangential ; the rays of each spicule uniting with those of adjoin- 

 ing spicules to form a framework. The typical spicule has six equal rays at right 

 angles to each other, with an axial canal in the centre of each. When four of the 

 six rays disappear, leaving only a glassy rod, the history of such a spicule is 

 betrayed by the presence of a minute cross, which is all that remains of the axial 

 canals of the atrophied rays. Endless modifications of the typical form may occur. 

 One or more of the six rays may develop more than the rest ; one or more may be 



