Spicules from multiple species of sponge, arranged in an Exhibition Slide. Spicules are the structural components of a sponge, or the “bricks,” and the shapes, sizes, and composition are unique for each species. Together, you can look at these features under a microscope to make a positive identification.
From Wikipedia: Sponges are a species-rich clade of the earliest-diverging (most basal) animals. They are distributed globally, with diverse ecologies and functions, and a record spanning at least the entire Phanerozoic.
Most sponges produce skeletons formed by spicules, structural elements that develop in a wide variety of sizes and three dimensional shapes. Among the four sub-clades of Porifera, three (Demospongiae, Hexactinellida, and Homoscleromorpha) produce skeletons of amorphous silica  and one (Calcarea) of magnesium-calcite. It is these skeletons that are composed of the elements called spicules. The morphologies of spicules are often unique to clade- or even species-level taxa, and this makes them useful in taxonomic assignments.
Sponges have been receiving special attention from researchers since the introduction of molecular biological techniques at the turn of the century, since findings point to sponges as the phylogenetically oldest animal phylum. New information has accumulated concerning the relevance of this phylum for understanding of the dynamics of evolutionary processes that occurred during the Ediacaran, the time prior to the Cambrian Explosion which can be dated back to approximately 540 million years ago.