An Evolutionary Road Map Animals arose from protistan-like ancestors in the 200 million years before the Cambrian era. The One Placozoan Trichoplax adhaerens is the only known species of this group. Sponges are mostly marine animals of widely varying size and shape. Sponges have an asymmetric body with no true tissues, no organs. 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.