(1) Branchial pharynx: The dorsal portion of Pharynx is called branchial pharynx. It is dorsolaterally perforated by a longitudinal series of numerous U-shaped gill slits.
Their number varies and increases as the animal grows older. The extensions of pharyngeal wall into the gill slits are called tongue bar, while the extension between two adjacent gill slits is called gill septum. The tongue bars are hollow and enclose coelomic cavity but the gill septum is solid. Both are connected together with transverse or horizontal connections, the synapticulae. The tongue bar and grill septum are supported by pharyngeal skeleton. Gill slits are richly lined by cilia, called lateral cilia. Development and arrangement of gill slits is identical with that found in Branchiostoma.
(2) Branchial sacs: Branchial sacs are pouch like structures into which gill slits open. They lie between the body wall and the pharynx and open outside through gill pores. Collar coeloms also communicate with the common branchial sac of its side through a collar canal. Gill pores are visible externally in two longitudinal rows in the branchiogenital region of trunk. Mechanism of respiration: The main physiological base of respiration is food cum respiratory water current which is set up by lateral cilia of gill slits. This water current passes through gill slits and branchia and leaves through the gill pores.
Tongue bars are richly vascular and participate in gaseous exchange. Blood of their capillary network takes up O2 dissolved in water and releases CO2 into it.