Figure 2 — Homology and diversity of the nose openings in tetrapods. In most tetrapods, the nostrils are at the front of the snout: (A) Iguana (Iguana iguana, reptile); (B) Common nighthawk (Chordeiles minor, bird); (C) Gorilla (Gorilla gorilla, mammal). In a few groups, however, this position has changed: (D) Pakicetus, with nostrils (yellow arrow) at the front of the snout; (E) Protocetiids, with nostrils further back; (F) extant cetaceans, including whales and dolphins, with nostrils at the top of the head.