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 



/ Prinoipalia -- ---- -< I V--,. ] x 



Comitalia --/-■: •-fv."-'l\ 



Intarmedia 



m ^'^ 



Dermalia 



Canalaria ---•■■■■ -if ...';;- 

 Autbderm..il 1",^ { " 



Hypoderm - 



-% 



Dictyonalia 



Prostalia 



basalla 



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

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

 dermlalia'], microscleres in the dermal membrane ; D. Hypodermlalia], moi-e deeply 

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

 skeletal framework of Dictyoniua ; Gastralia Autogastr^aKa], microscleres in tlie 

 gastral membrane ; Gastralia Hypogastr\alia'], more deeply situated gastralia ; 

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

 Comitalia, slender diaotine or triactiue spicules accompanying the last ; P. Intermedia, 

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

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

 rim ; P. pleuralia, defensive spicules, from the sides. (Prom 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, 



