From the nifty new book my brother just gave me, The Ultimate Encyclopedia of Mythology:
Apis or Hapi was the most renowned of Egypt's sacred animals. He was worshiped in Memphis, where his temple lay opposite that of the great creator god, Ptah. Apis, in the form of a real black bull, was thought to be the reincarnation or "glorious soul" of Ptah, who was said to have inseminated a virgin cow in the form of fire and to have been born again as a black bull.
Each day, Apis was let loose in the courtyard attached to his temple, and the priests would use his movements as a means for divining the future. Usually, the Apis bull was allowed to die of old age, but he was drowned in a fountain if he reached the age of 25. The bull was twice assassinated by the Persians.
Ptah's priests were said to be able to recognize the next holy bull by discovering certain markings on the creature's body, including a white triangle on his forehead and a crescent moon on his right side. The extent of the reverence with which the sacred bulls were regarded can be gauged by the fact that their mummified bodies were buried with great ceremony in huge underground burial chambers.