PRINCIPAL GROUPS OF THE SPONGES. 



169 



simply held in position by the soft parts of the sponge, no fusion 

 taking place between them. 



The recent Hexactinellids are all inhabitants of the sea, and are 

 mostly found in deep water, and palaeontologists are now acquainted 

 with a very extensive series of fossil forms, dating from the Cam- 



Fig. 59. — a, Side-view of a specimen of ' Cccloptychiian Seebachi, of the natural size, from the 

 Cretaceous formation; B, Portion of the hexradiate skeleton of the same, enlarged 65 times. 

 (After Zittel.) 



brian period. The oldest known type of the Hexactinellidtz is the 

 genus Protosfiongia, species of which occur in deposits of Cambrian 

 age. In this genus, the sponge is cup-shaped, with a thin wall, 

 composed of large cruciform spicules arranged so as to form a quad- 

 rate network, the interspaces of which are filled up by similarly dis- 



