Aphrocallistes vastus, courtesy of the Marine Ecology Station

Hexactinellida is a class of sponges with spicules, which make up sponges, of silica. Silica is a mineralized substance, hence the common name for hexactinellids is glass sponges. These spicules join with each other at right angles, making the sponge appear artificial and thus they do not have the ability to contract. Hexactinellida is considered an early branch within sponges, and most species are extinct, but some do still exist.

Information provided by: http://www.sidwell.edu