Class Hexactinellida

Glass sponges occur worldwide, mostly at depths between 200 and 1000 m. This group of sponges are especially abundant in the Antarctic.

All glass sponges are upright, and possess specialized structures at their bases for holding fast to the ocean floor. Most appear outwardly to be radially symmetrical; they are typically cylindrical, but may also be cup-shaped, urn-shaped, or branching. The average height of a hexactinellid is between 10 and 30 cm, but some can grow to be quite large. A hexactinellid possesses a cavernous central cavity (the atrium) through which water passes; a cap of tightly woven spicules covers the osculum in some species. Coloration in most is pale. Glass sponges most closely resemble syconoid sponges, but they differ too much internally from other sponges to be considered truly syconoid.

It is upon close internal examination that glass sponges can be most easily distinguished from other sponges. The skeleton of an hexactinellid is made entirely of silica. These siliceous spicules are generally composed of three perpendicular rays (and therefore six points, so they are described as hexactine), and are often fused, lending hexactinellids a structural rigidity not typical of other sponge taxa. Strung between the spicules is a largely syncytial network of soft body cells. Incurrent water enters the body through spaces in the syncytial strands. Within the syncytia are units functionally similar to the choanocytes found in other sponges but these units completely lack nuclei, and so are referred to as collar bodies rather than collar cells. They are flagellated, and it is the beating of their flagella that causes the current to pass through the sponge. Within the syncytia are cells functionally comparable to archaeocytes in other sponges, but these cells seem to demonstrate only limited mobility. Hexactinellids lack myocytes completely, and so are incapable of contraction. While hexactinellids possess no nerve structure, they seem to be able to send electrical signals across the body through the syncytial soft tissue.