Sponges are either radially symmetrical or asymmetrical. They are supported by a skeleton made up of the protein collagen and spicules, which may be calcareous or siliceous, depending on the group of sponges examined. Skeletal elements, choanocytes, and other cells are imbedded in a gelatinous matrix called mesohyl or mesoglea. Food items are taken into individual cells by phagocytosis, and digestion occurs within individual cells. Reproduction by sponges is by both sexual and asexual means.
Asexual reproduction is by means of external buds.
Some species also form internal buds, called gemmules, which can survive extremely unfavorable conditions that cause the rest of the sponge to die. Sexual reproduction takes place in the mesohyl. Male gametes are released into the water by a sponge and taken into the pore systems of its neighbors in the same way as food items. The larvae may settle directly and transform into adult sponges, or they may be planktonic for a time. Adult sponges are always sessile.