Sollas, 1888
Definition: Porifera with siliceous spicules (Demosponge megascleres) (Demosponge microscleres) and/or a fibrous skeleton (fibres). The spicules are either monaxonic (either monactine or diactine) or tetraxonic (tetractine), never triaxone (hexactine). The axial filament is embedded in a triangular or hexagonal cavity. In most cases the spicular skeleton and fibrous skeleton form a combined reinforcement. Other groups lack spicular skeletons, but compensate that by an elaborate fibre skeleton. A few groups have no skeletal elements other than diffuse fibrillar collagen. The cellular elements are discrete, never syncytial. The aquiferous system is of the rhagon- (leucon-)type. The choanocyte chambers may be eurypylous, diplodal or aphodal. Larvae are mostly simple parenchymella, but in some groups blastula forms are produced. Both oviparous and viviparous strategies occur.
Remarks: The Demospongiae contain about 90% of living species, with a described fauna already consisting of about 6-7000 species and an estimated total extant fauna of between 14-15000 species worldwide. Most are marine, but several dozens of species occur in freshwater habitats all over the world (excluding Antarctica). Within this class, four subclasses have been frequently recognized in previous publications, but three of them (Tetractinomorpha, Ceractinomorpha and Sclerospongiae) have a compromised definition, so this taxon level is controversial and not used here. A rough division is possible, based on the mutually exclusive presence of aster microscleres and isodictyal reticulation of the skeletal elements, but for these no formal taxon levels are employed here.
There are 12 orders, 70 families and ca. 1000 nominal genera included, although only 500+ genera are presently considered to be valid. 480 genera include marine species and ca. 25 genera concern freshwater species.
Some "relict sclerosponge" forms have both a basal calcitic skeleton as well as free siliceous spicules). Some groups lack a mineral or fibrous skeleton entirely (the genus Oscarella in the order Homosclerophorida, the genus Chondrosia in the order Chondrosida, the genus Hexadella in the family Darwinellideae, and the family Halisarcidae). Collagenous filaments or fibrils (forming the ground substance of the intercellular mesohyl) are ubiquitous, spongin fibres (also composed of collagen) occur in most families, and histological organisation is always cellular (as opposed to syncytial in the Hexactinellida). Choanocytes occupy chambers that are spherical, hemispherical, elongate or branched.
Sources: Bergquist, 1978; Hooper and Wiedenmayer, 1994
Orders included here:
Order Astrophorida
Order Chondrosida
Order Dendroceratida
Order Dictyoceratida
Order Hadromerida
Order Halichondrida
Order Haplosclerida
Order Homosclerophorida
Order Poecilosclerida
Order Spirophorida
Order Verongida
(Dendroceratida, Dictyoceratida and Verongida may be conveniently united as superorder Keratosa, recognizable by the absence of a mineral skeleton and the possession of spongin fibres)
Orders not included:
Order Agelasida: verticillately spined styles (Agelas fibres) (Agelas spicule). The order includes also some of the sclerosponges, i.e. Demosponges with basal calcareous skeletons (basal calcareous skeleton) united previously in the order Ceratoporellida (Astrosclera willeyana), because the spicules are identical.
Order Lithistida: polyphyletic assemblage of sponges possessing desma spicules forming a desma skeleton.
Order Verticillitida: no spicules; a massive skeleton of limestone chambers.