The hexactinellids, or glass sponges, are characterized by siliceous spicules consisting of six rays intersecting at right angles, much like a toy jack. Hexactinellids are widely viewed as an early branch within the Porifera because there are major differences between extant hexactinellids and other sponges. In particular, much of their tissues are syncitia, extensive regions of multinucleate cytoplasm. Some discrete cell types do exist, including archaeocytes. Furthermore, whereas other sponges possess the ability to contract, hexactinellids do not. Moreover, hexactinellids possess a unique system for rapidly conducting electrical impulses across their bodies, allowing them to react quickly to external stimuli.