As we all know, a penis finishes in a head called gland.
Or so it should be ...
But not always.
Some are double-headed! Bifurcated penises are found in the most primitive living mammals: monotremes (which include platypus and echidna) and marsupials (opossums, Tasmanian devils, quolls, bilbies, bandicoots, koala, wombats, wallabies, potoroos, numbat, gliders and others).
In fact, amongst marsupials, only the two largest species of kangaroos are destitute of this peculiar trait. The bifurcated penises are formed by two distinct columns, and so the penis has two ends. And to get more, the echidna has a four headed penis ! (photo above).
Echidna does not use its penis for urination and it only gets out of the body during mating (monotremes have a cloacae organ, like birds and reptiles, where the penis is kept while resting).
The two penis heads may have different anatomies, according to the groups and species: some are short (photo center, in Tasmanian devil), others long, some are coiled between them (photo bellow, in sugar glider) and so on.
In marsupials, even the scrotum is placed in front of the penis, not behind! (in monotremes the testes are internal, like in reptiles and birds).
The marsupial male has a fine degree of control over how far to extend or retract the scrotum; they can fully retract the package when they have to protect these sensitive organs or hang them loose, to insure the testicles remain cooler than the rest of the body (only at temperatures lower than 36o C sperm cells do emerge, the reason why humans too have externall balls).
The two headed penis of the opposum started in America a myth that this animal ejaculate in the nose of the female! Funny, but wrong!
Because the peculiar penis anatomy in males is coresponded also by an odd female genital anatomy. The marsupial females not only have two uteruses (which are partially or totally fused in evolved mammals) but two vaginas also!
In some species, the vaginas may be partially joined, but never lose identity.
The double vagina, double-headed penis and testes before the penis are regarded by the scientists as primitive traits...
Photo credit: Gordon Grigg