Syconoid - sponges that exhibit the first stages of folding. Choanocytes no longer line the spongocoel, but instead line the evaginations or flagellated canals. Leuconoid - sponges that exhibit the highest degree of folding. Flagellated canals form flagellated chambers which connect to series of canals that lead to the atrium. Internal structures Sponges are hollow and circulate water within their body cavities or atrium.
Water enters through the ostia and exit out through the osculum at the top of the animal.
The mesohyl layer is found beneath the pinacoderm and is composed of a gelatinous matrix containing skeletal material and ameboid cells. These cells, called amebocytes, function in digesting food and forming the spicules or skeleton of the sponge. The spicules come in different shapes and are secreted by special ameboid cells called sclerocytes. Amebocytes function as a circulatory system by transporting waste and oxygen as well as in digestion. Archeocytes, another type of ameboid cell, are phagocytic and play a role in digestion.