Hexactinellidse, a group of sponges having a siliceous skeleton composed of spicules, which are either united by the tips (Didyonind), or which are separate from one another (Lyssakind). The spicules are six-rayed, and through the centre of the rays run three transverse canals. A series of many-rayed "flesh spicules" also occurs. The group is of great, interest as, with very rare exceptions, it is at present limited to the deep seas, while in former periods it was very widely distributed. Thus many of the sponges of the Chalk or of the Lower Greensand sponge banks belong to this group, while it is known in the Palaeozoic. The best known recent member of the group is the "Venus Flower Basket" (q.v.), or Eupledclla, which is a member of the Lyssakina. Hyalonema is another wellknown form. It is surrounded at the base by a tuft of glass-like fibres, whence it has gained its name of the "Glass-rope Zoophyte." These anchoring fibres have long been known from the Carboniferous. Hyalonema was once regarded as a coral.