26 GUIDE TO THE CORAL GALLERY. 



TableCase2A. 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]. 



High Case The Hexactinellida, which include many remarkable and beau- 



T 1 l^^Ca. ^^^^ forms, nearly all come from great depths, ranging from i)0 to 

 2 A, B. ;^,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, wdiich 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 Hexactinellid, 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. 



Eupledella 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- 

 rayed 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 



