In 1822, a French naturalist by the name of Geoffrey Saint-Hilaire cut open a lobster and noticed that its arthropod anatomy was essentially a mirror image of our own, along what is called the dorsoventral axis.
These are molecules that have two forms which are mirror images of each other. This is important in the drug industry, for the different versions, known as enantiomers, can have different effects in the body.
One influential study showed that the mirror-image network once thought necessary does not have to be an exact replica of the original for learning to take place (albeit more slowly for big networks).