In Scoliodon, the primary respiratory organ is five pairs of gills, a stream of water enters the mouth due to the contraction of the pharyngeal muscle. The stream of water goes to the pharynx and from there the water enters the gill pouches and from the gill pouches the water flows out through the external gill slits. Muscular contraction of the pharynx creates the pumping activity of the water current, and breathing takes place in two steps.
When the hypobranchial muscle contracts, the gill arch expands, causing contraction of the buccopharyngeal floor and enlarging the oropharyngeal cavity. The enlargement of the buccopharyngeal cavity creates a negative pressure, which creates a flow of water into the oral cavity.
When the stream of water enters the mouth, the abductor muscle closes the mouth and the interbranchial muscle reduces the volume of the buccopharyngeal cavity. It pushes the water inside the buccopharyngeal cavity into the gill pouches, through the gill pouches the water flows out through the external gill slits.
The spiracle is a stunted organ in Scoliodon, but in some cases the spiracle is used to draw water into the buccopharyngeal cavity.
The gill lamellae are highly vascularized with a fine network of capillaries, the afferent branchial artery passing through the gill lamellae mixes oxygen into the blood and diffuses carbon dioxide from the blood into the water. The capillary network of the gill lamellae emerges as an efferent branchial artery, this is how gas exchange takes place in the branchial system of Scoliodon.