Proifera - Sponges (Wiki)
"Sponges have no nervous systems, " However, sponges do have "... porocytes. These let water into the sponge. Myocytes, little muscular cells, open up the porocytes and close them. ", and some authors consider the myocytes to be "neuromuscular" cells. That is, they function as both nerves and muscles. This does not contradict the above statement that "Sponges have no nervous systems" since the myocytes act individually and are not organized into a 'system'. However, this is where the nervous system begins.