542-488 million years ago
RESPIRATORY AND DIGESTIVE SYTEMS
In cephalochordates, the pharyngeal arches became more developed.
In Amphioxus, the pharynx stretches for half the length of the body and contains 100-200 gill slits (depending on the species). Amphioxus performs significant gas exchange through skin in addition the gas exchange performed with the water which enters the mouth, passes over these gill clefts, and departs the body through a single atriopore opening on the ventral side of its posterior (Weichert, 1970, p. 211).
In lancelets, cilia move food to the intestine where flagella mix with digestive enzymes produced primarily from the hepatic cecum (Ruppert, from Harrison, 1997, p. 354).Among primitive deuterostomes, the use of cilia to propel food is commonly used. Primitive echinoderms and pterobranchs possess ciliated tentacles while enteropneusts possess a ciliated proboscis. Primitive chordates utilize cilia and mucus for food capture in their pharynx (in Amphioxus, the endostyle is a ciliated groove which makes mucus to trap food) (Barrington, 194). Modern vertebrates retain cilia in their pharynx and in the respiratory tract (which developed as an outpocket of the gastrointestinal tract in fish).