PORIFERA. DEMOSPONGIiE 41 



(fig. 9, a) consist of a single rod or axis, which may be 

 straight or curved, and with sharp or blunt ends; each 

 spicule may consist of two rays or of one ray only. In the 

 former the two ends of the spicule are alike and there 

 is a small swelling of the axial canal at the centre of the 

 spicule where growth commenced; in the latter the two 

 ends are dissimilar and the swelling in the axial canal is 

 at one end of the spicule, and growth went on in one 

 direction only. Microscleres or flesh-spicules may also 

 occur but are often absent. Since in this Order the 

 spicules are only united by spongin or other decomposable 

 material, it is extremely rare to find the form of the sponge 

 preserved fossil ; usually, detached spicules only occur. 



The earliest representatives of the Monaxonida are 

 found in the Silurian ; the Order becomes more abundant 

 in the Carboniferous, where the genus Reniera occurs. 

 The freshwater form Spongilla is found in the Purbeck 

 Beds of the south of England. A large number of Mon- 

 axonid sponges are still living. 



Order 4. Tetractinellida. The spicules (fig. 9, b, c) 

 consist of four rays given off from a common centre, the 

 angle between the rays, when the end of one is taken as 

 a central point, appearing to be 120°. The rays may be 

 equal or unequal in length ; frequently one is very much 

 elongated (fig. 9, c), and in such forms the three shorter 

 rays are placed near the surface of the sponge-wall and 

 the longer ray is directed inwards. Sometimes the termi- 

 nations of the rays are bifurcated. Spongin is either 

 absent or occurs in minute quantities only, and since the 

 spicules are not united, the Tetractinellids, like the 

 Monaxonids, are seldom preserved in anything like a 

 perfect condition as fossils. The oldest forms occur in 



