![]() Sponges capture food (detritus particles, plankton, bacteria) that is brought close by water currents created by the choanocytes. Skeletal elements, choanocytes, and other cells are imbedded in a gelatinous matrix called mesohyl or mesoglea. 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. Sponges are either radially symmetrical or asymmetrical. ![]() Water movement is driven by the beating of flagellae, which are located on specialized cells called choanocytes (collar cells). Other characteristics of sponges include a system of pores (also called ostia) and canals, through which water passes. This is the simplest kind of cellular organization found among parazoans. 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. Sponges are primarily marine, but around 150 species live in fresh water. Sponges are a diverse group of sometimes common types, with about 5000 species known across the world. ![]()
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