The North American porcupine, Erethizon dorsatum, is a large rodent belonging to the family Erethizontidae. Surprisingly diverse, there are about 24 porcupine species found throughout the Americas, Asia, Europe and Africa. The 30,000 or so quills distinguish these animals from other members of the order Rodentia. While the quills can not be shot like arrows, they are sharp and easily dislodged. The modified hairs are reinforced with thick keratin plates that are layered like the shingles of a roof. Each unidirectional plate terminates with a barb that allow the quills to easily slip into a nosey dog and stay put. Because the terminal edge of the plate comes to a point, the quills are very painful to remove.

Yong, Ed. "Why Porcupine Quills Slide in with Ease but Come out with Difficulty : Not Exactly Rocket Science." Not Exactly Rocket Science. Discover Magazine, 11 Dec. 2012. Web. 07 Feb. 2015.