Sharks

Sharks are a group (superorder Selachimorpha) of fish, with a full cartilaginous skeleton and a streamlined body, with normally 5, but up to 7 (depending on species) gill slits along the side of, or beginning slightly behind, the head (in some species, a modified slit called a spiracle, is located just behind the eye), dermal denticles covering the body to protect from damage, parasites and improve fluid dynamics, and rows of replaceable teeth in the mouth.

Sharks have keen olfactory senses, with some species able to detect as little as one part per million of blood in seawater. They are even more attracted to the chemicals found in the gut of many species, and often linger near or in sewage outfalls. Some species, such as Nurse sharks, have external barbels that greatly increase their ability to sense prey. The short duct between the anterior and posterior nasal openings are not fused like in bony fish