I just saw an interesting review article published in Physiological Reviews that discussed the presence of olfactory (i.e. smell) receptors located outside of the nose.
It turns out that these “smell” receptors are not unique to our nostrils. In fact they are found throughout our bodies. Those found in the heart may be responsible for regulating heart function, those in the immune system are thought to help destroy types of leukemia cells, in the skin they may help regenerate tissue during wound healing, and in the digestive tract they may cause chronic diarrhea, constipation or perhaps even improve digestion. These olfactory receptors also have implications in cancer as they are thought to reduce or promote cancer growth in the prostate, prevent cancer cells from spreading in the liver, and reduce colon cancer cell growth.
Some animals have olfactory receptors located outside of nostrils as well, although they are still responsible for sensing chemicals in the environment. An article from National Geographic explored how Oregon shore crabs (Hemigrapsus oregonensis) smell things in the environment using an external array of hair cells that resemble a toothbrush.
Snakes, on the other hand, sense smells by capturing scent molecules on their tongues and moving these molecules into pits in the roof of their mouths that can sense them.Flies have chemical sensing hairs on their mouths and feet (tarsi for an insect). So when they walk on your food, they really are giving it a taste. Similarly, octopuses have chemical receptors in their suckers. Source:
The APS press release site