Gas exchange happens through the walls of the gill structures, mainly in the vascular branchial epitheliumlocated in the branchial sacs that are abundantly supplied with blood (Mandal,2012), and also though the body surface (Biocyclopedia,2012) (figure 9).

Figure 9 - Location of pharyngeal clefts in the three regions of the body (Ferreira, undated)

Therespiratory system is composed by gills pouches that can be up to 700 pairs. Thereare row of gill pores located dorsolateral on each side of the trunk justbehind the collar (Biocyclopedia,2012). This respiratory system opensinternally in the pharynx by the gill pores and to the outside by the two rowsof gill slits (Biocyclopedia, 2012). This is similar to the one in chordatessince the gill slits connect the pharynx with the external environment (Biocyclopedia, 2012) (figure 10).

Figure 10 - Structure of branchial region.  bc, coelom. tb, tongue-bars. ds, mesentery. pr, ridge. vv, vessel. gp, gill-pore. dn, dorsal nerve. dv, vessel. œ, oesophagus. vs, mesentery. vn, ventral nerve. (Encyclopaedia Britannica, undated). 

The cilia currents maintain a constant renew supply ofwater that moves from the mouth to the pharynx and out the gill slits andbranchial chambers to the exterior (Biocyclopedia,2012).