Development of Homoscleromorpha of the Order Homosclerophorida Dendy, 1905

Abstract

The Homoscleromorpha is a very distinct group of Porifera. Homoscleromorphs are small sponges, encrusting or lumpy with a smooth surface (color plate XIII). The body consists of two layers, the pinacoderm and the choanoderm, with a thin mesohyl layer between them. The skeleton, if present, consists of small calthrops and/or their derivatives (diodes and triodes) (Fig. 4.1) evenly distributed in the sponge body. The spicule rays may branch. The aquiferous system is sylleibid or leuconoid, often with a vast basal exhalant chamber. Choanocyte chambers are large, usually eurypilous (Plakortis, Plakinastrella, Plakina, and Oscarella), and aphodal or diploid (Corticium, Pseudocorticium). The apopyle is always surrounded with a ring of flattened apopylar cells (Fig. 4.2a, b) having both the flagellum and the microvilli collar. The basal membrane underlies the choanoderm and the pinacoderm (Fig. 4.2c, d); exo- and endopinacocytes are flagellated (Fig. 4.2e, f). Homosclero­morphs are marine sponges, usually occurring at low depth. The group is composed of one family Plakinidae Schluze 1880 and seven genera.