HEXACTJXELLIDA 



arrangement of the spicules is best realised by means of a 

 diagram (Fig. 93). 



The deviations from tliis ground-plan of Hexactinellid struc- 



Prostalia 



( Principal 

 Parenchymalia J Comital 



marginalia 



Intermedia 





A 



Dermaiia <* Autoderm Nl 

 I Hypoderm---i 



Dictyonalia 



Canalaria 



Prostali, 





Fig. 9.3. — Scheme to show the arrangement of spicules in the Hexactinellid skeleton. 

 CancJuria, mic-roscleres in the walls of the exciirrent canals ; iJermalia Auto- 

 (ferm[aliu], microscleres in the dermal memlirane ; D. H)ipoderm[aHu\ nioie deeply 

 situated (lernialia ; Dictyonalia, parenchymalia which become fused to form the 

 skeletal framework of Dictyonina ; Gastralia Autogastr{alia\ microscleres in the 

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

 Pareiichi/inalia I'rinci^ndla, main supporting spicules between the chambers ; P. 

 CoinitaJkt., slender diactine or triactine spicules accompanying the hast ; /■". Intermedia. 

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

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

 rim ; P. plcuralia, 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, 



