Sponge spicule
Spicules are structural elements found in most sponges. They provide structural support and deter predators. Large spicules that are visible to the naked eye are referred to as megascleres, while smaller, microscopic ones are termed microscleres.
Spicules are found in a range of symmetry types.
Monaxons form simple cylinders with pointed ends. The ends of diactinal monaxons are similar, whereas monactinal monaxons have a different ends - one pointed, one rounded. Diactinal monaxons are classified by the nature of their ends: oxea have pointed ends, and strongyles are rounded. Spine-covered oxea and strongyles are termed acanthoxea and acanthostrongyles, respectively. Monactical monaxons always have one pointed end; they are termed styles if the other end is blunt, tylostyles if their blunt end forms a knob; and acanthostyles if they are covered in spines.
Triaxons have three axes; in triods, each axis bears a similar ray; in pentacts, the triaxon has five rays, four of which lie in a single plane; and pinnules are pentacts with large spines on the non-planar ray.
Tetraxons have four axes, and polyaxons more (description of types to be incorporated from ). Sigma-C spicules have the shape of a C.
Dendroclones might be unique to extinct sponges and are branching spicules that may take irregular forms, or may form structures with an I, Y or X shape.
Sponges can be calcareous, siliceous, or composed of spongin.
The meshing of many spicules serves as the sponge’s skeleton.
The composition, size, and shape of spicules is one of the largest determining factors in sponge taxonomy.
Spicules are formed by sclerocytes, which are derived from archaeocytes. The sclerocyte begins with an organic , and adds silica to it. Spicules are generally elongated at a rate of 1-10 μm per hour. Once the spicule reaches a certain length it protrudes from the sclerocyte cell body, but remains within the cell’s membrane. On occasion, sclerocytes may begin a second spicule while the first is still in progress.
...
Wikipedia