Design
Hexactinellids are clearly distinct from other sponges (see the Porifera page for an introduction to general sponge design) in two main respects: 1) Their adult soft tissues are largely syncytial, i.e. the majority of cells are fused, resulting in one large multi-nucleated mega-cell that is wrapped around the mineral skeleton. It has been demonstrated that the sponges can use this syncytium to propagate electrical impulses to regulate their filtering activity, analogous to a nervous system in higher animals. 2) Their spicules have a triaxonic and cubic symmetry, i.e. they are composed of three axes that are arranged at right angles to each other. The basic spicule form is the hexactin, which has all six rays (two per axis) fully developed – hence the taxonomic name, Hexactinellida. By reduction, branching, and ornamentation of rays, glass sponges produce a staggering array of different spicule types that can be aesthetically highly appealing and form the basis to distinguish species and higher taxa. Spicules are categorized into two basic groups: megascleres, which provide structural support and are often visible with the naked eye, and the much smaller microscleres, the function of which – other than to please taxonomists – is largely unknown. Glass sponge skeletons have recently attracted the attention of materials scientists because they possess a number of interesting properties, including very good light-transmitting capabilities (if the living sponges actually use their spicules to transmit light is questionable, though).