Spicules can be calcareous, siliceous or made of spongin fibers. External structures Sponges vary greatly in size and can be found as small as a grain of rice to exceeding a meter in height and diameter. Most sponges are irregularly shaped and have erect, massive branching growth patterns. Sponges range in grade depending on the complexity of the internal water canals within the sponge. The most common sponge species are brightly colored and can be green, yellow, orange, red and purple.
Sponges have three main tissue layers.
The outer epithelial layer is called the pinacoderm . The pinacoderm is composed of flattened cells called pinococytes. Openings to the external environment are called ostia and are found in the pinacoderm layer. Syconoid - sponges that exhibit the first stages of folding. Choanocytes no longer line the spongocoel, but instead line the evaginations or flagellated canals.