PORIFERA. 31 



ORDER 2. CERATOSPONGI^E. Sponges with a skeleton 

 composed of a fibrous network of spongin. This includes 

 the ordinary bath sponges. A few siliceous spicules are 

 occasionally present. Not definitely known in the fossil 

 state. 



ORDER 3. MONACTINELLID.E. Sponges in which 

 the skeleton consists of spongin and siliceous spicules. 

 The spicules (fig. 6, a) are uniaxial, that is to say, they 

 consist of a single rod or axis ; they vary considerably in 

 shape, being straight or curved, and with sharp or blunt 

 ends. Flesh-spicules may also occur. Since in this order 

 the spicules are only united by decomposable material, 

 it is extremely rare to find the form of the sponge 

 preserved fossil, usually detached spicules only occur. 

 The earliest forms belonging to this order are found 

 in the Silurian, they become 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 examples 

 are still living. 



ORDER 4. TETRACTINELLID.E. The spicules (fig. 6, 

 6, c) are siliceous and are held together by spongin, they 

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

 angle between each of the rays being 120. The rays 

 may be equal or unequal in length, frequently one is very 

 much elongated, and sometimes their terminations are 

 bifurcated. Like the Monactinellids, the Tetractinellids 

 are seldom preserved in anything like a perfect condition 

 as fossils. The oldest forms occur in the Carboniferous 

 Limestone, where they are represented by the genus 

 Geodites. 



ORDER 5. LrraiSTnxE. Sponges with thick stony 

 walls and very variable external form. The spicules 



