40 PORIFERA. DEMOSPONGIiE 



deep. Walls folded so as to form a series of vertical grooves and 

 ridges. Canal-system well developed ; the radial canals are large and 

 start from the central cavity, but end before reaching the outer sur- 

 face ; others start from the outer surface and end before reaching the 

 central cavity. Spicules six-rayed and fused with one another so as 

 to form a mesh-work. The node where the axes cross is hollow, having 

 the form of a negative octahedron, the central part of each face of 

 which is absent ; the axial canals cross in the centre of the octa- 

 hedral space. The sponge was provided with a root consisting of 

 siliceous fibres. Chalk. Ex. V. radiatus, V. impressus. 



Plocoscyphia. Sponge formed of tubes and laminae which 

 anastomose, forming an irregular or rounded mass. Canal-system 

 imperfect. Upper Cretaceous. Ex. P. fenestrata, Upper Greensand 

 and Chalk Marl. 



CLASS II. DEMOSPONGLE 



The skeleton consists of siliceous spicules, or of 

 spongin, or of both spicules and spongin. In some forms 

 there is little or no spongin, but in others the entire 

 skeleton consists of spongin with no siliceous spicules ; 

 between these extremes there is a complete passage. The 

 spicules are never of the hexactinellid type. In some few 

 cases both spicules and 'spongin are absent. 



Order 1. Myxospongida. Sponges with no skeleton 

 or occasionally with a few isolated spicules. Not known 

 in the fossil state. 



Order 2. Ceratosa. Sponges with a skeleton 

 composed of a fibrous network of spongin. This Order 

 includes the ordinary bath sponges, etc., and is unknown 

 in the fossil state. 



Order 3. Monaxonida. The skeleton is formed of 

 spongin and spicules in varying proportions. The spicules 



