Order Homosclerophorida Dendy, 1905, Family Plakinidae Schulze, 1880

Abstract

Homosclerophorida Dendy (Demospongiae, Homoscleromorpha) contains a single family Plakinidae Schulze (including Oscarellidae Lendenfeld and Corticiidae Vosmaer), with seven valid genera and about 60 valid species worldwide. Species live mainly in shallow waters but a few have been recorded from abyssal depths (up to 2460m). Species are often encrusting, lobate, but massive species are common in some genera (Plakortis, Plakinastrella); surface is usually smooth or microhispid and consistency varies from soft to cartilaginous. All genera possess flagellated exo- and endopinacocytes, a basement membrane lining both choanoderm and pinacoderm, oval to spherical choanocyte chambers with a sylleibid-like or leuconoid organization, and a unique incubated cinctoblastula-type larvae; spicules, when present, are peculiar tetractines (calthrops) and derivatives. Genera are distinguished mainly by four morphological characters: presence of a siliceous skeleton; presence of a cortex associated with a leuconoid aquiferous system and well-developed mesohyl or a sylleibid aquiferous system with poorly developed mesohyl and ectosome; number of spicule size classes; and presence and type of ramifications in the actines of calthrops (tetractinal spicules), with three distinct general morphologies recognized.