During gastrulation, animals form a structure called the blastopore, which is where migrating tissues tuck themselves inward to establish the three germ layers of the embryo. In deuterostomes, the blastopore will eventually become the anus. In the complementary category, the protostomes, which includes annelids and arthropods, the blastopore develops into the mouth.
When I read that, my twisted mind thought of Catullus' attack on a sufferer from halitosis (97.1-3):
I did not (so may the gods love me) think it made any difference whether I smelled Aemilius' mouth or his anus. The latter is not a bit cleaner, nor the former a bit dirtier.
Non (ita me di ament) quicquam referre putavi,
utrumne os an culum olfacerem Aemilio.
nilo mundius hoc, nihiloque immundius illud.
The rest of Catullus' poem is too filthy to quote in this family-friendly blog.