The sponge's adaptations include being an excellent filter feeder. Because a sponge is sessile, its body is full of pores that allow water rich with nutrients such as plankton to enter its body cavity. Once there, the plankton is filtered out, and the water is expelled.
Most types of sponges have internal cells known as collar cells. They get the name because they have a collar of tentacles that ring a flagellum, which resembles a tiny tail or whip. The flagellum moves water into the sponge's body while the collar traps the food.
Since they can no more search for mates as they can search for prey, sponges also have the ability to release eggs and sperm into the water. The eggs are fertilized in the water and develop into free-swimming larvae that eventually settle on a substrate. Sponges also reproduce by budding. Pieces of a sponge that break off can also grow into another animal.
Though sponges have rudimentary skeletons based on silicon or calcium and a network of collagen fibers, they have no real internal organs or tissues, no nervous system and very few if any muscle cells. For a while, scientists believed they were plants, but they are animals.