A classification key is a way of categorizing living things by determining and arranging them according to their common qualities. Dichotomous key is a key for determining the identity of the organism based on two choices in each set of characteristics. The dichotomous key for sharks are the shape of the body, number of gill slits, position of mouth, and shape of caudal fins. Other characteristics include length or number of dorsal fins, presence or absence of pelvic fins, anal head, and shape of head.