SPONGES 119 



or hypodermalia, according as their axial cross is placed within, or 

 beneath, the dermal membrane. 



(3) Gastralia. Spicules similar in form and function to the last 

 named, but supporting the gastral membrane. 



(4) Parenchymalia. Spicules supporting the general parenchyma 

 and the chambers between the dermal and gastral membranes. In 

 the most primitive types of skeleton, as seen in Holascus and Farrea, 

 the parenchymal skeleton consists of large regular hexactines 

 (principalia), arranged to correspond with the intervals between the 

 thimble-shaped chambers, two rays being disposed radially and four 

 tangentially (Fig. 77,prc). This primitive type of skeleton may 

 become much modified in various ways, both as regards arrange- 

 ment and composition, the primitive hexactinal principalia becoming 

 modified in form, and supplemented by other spicules (comitalia). 

 In the sub-order Dictyonina and in many Lyssacina the principal 

 spicules of the parenchyma are united into a continuous framework, 

 and distinguished as didyonalia. 



Union of the Spicules. In many Hexactinellids the spicules re- 

 main separate from one another and simply interlock. In other 

 cases some of the spicules of the parenchyma become united to 

 form a continuous framework. This union is always effe^ed by 

 secondary deposits of silica, never by spongin. 



In the simplest method of union, characteristic of Dictyonina, 

 two parallel rays become apposed and united by concentric layers of 

 silica into a beam, in which the primitive component rays ,are dis- 

 tinguishable by their separate axial canals. In other cases the end 

 of a ray of one spicule becomes soldered to the central node, at 

 which the rays intersect, in another. In other cases again the rays 

 of adjoining spicules crossed in any direction are bound together 

 by web-like lamellae of silica. When two rays are not in contact, 

 cone-like elevations grow out from the sides of opposite rays, meet, 

 and finally fuse to form a connecting siliceous bridge or synapticula. 

 Since all these secondary deposits of cementing siliceous material 

 are without axial canals, they can easily be distinguished from the 

 true spicules. 



In the Dictyonina the principal spicules of the parenchyma 

 become united early into a framework, and are separate only in the 

 growing portions of the sponge. Their union imposes a check on 

 the growth of the sponge in a lateral direction, but it can continue 

 to grow in length or at the free margin ; hence the occurrence in 

 this group of tubular, plate-like, or cup-shaped sponges, the former 

 often very similar in form to those in the calcareous family Clath- 

 rinidae. 



In the Lyssacina the spicules either remain separate . (Hyalo- 

 nematidae, Holascinae\ in which case the sponge may attain to a huge 

 size (Poliopogon gigas, and others), or they may become united into 



