38 PORIFERA. HEXACTINELLIDA 



posed by different authors ; in the one adopted here the 

 divisions are based primarily on the characters of the 

 skeleton. Three classes are recognised, (1) Hexactinellida, 

 (2) Demospongise, (3) Calcarea. 



CLASS I. HEXACTINELLIDA 



The spicules in the Hexactinellida (fig. 9, f) consist 

 of three axes crossing at right angles to one another ; in 

 primary forms there are consequently six rays of equal 

 length proceeding from a centre. Each ray is traversed 

 by an axial canal, and these unite at the point of junction 

 of the six rays. Various modifications are produced by 

 some of the rays being longer or shorter than the others, 

 or almost absent; and also by the branching of the rays 

 and the occurrence of spines, knobs, etc. The spicules may 

 remain free or they may be fused with one another by 

 a deposit of secondary silica, but they are never united by 

 spongin. When spicules with equal rays are united end 

 to end, skeleton-cubes are formed, each cube consisting of 

 eight spicules (fig. 9,f). Flesh-spicules are abundant, but 

 are seldom found fossil. Some of the spicules form a layer 

 near the external surface of the sponge for the support of 

 the dermal membrane ; others form a similar layer near the 

 internal surface ; the spicules which constitute the main 

 part of the skeleton occur in the middle of the sponge-wall 

 and serve to support the canals and flagellated chambers. 

 The spicules which form the root- tuft by which many 

 Hexactinellids are fixed, are long and thread-like. The 

 canal-system is usually simple. 



The earliest form is Protospongia from the Menevian 

 Beds of St David's ; Hyalostelia is found in the Tremadoc, 



