Dasychalina cyathina , a pale pink subtidal sponge. Sponges are a diverse group of sometimes common types, with about 5000 species known across the world. Sponges are primarily marine, but around 150 species live in fresh water. Sponges have cellular-level organization, meaning that that their cells are specialized so that different cells perform different functions, but similar cells are not organized into tissues and bodies are a sort of loose aggregation of different kinds of cells. This is the simplest kind of cellular organization found among parazoans.
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.