So occasionally, whether due to radiation, toxins or just plain randomness and probability, embryos and foetuses can experience odd mutations. Usually, these result in a nonviable organism that either dies in utero, dies in early infancy or is sterile. But sometimes, weird things happen. Two of these 'weird things' (and I apologize for not having a better term) are parasitic and conjoined twins.
First off, conjoined twins. I realize that someone already posted on these girls (was it this comm?), but it's an unusual case; both girls have not only survived, but thrived into sexual maturity. Most conjoined twins tend to miscarry or die in infancy.
A set of conjoined twins is a pair of conscious individuals sharing a single body. This occurs when the egg fails to split into two complete, individual zygotes or when one identical twin 'attaches' to the other in the zygote stage. Many different types of conjoinment exist, varying between having two whole, separate bodies connected by tissue to as little difference as a second face. The most common types of conjoinment are:
Thoracopagus (connected at the anterior abdomen) 40% (Chang and Eng Bunker, the most famous conjoined twins in history, were thoracopagic (source))
Omphalopagus (connected at the sternum (chest)) 33%
Pygopagus (connected at the buttocks) 18%
Ischiopagus (connected at the lower back) 6%
Craniopagus (connected at the top of the head) 2% (Krista and Tatiana Simms, the only conjoined twins in Canada, are craniopagic (source))
There are also conjoined twins in the animal kingdom. Two-faced kittens are usually the ones who're cute enough to be reported on, but there are two-headed crocs, two-headed calves etc. in museums and online images.
Additional sources and more reading:
Some pictures may be disturbing.
More info and diagrams.
Video of Krista and Tatiana Simms by the Globe and Mail.
Wiki with additional links.
Parasitic twins, in a way, are the opposite of conjoined twins. They are kind of a middle ground between vanishing twins or 'chimaera' and conjoined twins. Someone with a parasitic twin may have as little as three extra toes (which usually get removed or overlooked) or as much as a whole extra body. In cases where a fraternal twin has attempted to attach, one twin becomes nonviable and may be entirely absorbed into the other twin and 'vanish' (hence 'vanishing twin'), or may remain detached from the twin, but develop inside their body, as in the case of this man, Sanju Bhagat:
More commonly known as the man who gave birth to his brother, Mr. Bhagat, in his 36 years, had always had an extremely swollen abdomen, and had no idea why. Many thought it was a tumour. But, finally, when Mr. Bhagat began to have trouble breathing due to the mass pressing on his diaphragm, he went to the hospital. Prior to surgery, doctors did a CT scan, which turned up an unidentifiable mass. Confused, they decided to go ahead with the surgery. What did they find? A human that, although it had an extremely malformed head, was otherwise well articulated and developed, with hands, feet, long fingernails and even genitalia intact. (After surgery, Mr. Bhagat weighed in at 84 lbs.) Source - link to two similar stories inside.
This specific form of parasitic twin is known as 'foetus in foetu.'
Other forms include these:
Source - rabid atheist blog, sorry.
And this little girl, Lakshmi Tatma, who was born with eight limbs and named after a similarly-structured Hindu goddess of fortune.
Source - another blog, sorry.
She underwent a 27-hour surgery at the age of 2 to have the extra four limbs removed and her body realigned because, with the twin attached, she could only expect to live into her teens. The surgery involved removing the limbs, realigning her coccyx, pelvis and femurs, taking out her dead kidney and replacing it with the dead twin's living kidney, and constructing genitalia.
Look! An entire post in freaksofnature!
Very detailed blog.
A case study; a couple of pictures might be a little gross to some.
Wiki with additional links.
It is not yet known exactly what causes conjoined and parasitic twins, and it's not clear exactly how they form, but I just have a (probably unhealthy) interest in them. And, as something that happens in nature and due to natural causes, and due to its occasional wtfyness, I thought it belonged here. Correct me if I'm wrong.