These are sponges and are the simplest of the many-celled animals.
Living sponges secrete a skeleton which may be composed of chitin,
silica, or calcium carbonate. These substances are commonly found in the
form of spicules—tiny hard parts that are used to help support the soft
tissues of the animal. These spicules take on a variety of shapes (Pl.
14) and are occasionally found as microfossils in some marine sediments.
Although sponges are not particularly common fossils, their remains
occur in some parts of the State. Sponges have been collected from
Paleozoic and Mesozoic formations of north and Trans-Pecos Texas, and
their spicules have been reported from well cuttings.