32 



PORIFERA. 



(fig. 6, d, e) are siliceous and consist of four rays, or are 

 irregular in form, the extremities of the rays branch or 

 expand and firmly interlock with one another but do not 

 fuse together. In addition to the skeletal-spicules there 

 is generally a surface layer of trifid spicules or of discs. 

 Flesh-spicules are also present. Several different types 

 of canal-system occur. Owing to their solidity the 

 Lithistids are preserved abundantly as fossils. They are 

 rare in the Palaeozoic, the earliest form being Archceo- 

 scyphia found in the Upper Cambrian ; in the Ordovician 

 and Silurian we get Astylospongia. No forms belonging 

 to this order have been found in the Devonian, Permian, 

 or Trias, they are numerous in the Jurassic, attain their 

 maximum in the Cretaceous, and are scarce in the 

 Tertiary. 



Siphonia. (fig. 7.) Pear-shaped, usually provided 

 with a stalk, which is given off from the broad end of 

 the body and terminates in rootlets. The incurrent 

 canals are small, slightly curved, and extend radially from 



FIG. 7. Siphonia tulipa. Upper Greensand, Warminster. A, vertical 

 section. B, horizontal section, e, excurrent canals; f, incurrent 

 canals. Two-thirds natural size. 



