Fig. 9

Mouth formation in deuterostome clade. Ambulacrarians retain ancestral deuterostomic mouth formation, and amphioxus and olfactoreans acquired new methods of mouth formation after dorso-ventral inversion. Amphioxus opens mouth by using coelom and its canal, which has a common origin with ambulacrarian coelomic pore-canals (asterisks). Olfactoreans developed a placode or its equivalent at the anterior extremity to form a stomodaeum