The spiracle is a vestigial first gill slit. It appears as an opening behind the eye, as in the spiny dogfish photo below. It is absent or reduced in many sharks, especially the fast swimming sharks and is usually larger and present in sedentary or bottom dwelling sharks. The spiracle in sharks is used to provide oxygenated blood directly to the eye and brain through a separate blood vessel. In the rays, the spiracle is much larger and more developed and is used to actively pump water over the gills to allow the ray to breathe while buried in the sand.

The spiracle of a spiny dogfish