Order Lyssacinosida Zittel, 1877

Abstract

Recent Lyssacinosida Zittel (Hexactinellida, Hexasterophora) contains three families and 53 valid placed genera, plus two genera presently unplaced and treated here as incertae sedis. The group is characterized by a choanosomal skeleton of spicules that remain separate (unfused) through life, or where fusion takes place, it does not involve hexactine megascleres – a typical dictyonal framework is not formed. These sponges are typically single oval, cup-form or tubular sponges with soft bodies and a single terminal osculum. They may be attached to hard substrate by basidictyonal plate (basiphytous) or rooted by anchors into sediments or onto irregular surfaces (lophophytous). Two patterns of surface structure are characteristic: (1) large pentactine or hexactine dermalia without specialized hypodermal supporting spicules and (2) small dermalia varying from hexactins to diactins supported on large hypodermal pentactins. The three constituent families are most easily differentiated by the form of the predominant choanosomal megascleres: hexactins or diactins or a mixture of stauractins, tauactins and diactins