Sponges are attached to the sea bed by a holdfast that may resemble roots or a series of fine hairs. They can be identified easily in thin section. Spicules are the most commonly preserved evidence for the presence of sponges in fossil communities. These are formed into skeletal frameworks with cubic symmetry. Hexactinellid sponges produce skeletons from a series of six-rayed spicules with each spine arrayed at 90° to its neighbors. Demosponges, the most common modern group, usually have unmineralized skeletons, but can produce siliceous spicules, or calcareous bases to their colony. Spicule type is very varied within the group. Most commonly, networks of such chambers are developed within the thick wall of the sponge and are linked to the paragaster by a series of canals (the leucon grade of organization).Ĭalcareous sponges grow skeletons composed of entirely of calcium carbonate spicules.
In more complicated morphologies, the feeding and water-moving cells are arranged in multiple chambers, linked to a common central area or paragaster (this is the sycon grade of organization). In the simplest case, archaeocytes line apertures in the wall of the cup-shaped sponge (this is known as ascon-grade organization). Feeding occurs in the walls of the colony. This has the hydro-dynamic effect of facilitating a good flow rate with less work from the flagella-bearing cells. They construct a skeleton with narrow openings, called ostia, through which water can enter and a broad opening through which water leaves, called the osculum. Sponges have a simple body shape, characterized by their functional needs as filter-feeding organisms (Fig. The movement of many flagellae, from many choanocytes, helps to move water through the colony. They have a funnel-shaped end, with a long flagel-lum, or whip-like filament, extending through the funnel and into the water beyond. Cho-anocytes are the cells that generate feeding currents through the sponge. Sclerocytes secrete mineralized elements of the skeleton, while spongocytes secrete the organic parts of the skeleton. These cells are feeding cells and can also change into another cell type if required. Arch-aeocytes are cells shaped like amoebae, able to move within the colony and lacking a fixed shape. Sponges are characterized by four important cell types.
0 kommentar(er)
