Order Hexactinosida Schrammen, 1903

Abstract

Recent Hexactinosida Schrammen (Hexactinellida, Hexasterophora) contains seven families and 36 valid placed genera, plus five unplaced genera here treated and summarized as Hexactinosida incertae sedis. The group is characterized by formation of a rigid dictyonal frame- work of simple (non-lychniscid) hexactins either by fusion of rays of adjacent dictyonalia arranged side-by-side, or by point of contact fusion of ray tips to centra, or to ray tips of adjacent dictyonalia. The length of rays is strictly limited and thus determines length of mesh sides, usually 150–400m but may be extended to over 1mm in some species; dictyonal rays are not extended indefinitely as is the pattern in Aulocalycoida. Body form and channelization are highly variable, including diplorhyses, diarhyses, schizorhyses, amararhyses and extradictyonal epirhyses, with or without aporhyses. Dictyonal framework patterns include those previously characterized as farreoid and euretoid, but some as yet uncharacterized patterns are formed by members presently included in Hexactinosida