Concept
In a zoological context, spines are hard, needle-like anatomical structures found in both vertebrate and invertebrate species. The spines of most spiny mammals are modified hairs, with a spongy center covered in a thick, hard layer of keratin and a sharp, sometimes barbed tip. Spines in mammals include the prickles of hedgehogs, and among rodents, the quills of both New World and Old World porcupines, as well as the prickly fur of spiny mice, spiny pocket mice, and spiny rats. They are also found on afrotherian tenrecs of the family Tenrecinae (hedgehog and streaked tenrecs), marsupial spiny bandicoots, and on echidnas (of the monotremes). An ancient synapsid, Dimetrodon, had extremely long spines on its backbone that were joined together with a web of skin that formed a sail-like structure. Many mammalian species, like cats and fossas, also have penile spines. The Mesozoic eutriconodont mammal Spinolestes already displayed spines similar to those of modern spiny mice. Spines are found in the rays of certain finned bony fishes including scorpion fish. The sting that is found in a stingray is a type of barbed spine. Spines are also found in invertebrate animals, such as sea urchins. They are a feature of the shell of a number of different species of gastropod and bivalve mollusks, including the venus clam Pitar lupanaria. Spines are also found in internal organs in invertebrates, such as the copulatory spines in the male or female organs of certain flatworms. In many cases, spines are a defense mechanism that help protect the animal against potential predators. Because spines are sharp, they can puncture skin and inflict pain and damage which may cause the predator to avoid that species from that point on. The spine of some animals are capable of injecting venom. In the case of some large species of stingray, a puncture with the barbed spine and the accompanying venom has occasionally been fatal to humans. Animals such as porcupines are considered aposematic, because their spines warn predators that they are dangerous and potentially toxic.