26 
GUIDE TO THE CORAL GALLERY. 
Table Case 2a. 
High Case 
III. 
Table Case 
2 A, B. 
The Compressed Grantia ( Grantia compressa ), which forms com- 
pressed sacs resembling little paper bags, is also common round the 
British coast. 
Sub-Class I. Hexactinellida [Six-Ray or Glass Sponges]. 
The Hexactinellida, which include many remarkable and beau- 
tiful forms, nearly all come from great depths, ranging from 90 to 
3,000 fathoms. 
The skeleton is built up of siliceous spicules, each typically 
possessing three axes and six rays, or of spicules derived from this 
type ; three bars of equal length crossing each other at right angles 
through a common centre would give the typical form of a regular 
six-rayed spicule. Endless modifications of this form occur ; the 
rays may be curved or branched, or one or more of the rays may 
disappear, giving rise to five-rayed, four-rayed, three-rayed, two- 
rayed, or one-rayed forms (Fig. 7). The spicules may be roughly 
grouped into two kinds — large “ skeleton ” spicules, which form the 
bulk of the framework, and scattered flesh-spicules of microscopic 
size. 
The soft tissues are arranged as follows : in the wall of a 
typical cup-shaped H exact inellid, a layer of relatively large thimble - 
shaped whip chambers is separated from an outer dermal and 
an inner gastral membrane by loose reticulate tissue. Currents 
always enter by the dermal membrane, pass through the convex 
surfaces of the whip chambers, and leave through the gastral 
membrane. The large central cavity, so often present, is termed 
the gastral cavity. Hexactinellida are divided into two sub-orders. 
Sub-Order I. Lyssacina. In this group the skeleton spicules 
are separate throughout life, or, in cases where they are more or less 
fused in later life, were separate in early stages. 
Euplectella aspergillum , or Venus’ Flower-Basket (Figs. 5, 6 ; 
and specimens in Case III. 2, and Table Case 2a), forms an elegant 
cornucopia-shaped skeleton, now often seen as an ornament. In life 
the skeleton is concealed by a gelatinous flesh. The lattice-like frame- 
work of the skeleton is formed of longitudinal, transverse and oblique 
strands, the last forming the prominent ridges on the surface ; the 
strands are built up of the fused rays of very large four-rayed and three- 
raved spicules. At the lower end is a matted tuft of spicules, by means 
of which the sponge is rooted in the mud. A surface layer of separate 
sword-shaped spicules, each with its handle tipped with a lovely little 
