Evolution and systematics
Hexactinellids (glass sponges) are deepwater marine sponges that have skeletons of siliceous (glass) spicules with a distinctive triaxonic (cubic three-rayed) symmetry. Unlike the other two main classes of sponges (Calcarea and Demospongiae), glass sponges lack either a calcareous or organic skeleton. Furthermore, glass sponges are highly unusual in that their major tissue component is a giant "syncytium" (see below) that ramifies throughout the entire body, stretching like a cobweb over the glass skeleton. As their skeletons are both made of glass rather than calcium, early classification schemes grouped hexactinellids with the demosponges; however, at present hexactinellids are separated from cellular sponges (the Calcarea and Demospongiae) in the subphylum Symplasma because of their unique (syncytial) structure. Nevertheless, recent molecular evidence suggests that whereas modern hexactinel-lids are descended from the most ancient multicellular animals, they are more closely related to demosponges than either group is to the calcareous sponges or other metazoans. There are approximately 500 species of hexactinellids in two subclasses containing five orders, 17 families, and 118 genera.
Hexactinellids have left the oldest fossil record of multi-cellular animals on Earth. Their triaxonic spicules are known from the Late Proterozoic of Mongolia and China. The group thrived during the Middle Cambrian and reached its maximum radiation and diversity during the Cretaceous, when hexactinellids formed vast reefs in the Tethys Sea. Their fossilized skeletons now make up the stony outcrops upon which many castles are built from southern Spain through France, Germany, and Poland to Romania.
Within each of the two subclasses of hexactinellids are sponges with loose skeletons—spicules held together by liv ing tissue—and sponges with fused skeletons. Sponges of both designs are essentially vase-shaped, with a large central or atrial cavity, usually with one opening, the osculum. The species with fused skeletons often have mittenlike or fingerlike protrusions of the body wall, and some form platelike structures; these species have oscula on each of the projections. The tissue of hexactinellids generally is creamy yellow to white. Some animals are quite clean, so that the whiteness of their tissue looms out of the darkness of their deepwater habitats. Others tend to accumulate particulate matter on the outside and can look quite dirty. The diverse species of hexa-ctinellids vary in length from 0.2 to 5 ft (0.5 cm to 1.5 m); many of the largest hexactinellids are as wide as they are tall.
The body wall is composed of three parts: both the inner and outer peripheral trabecular networks, and the feeding region, which is called the choanosome. Large incurrent and excurrent canals meet at the choanosome, where the fine, branchlike endings of the incurrent canals contact oval, flagellated chambers that create the feeding current through the sponge.
While sponges of other groups are constructed of single cells, each with a single nucleus, the greater part of the soft tissue in a hexactinellid consists of the trabecular reticulum, which contains thousands of nuclei and cytoplasm that is free to move as it is unimpeded by membrane barriers (the "syncytial" condition). The trabecular reticulum hangs from the skeleton in thin strands, resembling a cobweb, and stretches from the outermost layer, termed the dermal membrane, to the innermost layer, the atrial membrane. Single cells, specialized for particular functions, are also present. Cells are attached to one another and the trabecular reticulum by a unique type of attachment structure referred to as the "plugged junction," which is called so because it is slotted into the neck of cytoplasm between two regions, resembling a plug. It is not an extension of the lipo-protein cell membrane, but is a multi-
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