SPONGES 



143 



as to the true nature of these structures. Their chemical nature has 

 been shown to be different from that of spongin (Schulze) ; but while 

 some authors are inclined to regard them as foreign to the sponge, and 

 probably organisms of a symbiotrc or parasitic nature, others consider 

 them as true products of the sponge tissues. Haeckel, amongst the latter, 



Fio. 1)4. 



Spongin fibres of Spongelia avara, loaded witli foreign particles, yr.f, principal fibre; 

 conn./, connecting fibre. (After F. E. Schulze.) 



compares them with the fibrillae of Stannomidae, while Fol professes to 

 trace their origin to fusiform cells of the connective tissue layer, and 

 considers that the family Filiferae (0. Schmidt) should be reinstated for 

 the horny sponges characterised by the possession of filaments. Loisel 

 suggests that they are intracellular spongin filaments of the same nature 



