Flattened cells cover the exterior. Collar cells line the interior chambers. By means of their beating flagella, these cells move large volumes of water in through body pores and out through the large osculum at the top of the body. They also trap suspended food particles in their microvilli collars and transfer the food to amoeba-like cells in the matrix. Between the two layers of cells there is a semifluid matrix with needlelike structures for support.
Sponges reproduce sexually and asexually.
Sperm are released into the surrounding water to be picked up by a nearby sponge and directed to the egg within the matrix. The zygote develops into a free-swimming larva. Cnidarians are radial, tentacled animals that live mostly in the seas. Unique stinging devices called nematocysts discharge threads to capture prey and fend off predators. An outer epidermis covers the body, and an inner gastrodermis lines the digestive cavity.