TRIAXONIA. 



91 



the rays, or to the suppression of some of them. The variations 

 in the form of spicule may be classed in six main groups hexacts 

 (Fig. 78, 2\ pentacts, tetracts, triacts ('7), diacts, and monacts. 

 The spicules when united are generally bound together by a fine 

 laminated silicious substance. 



FIG. 78. Spicules of Hexactinellida. 1, an auto-gastral pinulus of Sympagella mix; 3, hexact 

 of Holascus fibulatus to which are applied small diacts; 3, pinulus of Caitlophacus latus; 

 U, pentact pinulus of Hyalonema lusitanicum ; 5, discohexact of Tcegerfa pulchra; 6, disco- 

 hexacter of Dictyocalyx gracilis ; 7, triact of Hyalonema gracilis ; S, scopula of Eurete semper i ; 

 9, amphidisc of Hyalonema Sieboldii. (From Perrier after Schulze.) 



Prostalia are the spicules which occur over the outer surface of the sponge. 

 They are of three kinds : basalia or rooting spicules, pleuralia at the sides, 

 and marginalia round the osculum. 



Dermalia are spicules in relation with the bounding membrane of the sponge, 



