Either of the two penises possessed by the male members of order squamata (snakes, lizards and amphisbaena).

Hemipenes are a set of symmetrical, paired penises that are usually carried inverted in the cloacae. During copulation, one of the hemipenes becomes erect and um... things proceed as they do in most other species.

Hemipenes do not have a urethra; instead they have an open canal running along their length. The testicles are secured deep in the body -- true internal organs. Each hemipenes draws from a different epididymis, meaning that even after one runs dry, the other is ready to go. The male will often alternate between the two, but not always.

Hemipenes often have spines or hooks to keep the male firmly attached to the female until he's finished*; many squamata also ejaculate a mating plug (a gob of thick secretions that blocks other males' sperm from getting in), which may increase the length of time needed for a fully successful copulation. Some hemipenes are forked, and have two heads (yes, that's a total of four heads in all).

If you have a squamata and wish to know its sex, you may need to check for a hemipenis. This is not easy, and you should get someone who knows what e is doing to help you:

  • First, figure out the species that you are dealing with. If you have a species with a good level of sexual dimorphism, you may be able to skip the following activities.
  • If you have a young squamiño, under two weeks of age, you can try popping it: place a finger firmly below (towards the tail) the cloacae and press gently upwards. If there are hemipenes present, they should pop out. This is only safe with very young snakes and lizards, as the tendons that hold the hemipenes in will start to firm up as the animal ages, and may be damaged by popping.
  • If you have an older reptile, you may be reduced to cloacal probing. This is basically sticking a small stick down the cloacae, and seeing how far it goes. Males have a pouch in the tail, while females do not. You need to have the right size probe (as large as will comfortably fit), and you need to be gentle. If the probe travels only as far as four or less subcaudal scale rows, it's a female, and six or more means it's a male. More than ten means you've impaled your subject. Don't do this without the proper training!

If the hemipenes are hanging out of your little scaled friend, this is called paraphimosis (it means something rather different for humans), and is very bad. It is likely the result of something bad (poor popping technique, infection, etc.) and will likely result in other bad things, like a secondary infection and necrosis. Amputation may be necessary. See your local herp vet immediately.

Want to see some hemipenes? Of course you do. This site has some good pictures, and this one has another, but beware, the second has actually dissected their snake**. For those of you who are not perverts seeking snake porn, this site has a nice simple pencil sketch that even a nun would approve of.

* Sounds painful doesn't it? It can also be painful for the male, who, being stuck to the female, may be passively dragged along the ground if she decides to go anywhere before he's finished.

** In case you care... All these photos, on both pages, are almost certainly of dead snakes, as you will never see a healthy hemipenis out in the open. Someone injected fluid into the hemipenial lumens, inflating the glans.

http://news.softpedia.com/news/Those-luchy-bastards-41906.shtml {sic}
Probing and popping: