Here’s a riddle: If you lived in your backpack with only your head and your feet sticking out, would you poop in your backpack?
Of course not. Don’t be ridiculous. You’d move your anus and poop out of your shoulder, of course. Just like the snail did.
All mollusks – the squids, the octopi, the clams, the snails – can trace their family tree back to a common ancestor. We hypothesize that this ancestor looks like HAM (Hypothetical Ancestral Mollusca). This mollusk was snail-like with shallow conical shell. The wide opening of the shell allowed its alimentary system (the tube that starts with your mouth and ends at your other end) to be distributed how it is in many animals: mouth at one end and anus at the other.
Then Nature grabbed the shell of the snail and twisted it into an ice cream with a harder-than-candy shell (a process called torsion). As a result the opening through which everything happened (eating, pooping, crawling) got smaller. As the shell twisted, so too did its alimentary canal. The anus still needed to exit out the same, now smaller opening (because you don’t want to poop in your own backpack). The arrangement made the anus and mouth next door neighbors. If you’re looking for the anus of snail, look behind his head or just over his ‘shoulder’.
Maybe you’re having a hard time visualizing all of this. Or maybe you’re stuck in your backpack still trying to solve the riddle. Either way, I have more visual aids courtesy of an amber snail I found doing his business right there along the side of the road in Maine (such shameless little molluscs). You’ll notice his business is large proportional to the size of his body (far larger, proportionally, than anything any of your friends have bragged about).
Now that we know where the anus is, what about the snail’s breathing hole? Land snails have primitive lungs and breathe through a pneumatophore, his ‘nostril’. Real estate is tight so the snail’s ‘nostril’ is near the anus. In the picture below, the hole just above the, ahem, business, is his pneumatophore (follow the arrow). And no, I doubt he held his breath the entire time. He’s outdoors. It’s well-ventilated.
Amber snails have a relatively large aperture (opening) to their shells so they don’t evacuate on their heads, but many land snails do.
Middle-school jokes and nicknames begin in…no, onto more science!.
How about the penis! Behind the head. Vagina? Often next to the penis. Where else would you put it? Yeah, many snails, including the amber snail, are hermaphrodites. I used ‘he’ in the post because that’s the trouble with pronouns.
If you’re interested in your own alimentary canal and it’s adventures, then you should read GULP by Mary Roach. It’s a rootin’ tootin’ good time!