In elasmobranchs (sharks and rays), a spiracle is found behind each eye, and is often used to pump water through the gills while the animal is at rest (Fouts, 2003). A spiracle is also found in primitive bony fishes as the chirrups.
Insects and some more advanced spiders have spiracles on their exoskeletons to allow air to enter the trachea (Solomon et al., 2002). In insects, the tracheal tubes primarily deliver oxygen directly the animals' tissues. The spiracles can be opened and closed in an efficient manner to reduce water loss. This is done by contracting closer muscles surrounding the spiracle. In order to open, the muscle relaxes. The closer muscle is controlled by the central nervous system but can also react to localized chemical stimuli. Several aquatic insects have similar or alternative closing methods to prevent water from entering the trachea. In spiders, however, the oxygen diffuses into the hemolymph (Foelix, 1996). A similar diffusion effect also occurs in some insect caterpillars. In these groups the respiration is more reminiscent of lungs. In spiders and other arachnids they have structures called book lungs.