THE PORIFERA AND PROTOZOA 317 



fine central canal. Some of the most characteristic 

 forms of spicule are shown in Fig. 95. Loose siliceous 

 spicules of this kind accumulate in such amounts on the 

 sea-floor to-day as to form a "spicular ooze," as, for 

 instance, off Kerguelen, at a depth of 120 fathoms. 

 Similar deposits among the stratified rocks have passed 



FIG. 95. PORIFERA. 



a, Mbnactinellid (Xis), and b, Tetractinellid (Xio) spicules from interior 

 of hollow flint, c. Dermal spicule of a Lithistid, Upper Chalk. (X 32.) 

 d, Siphonia tulipa Zittel, Cenomanian (Upper Greensand), War- 

 minster (Wilts). Vertical median section (stalk broken off) showing 

 vertical canals opening into central cloaca, e, Fragment of hexac- 

 tinellid meshwork, showing central canals (black), Neocomian. (X3O.) 

 /, g, Ventriculites infundibuliformis S. P. Woodward, Upper Chalk. 

 /, Side view of imperfect specimen ; g, cross-section. (Both xj.) (All 

 after Hinde.) 



into cherts, the spicules becoming cemented by a secondary 

 deposit of chalcedony into a very hard rock. Such 

 sponge-cherts are found in Britain in the Carboniferous 

 Limestone, at several Jurassic horizons, and particularly 

 in the Lower and Upper Greensands of the Cretaceous. 

 In the higher orders of sponges the spicules are united 



