Sponges were originally classified as plants because they were immobile and seemed to lack the basic animal characteristics of muscles and nerves. However, in the 1820s, scientists began to question this classification and discovered that sponges possess certain characteristics that are more characteristic of animals. For example, sponges have cells called choanocytes, which have flagella and are responsible for creating water currents within the sponge. These cells are similar to the collar cells found in the simplest animals, known as choanoflagellates. In addition, sponges have cells called amoebocytes that can move and digest food, which are characteristics of animal cells. These and other discoveries led scientists to reclassify sponges as animals and place them in the kingdom Animalia.