Fossil Hexactinellida

Extract

The structure and evolution of the hexactinellid skeleton has been conditioned by: (1) the rectangular form of the spicule (ultimately due to the square cross-section of the axial canal), and (2) the sheet-like form of the choanocyte-membrane, properly the choano-syncytium (Reiswig, 1979) as individual choanocytes with proper nuclei do not exist, which is a continuous syncytial sheet bearing outpocketings (diverticula) that correspond to the choanocyte chambers of other sponge groups. In most species the diverticula are separate, and attached to one another by syncytial filaments, but their arrangement as part of a sheet-like structure is still apparent. In a few species the choanocyte membrane becomes a labyrinth of anastomosing tubes. Accompanying these are the essential absence of mesogloea and the paucity of collagen (spongin), which results in the soft parts being essentially confined to the space occupied by the skeleton. The place of mesogloea is taken by a syncytial network of filaments which also form the dermal and gastral membranes.