Development of Sponges from the Class Hexactinellida Schmidt, 1870

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Abstract

Representatives of the class Hexactinellida, commonly called glass sponges, are very variable in shape. They may be tubular, cup-shaped, lumpy, branching, or lobulate (color plate III); only encrusting forms are lacking. Hexactinellid sponges have silicate triaxial spicules or their derivatives. Typically, spicules are represented by hexactins, with three axes crossing at regular angles. A loss of one or more rays results in pentactins, tetractins (stauractins), triactins (tauactins), and diactins; rarely, monactins also occur (Fig. 2.1). The axial filament of the spicule resides in a quadrangular cavity. Spicules are divided into micro- and macroscleres; the latter, often fused together, form rigid skeletal lattices (Fig. 2.2). Dense spongin or nonspicular skeletons do not occur. Living tissues of glass sponges are syncytial and consist of the dermal and the atrial membrane, the internal trabecular reticulum enclosing cellular components of the sponge, and flagellated chambers (Figs. 24, 25). Separate nucleated cells, which are situated in syncytial pockets or capsules, may be connected by specialized contacts, porous plugs. Large eurypilous flagellated chambers are organized according to leuconoid type. All glass sponges are ovoviviparous, with the trichimella larva.