GLASSY GROUP. 
537 
size of the thimble-shaped flagellated chambers, which attain an average length 
°f 250 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 subdermal 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 
