HEXACTINELLIDA 



201 



arrangement of the spicules is best realised by means of a 

 diagram (Fig. 93). 



The deviations from this ground-plan of Hexactinellid struc- 



Prostalia 



Principalia t 



Parenchvmalia <J Comitalia 4-- 



Intermedial: 



A 



Perm alia I Autoderm- H 



Hypoderm. ^L- 



Dictyonalia 



Canalaria -/.'. ...4 



Prostalia 



basalia 



Fig. 93. Scheme to show the arrangement of spicules in the Hexactinellid skeleton. 

 Canalaria, microscleres in the walls of the excurrent canals ; Dermalia Auto- 

 derm[alia], microscleres in the dermal membrane ; D. Hypoderm[alia], more deeply 

 situated dermalia ; Dictyonalia, parenchymalia which become fused to form the 

 skeletal framework of Dictyonina ; Gastralia Autogastr[alia], microscleres in the 

 gastral membrane ; Gastralia Hypogastr\alia\ more deeply situated gastralia ; 

 Parenchymalia Principalia, main supporting spicules between the chambers ; P. 

 Comitalia, slender diactine or triactine spicules accompanying the last ; P. Intermedia, 

 microscleres between the P. principalia ; Prostalia, projecting spicules ; P. basalia, 

 rooting spicules, from the base ; P. marginalia, defensive spicules, round the oscular 

 rim ; P. pleuralia, defensive spicules, from the sides. (From Delage and Herouard, 

 after F. E. Schulze.) 



ture are few and simple. They are due to folding of the chamber- 

 layer, or to variations in the shape of the chambers, and to increasing 

 fusion of the spicules to form rigid skeletons. A simple condition 

 of the chamber-layer, like that of the young sponge of Fig. 89, 



