We’re starting out relatively low on the weirdness scale, though if you look closely at enough bats you might disagree with me. The reason I chose bats for this list is that they are an example of a familiar anatomy—that of a hairy, warm-blooded mammal—taken to extremes. While other ‘flying’ mammals like flying squirrels can in fact only glide, bats have evolved true powered flight, utilizing hairless skin stretched between grotesquely elongated fingers. Unnerving, huh?

Wings aren’t the only weird anatomy bats have; almost everything is at least a little off. Their legs are turned around, so that their knees point backwards. Their nipples are in their armpits. They spend their lives hanging upside-down, and in fact get dizzy if they spend too much time with their heads elevated.

But my favorite ‘alien’ part of bat anatomy is their faces. Few groups of animals have as much diversity of facial features as bats do. Some look nearly normal, like little flying mice or foxes. But others look as though they were mangled in an episode of Monster Factory, with ridiculously huge ears, squashed or stretched snouts, and noses that defy understanding. Take a look through ecologist Merlin Tuttle’s bat portrait gallery to see some of the greatest extremes that mammal faces can be pushed to.

RELATED: 7 Weird Natural Phenomena That Prove Our Planet Is Absurd

Okay, so spiders aren’t exactly untapped source material for creepy aliens (or creepy movies). But spiders have more to offer than just their eight legs, eight eyes, and general hairiness.

Spiders don’t have a hard exoskeleton, meaning that their bodies are more flexible than most arthropods (a group that includes insects, crabs, scorpions, and other crawly critters). This means that their bodies can expand after eating, a little like a tick’s. This also means, however, that when they are wounded, they sort of … pop. Yes, spiders are pressurized—they actually move their legs using hydrostatic pressure!

There is a lot of other weird spider anatomy trivia you may not be aware of. For instance, they breathe through holes in their abdomens; the males store sperm in turkey baster-like organs on their faces; and their brains are so large they extend into their legs. But there’s also more than meets the eye to spider hairiness. The thin little prickles all over a spider are actually called setae, and they’re nothing like the hairs on our heads. Spiders use them as a kind of catch-all sensory organ: touch, hear, smell, and even taste. So when a spider scrambles across your leg, it’s also doing the spider equivalent of licking you. Imagine experiencing that little faux pas while shaking the hand of an alien envoy!

The velvet worm is not really a worm, but a member of its own phylum called Onychophoran. It also has the perfect little ugly-cute face. It looks so squishy and friendly! Be warned, though, that any attempts to cuddle this four-inch creature will get you squirted with the slime they eject out of the tubes on either side of their mouth. This slime is also used in prey capture, where it works like quick-drying glue.

The velvet worm has a lot of other little fun bits. Inside their soft-looking little legs are pairs of retractable claws, used for climbing through the undergrowth. Their ‘teeth’ are actually just more of these same claws, modified slightly for tearing apart prey into chunks that they can liquify before ingesting.

Some species of velvet worms live in surprisingly complex family groups, with females as the more aggressive sex. Many species give live birth with gestation periods that can be as long as 15 months. At least one species has a very upsetting way of mating: The males stick packages of their sperm on the female’s skin, which actually decomposes in spots to let the sperm inside. Other species make do by having the male stick his sperm package on a spear or axe-shaped appendage on his head, which he then inserts into the female’s genital opening.

Kinky.

These lesser-known cousins to jellyfish are unambiguously the prettiest critters on this list, in spite of how bizarre they are. Also known by their scientific name ctenophores, comb jellies are semi-transparent creatures mostly shaped like bells or cylinders. Like jellyfish, they have tentacles, but ctenophores only have two, which coat prey in goop rather than sting it. Also unlike jellyfish, ctenophores move not by undulation but by rows of cilia called combs, which flash in iridescent colors as they beat. It allows them to glide seamlessly through the ocean, and also makes it look as if they have glittering rainbow lines on their sides.

Some ctenophores diverge from the typical boxy shapes to make some truly weird configurations, such as the species known as Venus’ girdle, which resembles a flat, transparent ribbon that slowly snakes through the ocean. Unlike any other sensible animal, the girdle’s mouth isn’t at one end of its body but is rather a long slit right in the middle of the ribbon. On the plus side, this means that the girdle can wiggle in any direction it wants without having to worry about things like ‘backwards’ or ‘forwards.’

Another weird-shaped group of ctenophores are the platyctenida, which are the only bottom-dwellers of the phylum. These fellows actually move along the seafloor mouth down and butt up (well, what passes for a ctenophore butt). They move by everting their pharynx — or, throat—and using it like a foot. These are among some of the prettiest-looking ctenophores of all, if you manage to ignore the fact that they’re squidging along the seafloor by turning their throats inside out.

Finally, we’ve reached the only non-animal on the list. Radiolarians are members of the kingdom known as Protista, a kind of biological grab bag of single-celled organisms that don’t fit anywhere else. Protists have astonishing diversity of form, in spite of only working with a single cell.

I chose radiolarians to end this list because they prove that life can truly come in any imaginable shape. Radiolarians don’t look like organisms, they look like modern art—beautiful tiny sculptures with skeletons of glass, or strontium sulfate. Sometimes these skeletons look like honeycombs, sometimes they look like snowflakes, sometimes they look like miniature rocketships. But inside of these inorganic shells are living cells, which extend their soft pseudopodia through the gaps to grab food and eject waste. Many reproduce by cracking their skeletons in half, so that each new cell has to make only half a shell.

Radiolarians are microscopic organisms, and when they die, their tiny skeletons form a thick sediment that covers large portions of the ocean floor in what appears to be grayish clay. You’ve probably seen it in videos of deep-sea exploration without even realizing that beneath the remains of a shipwreck or reef were the remnants of millions upon millions of bizarre, beautiful, and alien lives.