3512 



THE SPONGES 



SIX-RAYED OR GLASS-SPONGES Class Hexactinellida 



In the sponges of this group the skeleton is built of spicules with three axes 

 and six rays, intersecting one another at right angles through a common centre. 

 A second characteristic consists in the comparatively simple arrangement and large 

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

 one-two hundred and fiftieth of an inch in Euplectella. leaving out of 



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

 TERSECTION NODE OF A FOSSIL VENTRICUWTE SPONGE. 



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 sus- 

 pended between and supported by subdermal and subgastral 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 support a 



