Human Outside, Alien Inside

"Kim: I've got to be honest, I wasn't expecting something so different.

Tal: Neither was I. Our species look so similar...well at least on the surface.

Kim: I would've never guessed when it came down to the basics. Well, let's just say the birds and bees will be very confused."

- Star Trek Voyager ("The Disease")

Let's face it: Building a truly elaborate alien costume costs money, more than the budget will safely allow. However, Rubber Forehead Aliens, cheap as they are, just won't cut it forever.

One safe way to get around the latter problem is to suggest that while on the outside they're exactly the same, on the inside of your alien, anything goes. If you were to dissect the average TV alien, it would look like someone had torn a squid apart and stapled it to the remnants of a rump roast.

Once this is done, the alien sounds suitably... alien-y. Of course, now the producers have to make sure the creatures are gutted only off-camera.

This not only applies to physical organs, but biochemistry as well. Funny colors of blood, odd allergies, complicated mating rituals (or mating organs), Bizarre Sexual Dimorphism, funky dietary requirements (or intoxicants) or interesting bowel movements are all common. Sometimes even their minds are totally different, making communication with them problematic.

The less human-looking the alien is outside, the less likely this trope will be resorted to. This can lead to the ironic trap of Serkis Folk and other more elaborate aliens falling on the other side of the Uncanny Valley and thus seeming, on some level, more human than the Human Aliens or Rubber Forehead Aliens if they appear in the same series.

Aliens may also be shown to have immunity from things that are plenty lethal to your average person, being able to survive heat and cold extremes unprotected, tolerate massive doses of radiation, or be unaffected by poisonous materials. On the other hand, they could also be negatively affected, or perhaps even injured or killed, by a substance that would be completely harmless to an Earthling.

Interestingly, this isn't inverted very often. Humans, even when they're the "aliens" of a story, almost never have flashy biological differences that make them anything but weaker than the rest of the cast.

For more literal and squicky examples of an alien "inside" a human, see Mobile Suit Human, Genuine Human Hide, and Puppeteer Parasite.

Anime & Manga

 * The aliens of Parasyte definitely fall under this trope. Apart from their Body Horror-riffic maturation process, there's the fact that they can split their body mass into multiple pieces(although pieces that are too small will wither and die quickly), that they're made of a singular type of cell that they can turn into any variety of bodily tissue they need, that fire and acids cause them to undergo the equivalent of a grand mal seizure, and that they don't know how, or even IF they reproduce.
 * Although according to Word of God, the Parasytes were not necessarily meant to be aliens. The negation of No Biochemical Barriers means it's more likely that they originated on Earth. But either way they do have some... interesting biology.
 * One episode of Urusei Yatsura features Lum and Ten eating pickles and realizing too late that the brine has an effect on them that alcohol does not. (Arguably a Not Himself episode, as we never see them completely drunk at any other time in the series.)
 * Would that be the same episode that saw them get drunk on umeboshi (a kind of pickled plum)?
 * Similar yet different, from the same series: Lum has no clue, thanks to her species' increased tolerance for spice, that most of the recipes she knows are hot enough to cause temporary mental trauma.
 * In at least the anime (episode "What A Dracula"), the Oni can't stand the smell of garlic and are repulsed by those who've eaten it. Ataru explicitly compares Lum to Dracula after proving that, yes, his garlic-scented breath does act like tear gas on her, going so far as to jokingly try and ward her off with a cross after pointing out she hates garlic, has fangs, and bites his neck/sucks his blood.
 * The natives of The Twelve Kingdoms are apparently indistinguishable from Earth humans, but instead of reproducing "normally" they are born out of large fruit that grow on special trees. They can have sex, but it looks like it's for fun and not for reproduction.
 * It doesn't end there: Sometimes still-growing egg-fruit accidentally fall from their branches and into shoku (storm-induced portals to the real world), where they are "somehow" transported into the wombs of pregnant human women. That's how Youko Nakajima, the main heroine, was born.
 * Lampshaded in the novels, where Rakushun admits that the concept of the eggs being transported from the middle of the ocean to a person womb is bizarre.
 * Likewise, the inhabitants of the world of Simoun are otherwise indistinguishable from Earth humans, except that they're all born the same sex, develop feminine bodies when they reach adolescence and choose a sex at age 17 by wading through a particular spring. Reaching such a state through evolution is of course extremely unlikely, but this is a fantasy/steam-punk series that needs a justification for making all the main characters Schoolgirl Lesbians.
 * Simoun is carrying on in the grand bizarro-feminist tradition of Doris Lessing, in whose The Cleft (which actually came out a year after Simoun) someone being born male nearly destroys the primordial human race.
 * The track names of the OST suggest a Gendercide of unspecified origin or nature.
 * In the decidedly more mature manga version of Mermaid Melody Pichi Pichi Pitch, mermaids can get drunk on anything carbonated. Both versions also somehow allow for an instant transformation from human to mermaid upon being immersed in water.
 * The mermaids from Seto no Hanayome are the same way with water, but are otherwise human, though exceptions can be made for Shark Fujishiro and Kai Mikawa, who transform into a shark and an orca, respectively, when touched by water.
 * And in Shark's case, he can part way transform whenever he wants to attempt to eat Nagasumi.
 * The Goddesses from Ah My Goddess could fall under this. Urd uses alcohol as an "alternate energy source", but seems to suffer no deleterious effects. Skuld is the same with ice cream. Belldandy can drink a distillery and be completely unaffected, but gets blitzed off of soda. Of course, this just makes her an even sweeter and more empathetic Literal Genie.
 * Invoked in the 21st episode of Irresponsible Captain Tylor, "Paco-Paco Junior". Having gone to sleep beside Tylor at the end of the last episode, Azalyn/Empress Goza XVI claims that she's pregnant with Tylor's child, due to the differences of the Raalgon biology compared to those of the human.
 * Parodied in Kenichi the Mightiest Disciple with Nijima, who apparently has some biological irregularities.
 * Used to a small degree in My Two Wings, in which members of  are all hermaphrodites, for starters. In an inversion, it's subtly implied that humans are a fair bit hornier in general.
 * Celty in Durarara has most of the same organs as a human, but they're vestigial and nonfunctional (to the bewilderment of a character who vivisects her.)

Comic Books

 * Classic comic example: An alien woman in an old EC title did in her earthling husband on her wedding night after seeing his navel and realising he was a mammal. This entirely Caucasian-looking blonde-haired woman with nice, perky breasts was supposed to be from an egg-laying species who find the mammalian lifestyle disgusting.
 * The Kree of the Marvel Universe have black-widow-like mating practices; females are "neutered" to prevent this. They also require a special gas or other life-supporting technology to breathe on Earth.
 * Superman has many handwaves for his many powers, but the main thing to consider is his kryptonian ability to emit microwave radiation and levitate, all from exposure to higher energy solar radiation.
 * The Ultimate Universe version of the Fantastic Four used this to explain some of the team's powers. Reed, for instance, no longer has internal organs, merely possessing a "bacterial stack" in its place. Oh, and he no longer craps.
 * In ~Sergio Aragonés~' "A Mad Look At Star Wars", Luke Skywalker is in Mos Eisley, and asks to use the bathroom. In there, he finds several different varieties of urinal.
 * A rare inversion: The three-eyed altered humans who show up in a few issues of Tom Strong can no longer eat at least certain terrestrial foodstuffs without getting the pukes. However, coffee doesn't bother them...and normal humans seem to be able to eat their food without adverse effects.
 * Karolina Dean and family from the Runaways. On the surface they look like normal humans, but their bodies apparently store and then expel solar energy and a dying vampire notes that Karolina's blood "burns like sunlight." This never being picked up by a doctor could be explained by her parents apparent natural living kick which might have involved never taking Karolina to a doctor or having her vaccinated.
 * Some aliens in Empowered's verse have two left kidneys. And one kind of alien (maybe the same one) has kidney equivalents that filter the bloodstream using coherent light - essentially, Frickin' Laser Beams.

Fanfiction

 * Every Neon Genesis Evangelion fan knows that Rei isn't fully human but it's not shown exactly what differences being a half-breed means in her case. One fanfiction (this troper doesn't remember the name) states that she doesn't have inner reproduction organs; instead, she has some kind of "blood valves" that serve to nourish her S2 Engine which houses her soul and is transplanted whenever she "dies". Once Rei IV (taken from Unit 01's dummy plug) started to reject Lilith's DNA, Rei V (made by substituting Lilith with Shinji) was genetically modified to slowly morph the S2 Engine into... well, what she should have if she were a normal 14 year-old girl (though she also lost her albinism). This troper makes it wonder whether it means she will also become fertile and have periods - it is canonical that she "doesn't bleed".
 * Well, she could be either infertile, or hyper-fertile (IE: She doesn't have periods because she doesn't replace her uteral lining every month).
 * At least two WoW fan authors have ruled that draenei (who canonically have blue blood) have two hearts. A few others have speculated on the reason why the males are so much bigger (even if Garona's canonical origin has kind of Jossed that latter).
 * It's Fanon and the subject of at least one fanfic (probably more) that Vulcans get drunk on chocolate.

Film
"Kirk: I was lucky that thing had knees!
 * Galaxy Quest. The cephalopodic Thermians use holographic projectors to Cosplay as human.
 * One of the humans, Fred, falls in love with a Thermian. She deactivates her holoprojector, and they proceed to do something that makes Plucky Comic Relief dude Guy exclaim, "Oh, that's not right!"
 * In the comics spin off they're trying to have a kid.
 * Also a case of Bizarre Alien Psychology, given their near-total incomprehension of deception or fiction.
 * Played with in the Thermians' sincere but stomach-churning attempts to accomadate the preceived needs of "fellow extraterrestrial" Dr. Lazarus, who is actually a human actor.
 * A deleted scene on the DVD has a Thermian showing "Dr. Lazarus" to his quarters, where there is a large bathroom equipped with bizarre fixtures. "You're very... complicated." the Thermian explains.
 * Star Trek VI, after a "welcome fight" that Kirk has with another Rura Penthe prisoner:

Martia: Those weren't his knees. (Kirk does a Double Take) Not everyone keeps their genitals in the same place, Captain."


 * Invasion of Alien Bikini, a rather strange but entertaining new offering from Korea, basically lives this trope.

Literature
"Me: May I touch that?
 * Inversion: In March Upcountry by David Weber and John Ringo, aliens poison the food of a squad of human soldiers and try to blackmail them with the antidote. Not only is the poison tasteless to the aliens but strongly flavoured to the soldiers, it also turns out to not affect humans. In a further inversion, the human's doctor discovers another poison (an obvious expy of fugu venom) can be used to produce the several nutrients the humans need for their own health but they hadn't been able to find up to that point in the planet's flora and fauna; and just in time, as their reserve of supplement pills were running out.
 * Their gender also causes confusion. The 'males' are smaller and act as Mister Seahorse carrying the young to term. The 'females' are larger and have a very pronounced ovapositor. Consequently the gender responsibilities in the society are switched relative to human society. When they figure this out the humans get very confused before eventually deciding to keep calling the females males and males female to avoid confusion.
 * Another inversion occurs in Josepha Sherman's novel A Strange and Ancient Name, where iron is deadly to elves and even the tiniest scratch means certain doom. When the half-human prince gets stabbed by an iron weapon, the kingdom goes into mourning, waiting out his long and painful death. Until the (human) love interest points out in a rage that maybe his despised heritage makes him less susceptible to the effect, and the healers decide to actually try to help him.
 * The Star Wars Expanded Universe often describes the unusual biology of aliens. In the films, the details of a species are rarely gone into in any depth.
 * A bit of bizarre alien psychology: Thakwaash, a horseheaded species of humanoids, easily and healthily form multiple personalities, to the point where they are "We" and not "I." Runt from the X Wing Series washed out into Wraith Squadron because his incoherent, borderline suicidal "pilot mind" wouldn't obey orders and in simulators tended to get killed.
 * Another multiple-personalitied alien race are described by Diane Duane in Spock's World; they have "octocameral" brains, so they have eight personalities each.
 * Twi'leks, those very humanlike aliens with odd-colored pigmentation and a pair of braintails instead of hair. The braintails are mobile enough that they are used in a low-grade Starfish Language and in some adaptations seem to be prehensile, and they actually do have some brain tissue in them. Aayla Secura, after going completely amnesiac thanks to a Dark Jedi, has her memory restored later when someone does something to her braintails. Twi'leks can survive losing part or all of a braintail, but this is really painful.
 * The aliens from Neal Stephenson's Anathem cannot metabolize food from the planet in which the story is set, and can only marginally breathe the air.
 * The Xenogenesis series goes at length to describe just how inhuman the Oankali are to humans. For starters, they see, smell, and hear through hair-like tendrils all over their body. One that's met early on was bred specifically to look human...meaning that those tendrils are in the spots normally occupied by the eyes, ears and nose. And let's not even get started on how they mate.
 * They're also three-gendered...and the third gender has a second, boneless pair of arms...and that's just the most externally obvious and least bizarre trait.
 * In Zenna Henderson's sadly-underrated short story Subcommittee, Earth is invaded by fuzzy, brightly-hued Humanoid Aliens. After a huge war in space, representatives from each side begin peace talks. The humans are appalled when the aliens appear to be asking for all of Earth's oceans. Naturally,
 * The classic L. Sprague DeCamp short story Nothing in the Rules has a mermaid who gets drunk on fresh water, which would be fine if the protagonists hadn't just entered her into a swimming meet (hence the title)
 * S.L. Viehl's Stardoc series thrives on this. Just to give a relatively tame example: Jorenians (large, attractive, blue-skinned humanoids who can actually interbreed with humans) have green blood, twelve-chambered hearts, retractable claws, compartmentalized stomachs, and an unusually large and complex spleen (which is a vital organ to the species).
 * A large part of Extraterrestrial Sex Fetish by Supervert.
 * Scott Adams' non-fiction book The Dilbert Future is a realistic approach to predicting the future, with mankind's obvious faults taken into account. One section criticizes the idea that humans could have sex with aliens like in Star Trek. Scott's own speculated attempt is summed up with this delightful passage:

Alien: That is not an erogenous zone. It is a separate corporeal being that has been attached to my body for six hundred years.

Me: It's cute. I wonder if it would let me have sex with it.

Alien: That's exactly what I said six hundred years ago."


 * In Sergey Lukyanenko's Lord From Planet Earth trilogy, the entire galaxy is populated by Human Aliens, many of which look nearly indistinguishable from humans. In fact, the main character (a human) meets and later marries a woman from planet Tar, who easily passes for an Earthling. Other races have slight differences in external features, but may have radical differences internally. For example, Palians look like pale humans with elongated canines and yellowy eyes. They also exclusively consume blood and are sensitive to certain radiation emitted by yellow dwarf stars. Their visits to Earth resulted in the vampire myths. Another race has blue skin and is a lot less sensitive to extreme temperatures. The author explains it all with Panspermia, as all races in the galaxy but one are a product of seeding their homeworlds by the aptly-named Seeders. The various internal differences are simply the result of the same DNA evolving differently due to the various environments present on each planet. In fact, this forms the main plotline of the trilogy.
 * A fairly subtle example: In the Confederation of Valor series, Taykans become mature physically, sexually, and (to a limited extent, at least) socially long before they become fertile.
 * In The Alien Series, Alpha Centaurion may look human, but their internal organs are radically different. For one thing, they have two hearts.
 * Most of the Marra encountered in The Madness Season like to disguise themselves as human. Though this is mostly because the hero tends to visit human colonies rather than those of other intelligent species. They may superficially seem human but an FMRI shows some major differences.
 * The Starfleet Corps of Engineers stories do this while fleshing out the Ardanans, who were established in Star Trek the Original Series as looking just like humans (as did many of the series' aliens). The SCE stories describe the Ardanan brain and demonstrate that its structure and chemistry is very different from that of humans; this becomes a plot point.
 * Another Star Trek Novel Verse story, A Singular Destiny, mentions that Zaldans (who look human other than their webbed hands) have a different internal layout.
 * It's not really gone into much, but this is the case for Red Martians (and presumably the other three humanoid Martian races) in John Carter of Mars- though they look superficially human, they reproduce by laying eggs, live for hundreds (potentially thousands) of years, can survive extended periods without food or water (handy for living on what's largely a desert planet) and, in The Film of the Book, have bright blue blood. Oddly, this somehow does not prevent John Carter and Red Martian princess Dejah Thoris from having two kids.

Live-Action TV
"Trip: You think this is my fault?!
 * Star Trek likes doing this. A typical Vulcan has much larger, "nonhydraulic" heart located where a human has his liver, copper-based green blood, nictating membranes in his eyes, a body temperature over 105 degrees Fahrenheit, and a standing pulse rate of over 200 (And Pon Farr! Who can forget the Pon Farr, from which a million slashfics were born!). Klingons have evolved truly complex redundant biologies, making them next to invulnerable, though prone to many minor malfunctions. They also have thick, pink blood, though we have seen otherwise, including in the theatrical version of Star Trek VI, which introduced the idea (video releases restored a scene showing that the red-blooded Klingon was really a human wearing a mask). Kes, a very human-looking alien in Star Trek: Voyager, is shown in one episode giving birth through a sack in her back. There are certainly others.
 * This is lampshaded in The Big Bang Theory, where Koothrappali wonders out loud why no alien women ever told Captain Kirk to get his thing out of her nose.
 * Humans do have the advantage of living ten times or so longer than Kes's race. Yay us.
 * And we have better hearing than Cardassians too, hurrah.
 * We see much better than the Vorta, but their hearing is much better than ours to compensate. Also, they have a very limited sense of taste. Neener neener.
 * And the Trill are (two or) three species with separate degrees of this. The Trill symbionts are alien slugs that live for millennia. The humanoid Trill are, conveniently, fitted with a womb-like pouch inside the body that can bond with the symbiont Trill. When bonded, a Trill composed of a symbiont and a humanoid host now have one mind and one personality, to the extent that they are legally a brand new species.
 * Betazoids, despite looking entirely human (apart from their black irises), have an extra lobe in their brain to account for their telepathy.
 * The Denobulans have a distinct mating season (when the males fight for mates) and hibernation period. They also can puff up their heads like blowfish to scare predators.
 * Phlox's bizarre biology was an ongoing joke in the series. He has striped toenails that grow so rapidly they have to be cut once a week, a 16-inch tongue that needs regular scraping and a smile that the Joker would envy. Plus he hibernates five to six days a year.
 * Rajiin is an Oran'takun with the ability to do a complete biometric scan of someone, then wipe their memory of the event, though it's possible some of these abilities have been surgically implanted. And let's not forget the cum creature from "Vox Sola".
 * The episode "Two Days & Two Nights" brought us the second encounter with shape shifting aliens. Also a Crowning Moment of Funny:

Malcolm: You were willing to follow two strange aliens into a basement.

Trip: Gorgeous aliens! Don't forget they were gorgeous.

Malcolm: They were male!

Trip: Not at first!!!"

"Captain Gideon: It's an amazing thing, technology."
 * In the Star Trek Voyager episode "The Disease", Harry Kim has sex with a woman who looks human (as mentioned in the title quote) but prolonged sexual contact makes his skin glow, as well as making the two become physically addicted to each other.
 * Pregnant Bajorans have sneezing fits instead of morning sickness. And a complex network of veins bonds the mother to the child, which was the reason for why Nerys couldn't give the embryo back to Keiko (It Makes Sense in Context).
 * The freaking Ocampa. Lifespan of nine years. Body temperature around 16 degrees Celsius. Can only have one child. When ready mate, they exude a yellow substance from their hands and must have an hour long foot massage with 50 hours of the substance appearing to enable mating. Give birth standing up, from a sack between their shoulders. While their psychic powers probably helped, one still wonders how a species with such bizarre, not to mention mathematically troublesome, method of reproduction could have come about in the first place.
 * A Starfleet officer on DS9 named Vilix'pran (whose species has not been shown on screen) "budded" many children. Who had wings that had to be kept from tangling. Oh right, Vilix'pran takes the pronoun "he".
 * When Quark and his family were accidentally sent to 1947 Roswell, we learned that Ferengi biochemistry is sufficiently different from our own that sodium pentathol injections do nothing besides making them very sore at the injection site. Ferengi also have four-lobed brains, which somehow makes them immune to telepathy.
 * This is done with humans in the episode 'The Communicator' when Archer and Reed are captured on a pre-warp planet who initially think they are spies; when medically examined, the aliens comment on their "redundant renal organ" and "red blood with haemoglobin based on iron, a toxic element" and that they are missing four vertebrae.
 * It also helped expose Riker in 'First Contact' (the episode). Alien doctors examining an unconscious Riker are surprised to find his heart where his digestive tract should be, among other things.
 * The Centauri on Babylon 5 look enough like humans that they were able to briefly fool Earthers into thinking that they were two branches of the same species. However, among their known physiological differences are a lack of major veins/arteries in their wrists, two hearts, and radically different sex organs: males have six two-meter-long prehensile genitalia located on either side of the back, and which curl up under the arms when not in use, while females have six corresponding orifices. Consequently, Centauri can rate how far they've gotten from "one" to "six". They are, however, also seen kissing human-style.
 * According to the first Psi-Corps Trilogy novel, most humans of any importance were highly skeptical of the claim, and performed genetic analysis as soon as they could get a hold of some Centauri tissue.
 * In one episode, Londo mentions that he once caught the emperor (back when he was an adolescent) trying to peek up women's skirts. A fan commented that from what we've been told about Centauri biology, he was looking in the wrong place. JMS responded "tell that to a foot fetishist."
 * Male Drazi have a 'pouch' situated in their left armpit. It is clarified to be their reproductive organ.
 * Narns are marsupials. 'Pouchling' is an insult when applied to an adult. And MALES have the pouch!
 * Minbari, being from a planet that's much colder on average than Earth, also have a much lower tolerance of high temperatures than humans.
 * Their bone density is higher than humans (and not just the part that protrudes from their skulls), giving them more strength. They're also able to stand more physical trauma, as evidenced when Delenn was nearly bled out, and Franklin commented that if a Human had lost that much blood, he'd be dead. Alcohol causes them to fly into homicidal rages. And this might not be Canon, but it's been stated that Minbari have a weak sense of taste, and as a result their food is all very spicy.
 * In the episode where John cooks Delenn some Minbari food (that looks like cubes of tofu); Delenn hastily seasons her meal when John isn't looking. Supposedly this is because he prepared it too blandly. Or he's just a lousy cook (which he admits), and she tries to cover the unappetizing taste with spices.
 * Pak'ma'ra are carrion eaters, with prehensile Cthulhu-esque tentacles on their faces used for drawing in food. Under the tentacles is a beak, and they require translators to speak to other races. Also, according to the doctor in Crusade, they are not sexually compatible with humans.

"Lorne: I thought Our Lady of the Perpetual Seabreeze was the real deal, till the Divine Miss J. walked right through that door and into my ass. ... which is where my heart is. Physiologically. I could show you an X-ray."
 * Time Lords from Doctor Who have two hearts with a redundant circulatory system, a low body temperature, a respiratory bypass system, an ability to regenerate from death twelve times, a lifespan of potentially hundreds of years per body, and a complete additional sensorium tuned to temporal events. The show never addressed how the Coal Hill School nurse failed to notice that Susan was not human.
 * This trope is directly mentioned in Genesis of the Daleks. A Kaled scientists scoffs at the Doctor and his companion's claim that they're not from the planet Skaro. Then their test results come back, leaving the scientist stunned that, despite external appearances, their biology matches nothing native to Skaro.
 * Meanwhile, in The Dominators, the Dominators scan the Doctor's companion believing him to be a native of the Alien Planet of the Week. A later scan of an actual native reveals a totally different anatomy, which backs up the Doctor's earlier lie that there are two intelligent species on the planet.
 * The very rare inversion did happen once: in The Hand of Fear, Sarah Jane shakes off a "lethal" poison, as it turns out not to be lethal to Homo sapiens.
 * In The Daemons, the Doctor claims that an aspirin might kill him, though it's not clear whether he actually means it, or is just contriving a possible example of why human medical advice does not apply to him.
 * And even assuming he was telling the truth about his reaction to aspirin, it's not clear if this is true for all Time Lords, or if it's just that The Doctor is severely allergic to it.
 * In an earlier episode, the Doctor mutters about his different biochemistry to avoid being injected with an antibiotic cocktail by Liz Shaw, who promptly overrules him. He doesn't suffer any apparent ill effects.
 * In The Movie, the Seventh Doctor is shot three times in total, in the shoulder and the leg; dialogue makes it clear that these were not fatal wounds. The issue is that the medical team don't understand the Doctor's alien biology, think he's fibrillating wildly out of control due to his (for a human) incredibly high pulse rate of 300 beats per minute, and place a surgical probe in his circulatory system to try to find out why. Unfamiliar with the circulatory system of an alien with two hearts, the probe gets lost, damages the Doctor's body, causes him to have a seizure, and the doctors' panicked attempts to remove the lost probe and revive the Doctor kill him. The Doctor's regenerative process is inhibited by the anaesthetic for several hours (the Doctor himself conjectures that this is why he has amnesia for several more hours following his regeneration), and the scene in the operating theatre makes it clear that the medical staff are also surprised by how much anaesthetic it took to get the Doctor unconscious.
 * In "The Unicorn and the Wasp", the Doctor recovers from cyanide poisoning by ingesting a variety of different ingredients and shocking himself - the biological Techno Babble was actually vaguely in the right area for curing cyanide poisoning. "Smith and Jones" also has him survive radiation poisoning from an X-ray by expelling it all through his foot.
 * The Fifth Doctor claims that he wears the celery because it's an excellent restorative.
 * The Eleventh Doctor asks for celery after attempted anaesthetic from a Silurian.
 * When the Tenth Doctor regenerates into the Eleventh, the regeneration causes some sort of biochemical imbalance that the Doctor is trying to counteract by eating various foods. Finally frozen fish sticks dipped in custard do the trick.
 * Humans who spend a lot of time exposed to time travel acquire immunities to certain time travel phenomena. There is an implication that the Doctor's companions are slightly 'changed' after traveling with him through time due to the exposure to the energies of the TARDIS.
 * According to "A Good Man Goes to War",.
 * As the only alien species on Alien Nation, the internal parts and biochemistry of the Newcomers are well-documented and used in the story for humor and drama. Some notables:
 * Survival in hazardous environments.
 * Great physical strength.
 * Nerve plexus under the arm; their most sensitive area, a key pressure point in a deadly Overseer martial art.
 * Regular humans have a fairly major nerve cluster in the same place. Although attacking one won't kill, it can do some damage.
 * Males receive half-formed infants from their mates for further development, like a cross between a marsupial mammal and a seahorse. (George Francisco was the first male TV character to give birth on-screen.)
 * They become intoxicated from spoiled milk.
 * They cannot digest cooked animal proteins, and subsist on raw meats.
 * Saline water dissolves their flesh like acid.
 * Two redundant hearts; they can take a huge bullet in one of them and not stop attacking.
 * A frightening, monstrous metamorphosis triggered by an overdose of a unique narcotic.
 * A gland in their neck when implanted into a human inhibits the human aging process (but its removal kills the newcomer). Information buried to prevent genocide in quest for eternal youth.
 * There are several near- and full-inversions in Farscape: humans are far less sensitive to heat than the outwardly-similar Sebaceans, allowing them to survive in warmer temperatures. On the other hand, all of the alien races have much better eyesight -- though this too once proves to be an advantage to Crichton, as it renders him less susceptible to a mind-altering cosmic lightshow. Sebaceans also have a good number of internal differences like the lack of kidneys  and a smoother pregnancy.
 * Hell, Farscape is just about the only series in which humans are not the Jack of All Stats.
 * Of course, each race has its own differences; in one episode dealing with excessive radiation in Moya, Chiana talks about how she's got a heightened resistance to it, while Sikozu talks about how limited exposure would make a good number of her vital organs shut down.
 * Delvians are plants.
 * On the short lived NBC series Something Is Out There, the alien Ta'ra has a number of physiological differences: caffeine was an inebriant, and hands were apparently primary erogenous zones. Additionally, she felt the need to sleep in the nude and at one point became sexually aroused watching Crime Story. The latter two may have been more cultural than physiological, however.
 * Orkans can drink with their finger (and use various Psychic Powers focused through their index fingers), have three hearts, physically age backwards, and hatch from eggs which come from the male's bellybutton. And according to the pilot episode, their "parts" are interchangeable. In the season 2 opener, Mork takes some medicine designed to shrink nasal membranes, then says that Orkans are all membrane and shrinks down to the scale of an Earth-like nano-world.
 * The Witch Species from Sabrina the Teenage Witch have some incredibly bizarre biological quirks, far beyond what their magical powers would imply. Possessing certain character flaws can inflict them Anvilicious pun-based mutations (for example, a witch who acts "spoiled" will start to dissolve into foul-smelling ooze), their magic needs to be extracted and "tuned up" regularly to function properly, and their wedding vows involve exchanging ring fingers.
 * Krevlornswath ("Lorne") of the Deathwok Clan of Pylea on Angel looks quite humanoid, other than the green skin/red eyes/horns thing. You figure out they're called the "Deathwok Clan" for a reason when Lorne gets beheaded and remains alive as long as his body isn't destroyed.
 * Not to mention Lorne's hulking, bearded mom.

"Rimmer: Lister, just because they're aliens, doesn't mean they don't have to go to the little boy's room. Of course, they probably do something weird and alien-esque, like it comes out the top of their head, or something.
 * One of the few alien aliens to appear of Blakes Seven did a very good job of imitating humans, but reverted back to their original form after being shot. Well, mangled original form, due to being shot and all. Because of this, they could have a definitely non-human thing lying on the floor, oozing goo and gurgling without needing to be able to fit an actor inside.
 * My Hero (TV) did a fair number of jokes revolving around the quirks of Ultronian anatomy, ranging from "unpleasant reactions to everyday chemicals" to "blue urine" to "unpredictable bouts of coma".
 * In an early episode of Red Dwarf (which, in fact, is entirely devoid of actual aliens):

Lister: Well, I wouldn't wanna sit behind one in a cinema!"


 * There is only one known alien race in Time Trax, and they are shown and mentioned in only one episode. The Procardians appear to be your usual Human Aliens without any extenal differences, but have a number of biological oddities about them. They can glow and levitate like the aliens in Coocoon. Their voices sound very odd to the point where their vocal cords are incapable of producing human speech (and vice versa), although one attempts to say "Thank you" at one point and it sounds a little Helen Keller. They age in stages, meaning they can look exactly the same for several years before metamorphosing into an older equivalent. Since the change is so sudden, it can be difficult to recognize the same Procardian after one such change. Another bizarre feature is their equivalent to our fight-or-flight response to threat. They raise their arms in a combat-like stance, which fires an energy bolt at the threat. The effect of the bolt (on human, at least) is a coma-like state, which can be "cured" with shock therapy. This reaction is instinctive, as they have no adrenalin. Oh yeah, they also don't wear clothes.
 * Despite all these differences, Lambert doesn't hesitate before pumping them full of TXP, a highly toxic substance that is deadly to humans in three doses. He could've killed them. Instead, it works on them the same as humans, allowing them to be transported into the future.
 * Most of the aliens in the Stargate Verse are Transplanted Humans, so they rarely fit this trope, but there are a few aliens that definitely do.
 * Jaffa were genetically engineered from humans as a slave race by the Goa'uld. A Jaffa's lifespan is about 150 years or so and they have a symbiotic relationship with young Goa'uld that lives in a pouch in their belly and provides them with a supercharged immune system and great physical strength. If deprived of a symbiote or a medication that simulates its effects, a Jaffa will die in hours.
 * The Wraith are both insectoid and vampiric. Physically they look like Rubber Forehead Aliens, but their humanlike digestive system is vestigial. Instead, they feed on the life energy of humans through mouths on their hands, and when well-fed they are extremely strong, fast and tough. Their society resembles an ant colony, with a ruling queen and many subservient drones under her.
 * In Roswell, Max gets drunk with just a "little zip" of an alcoholic beverage in about two seconds, and gets back to his sober self without a memory of what he did, or any apparent hangover once he kisses Liz (or might have just been a coincidence). Michael, on the other hand, didn't get drunk the same way, but he got a sort of allergic reaction which made him have electrical lines running through his body, and every sense in overdrive. He did have a hangover. And although their biology is rather human except for their cells, it is said they can sustain a one hour climax, even if they don't go into details about how that exactly happens...

Myth

 * Many creatures in old folklore have odd habits and biology. For instance, faeries were vulnerable to iron (particularly worked iron). In some stories of vampires, they could not resist counting things such as scattered poppy seeds on the floor.
 * "Von! Von poppy seed! ... Two! Two poppy seeds! ... Three! Three poppy seeds! Ah! Ah! Ah!" *thunder and lightning*... Sorry, it was just sitting there.
 * Becoming a vampire gives you OCD?
 * Yes, oddly enough. In superstition, a vampire was compelled to count everything in a pile of objects... even more oddly justifying The Count.
 * Malkavians anyone?

Radio

 * In the Big Finish Doctor Who Companion Chronicle "The Darkening Eye", Adric survives a stab wound to the chest because of his species' Healing Factor and the key little detail that his assailant was aiming for the location of a human heart. Guess which side of the chest cavity holds an Alzarian heart?

Tabletop Games

 * Warhammer 40000 doesn't shy away from this one, and thoroughly explored it for some species in the short story Xenology. The Orks are one of the most obvious examples, being sentient fungi, but even the humanoid Eldar and Tau are revealed to have dramatically different internal workings from humans. Tyranids, of course, are more Nightmarish Alien Biology -- every Tyranid is designed and grown by the Hive Mind for a specific purpose, though most of them involve lots of claws and teeth. Even Tyranid weapons are separate alien beings, ditto their ammunition. Many of their troops lack digestive systems since they're not supposed to survive the battle they fight.
 * There are even Tyranid forms consisting entirely of large digestive pits, grown for any surviving troops to throw themselves into to serve as protein for rebuilding. And early stage genestealers are just Humans with a psychic command to worship the hive mind like a God.
 * Orks aren't actually sentient fungi, they're animals with symbiotic fungi and algae in their bodies. They're still very bizarre, though. The fungal tissue performs the functions of many human organs. They reproduce through spores, which grow into womblike plants that produces an Orkoid creature of some type (depending on the environmental conditions anything from a Squiq to an Ork). Their robust biology allows them to survive nearly anything that isn't instantly fatal (they have less important organs that might get damaged and they heal quickly). Ork "doktors" often hack off limbs from dead (or living) orks and stick them onto other orcs, after which they work perfectly fine. Orks can even survive a few hours after being decapitated (and will be fine if the head is attached back to the body).
 * Orks are also born with innate of knowledge of a certain skillset most just born knowing how to fight but for instance Mek Boyz Ork engineers inherently know to make everything from guns to war mechs. Of course everything they build wouldnt really function well if not for the Orks Clap Your Hands If You Believe based science.
 * The Space Marines essentially have bizarre biology added as part of their upgrade process, including an additional heart, lung and stomach, and organs that allow to them spit acid, see in the dark and interface with their signature Powered Armor.
 * The Kroot are quite strange when it comes to their bodily functions. They lack any orifices 'down there,' and so excrete waste through their skin. To reproduce, males rub their bodies against the females, who absorb their equivalent of gametes through contact. One of the female's extra stomachs turns into a womb, and at the end of the gestation cycle they vomit up the baby.
 * Invoked in Werewolf: The Apocalypse, with the Ananasi were-spiders. The splatbook mentions that while they look outwardly human, their internal organs are nothing of the sort, instead being a blend of human and spider organs.

Video Games

 * The asari in Mass Effect can literally mate with anything that has DNA to produce viable offspring. The actual mechanism is parthenogenesis coupled with the ability to scan DNA via biotics, plagiarise useful segments, and modify the genome of their offspring. Physical mating is optional.
 * For asari, mating within their own species can result in severe, highly dangerous genetic defects down the line in a manner loosely akin to incest. That's about as bizarre as biology gets.
 * This is far more rare than their cultural taboos would suggest. And it's a good thing since "pureblood" asari (an asari born with two asari as her parents) is more likely to be or give birth to an Ardat-Yakshi (a asari with a rare genetical disease that makes her become a sex-addicted murderous sociopath over time and causes death to her partners by frying their neural system). A textbook example is.
 * There's also a passing mention of the fact that krogan have four testicles. Fortunately we don't see this first hand.
 * The krogan have an extra set of everything, because it helps them survive Tuchanka. They even have an entire backup nervous system.
 * It is worth noting that the Widow sniper rifle is designed specifically to kill krogan. You know what else it is designed to kill? Tanks.
 * Pursuing any Interspecies Romance in ME 2 brings this subject up -- many alien races are humanoid, and a few are even primate-like mammals, but beyond that, all bets are off to the extent that the ship's resident scientist, a specialist in genetics and xenobiology, can give the player character medical advice concerning the physical... difficulties... such a relationship entails. Gets really uncomfortable when you realize that two of the romance options are a reptile and a (flightless) armor-plated bird/dinosaur, albeit humanoid...
 * While quarians seem to be mammalian and more or less human in appearance (it's hard to determine through the enviro-suits they need to wear to ward off infection, and it's implied they don't have ears like a human -- and they have only three fingers to either hand), they share the unique trait with turians that they are made from "dextro-amino acids". And No Biochemical Barriers is most definitely not in effect in the Mass Effect universe: anaphylactic shock is a very real possibility. In fact, talking to the doctor while pursuing the female human/male turian relationship has him discretely recommend "avoid ingestion".
 * Oh, and the aforementioned reptile alien romance? Sustained skin contact can cause rashes in humans, and humans who make oral contact with them can likewise experience hallucinations.
 * Samus Aran, despite having been infused with DNA from two different alien races, still looks like a perfectly normal human being.
 * In World of Warcraft, draenei have blue blood (confirmed both in-game and in tie-in novels), which implies rather exotic biochemistry..
 * Garona and her child are a little too exotic to classify as natural. Warlocks and Guardians did it. The other half-breeds, however...
 * Sims aren't aliens, but they aren't humans. They age rather quickly, have short lifespans, apparently have clear urine, and vomit pink no matter what.
 * Inverted in Sam and Max Freelance Police, Dr. Norrington said that The Great Old Ones are identical to humans, and by extension animal life on the inside. "We save the weird stuff for the outside".

Web Comics

 * In Freefall, Sam Starfall looks like a normal if not-quite-human humanoid. Turns out his head is a special type of space helmet, and from inside he can control the expression pretty extensively. This is why he never needs a helmet even when Florence, a genetically-engineered humanoid wolf, requires one. His entire suit is implied to work similarly. His actual appearance involves lots of tentacles.
 * "Apparently my race triggers a nurturing reaction in humans. They would take one look at me and immediately begin to regurgitate their last meal for me. I mean, how do you politely say 'no' to something like that?"
 * Sam's bizarreness extends down to the biochemical level as well, with the explanation that a lack of massive extinction events on his homeworld resulted in lessened evolutionary pressure, and a relatively simple biochemical makeup that makes him edible to all Earth life forms. Even cows finds him appetizing.
 * Schlock Mercenary has many species that qualify for this trope, looking generally human while explicitly not actually human. Author Howard Tayler loves to describe the details in footnotes for the strip.
 * One of the more obvious examples among the regular cast is Lt. Ebbirnoth, who has one large eyeball where one would find a head on a human. This becomes a plot point during the first Credomar story, where a human sniper targets this eyeball thinking that doing so would actually damage the brain, which for Ebby's race, called Uniocs, is actually located in their pelvis.
 * In El Goonish Shive, the Uryuom are sexless; they produce "eggs" which can accept and combine DNA from multiple individuals, even those of different species ("the current record is twelve parents, only nine of whom were Uryuoms"), Uryuom DNA need not even be part of the mix. They have limited Shapeshifting powers and use limited telepathy to have a common language with anyone in three seconds or so and instead of pheromones. They also have advanced shapeshifting technology that lets them not only mimic another race's outer appearances but also become biologically compatible.
 * In Drow Tales the Fae (drow, light elves, drowussu etc) are more biologically different than their humanoid appearance would suggest. They key to this is mana; the energy they generate, absorb and manipulate. It is the source behind their comparative resistance to, and lack of, diseases (they are not completely immune though). It is also what keeps them young and in a healthy condition, provided there are enough of them to generate a surplus of mana. Young drow age about half the rate of humans physically, but only lag behind 2 years mentally (a drow who is 30 would look 15 physically but be about 28 mentally). If they live with enough other fae they will stop aging at about 60 (the equivalent of a 30-year old human). They still need to eat and drink, but less so than a human of comparative size would and their bodies does not store fat they way a human body does. Their nature means that they also can "starve" even when they have plenty of food and drink - if they are alone. If that happens they will start to suffer from mana deprivation and starts to age much like a human would. They also get high on chocolate and the like; they evolved from cats.
 * Happens a few times with Aylee in Sluggy Freelance, though it's usually overshadowed by her more overtly alien features. When she first arrives on Earth, she has a hard time remembering that humans need to sleep and breathe occasionally. During one of her transformations she also develops an entirely potato based diet.
 * Played with in Penny Arcade, in reference to Mass Effect.
 * A similar example is this Perry Bible Fellowship comic.
 * Similarly, in Order of the Stick, it's offhandedly mentioned that a dwarf died from alcohol poisoning when "both his livers" gave out.
 * Homestuck. The trolls look like humans with gray skin and horns, but references are made them having insect-like physiology, and young trolls look like grubs. Individuals can have blood of almost any color of the rainbow (in fact, blood color is the basis of their caste system). We're not given much detail about their reproduction, but the act involves filling pails with genetic material--and individual trolls are required to fill pails with two different partners: a pairing based on mutual love and a pairing based on mutual hate. Troll biology sure is weird!
 * Chainsawsuit demonstrates some... side effects.
 * Eiffel, the alien in Fall City Blues, looks human but eats rocks.
 * Selkie has some odd dietary requirements and needs to wash her gills out regularly. She's also apparently ectothermic, despite the author stating that adult females of her species have mammary glands. The fans have gotten up in arms trying to figure her out.
 * According to Eerie Cuties, Succubi produce flying heat- male-seeking bombs exploding into clouds of magical pheromone (keyed to the specific succubus) on contact. If there's more than one of these... things, they spread to cover more victims. Observe!
 * Pretty much ALL aliens in The Accidental Space Spy has this. In lesser extent the author's other webcomics.

Web Original

 * The superheroine Tennyo in the Whateley Universe. She's got the weird organs. Something like anti-matter is in some of her cells, and she's fine with that. Her DNA is utterly non-human and isn't even in a helix. Her regenerative powers are so high that her bodily fluids are bio-hazards.
 * And then there's Sara Waite, who no longer possesses internal organs. The fact that she's more Eldritch Abomination than human has a lot to do with it.

Western Animation

 * On Invader Zim, despite the fact that Zim can pass as human with only a minimal disguise, it's implied that Irkens have none of the same internal organs as human beings. They do, however, possess something called a squeedlyspooch around where humans have stomachs. The episode "Dark Harvest" revolves around this: Zim, in an effort to perfect his human disguise, crams his body full of stolen organs, including six hearts and spleens in three different colors. Irkens (or perhaps just Zim) also have allergies to some very common items... like water (possibly just polluted water), meat, and barbecue sauce.
 * However, Gaz also claims to possess a squeedlyspooch.
 * In a DVD commentary, series creator Jhonen Vasquez stated that the squeedlyspooch is a master organ and that it carries out all body functions.
 * Starfire of the Teen Titans (at least, the TV show). Aside from her decidedly orange skin and her ability to fly and to shoot energy from her hands and eyes, she has nine separate stomaches, an oft-unused long, prehensile tongue, a temporary but extensive Metamorphosis triggered by puberty, and a nasty allergy to metallic chromium.
 * In the comics, despite looking almost exactly like a (very lovely) human, her species evolved from cats. Flying, orange, alien cats. Not surprising.
 * Starfire's "transformation" is implied to be extra-special; she points out that her sister's metamorphosis consisted of turning purple for a couple of days. The tradeoff, she's told, is that the more cracked-out a Tamaranian's bodily transformations become during this process, the more likely they are to form a chrysalis (a very rare event, probably to justify the fact that she didn't know about it), and a Tamaranian who forms a chrysalis will emerge very powerful indeed. Of course, she probably would have appreciated the whole thing more had she not been told this by a creature that was waiting for her to reach this stage so it could turn her into lunch.
 * To sum up, if ever the writers were stuck for a gag, they'd just trot out one more hitherto-unmentioned aspect of Starfire's cracked-out biology.
 * Inverted in the 2003 Teenage Mutant Ninja Turtles series, Baxter Stockman and Oroku Saki see a thermal camera image of the Turtles, which looks roughly like a human being's heat signature - only humanoid-turtle shaped. They also sweat. This implies that they have a human-like metabolism. They seem to be Turtle Outside, Human Inside.
 * Of course, they also have a full set of teeth, and eat human foods. If you have a turtle, don't give it pizza or Chinese food unless it too is a humanoid mutant. It's not good for them.
 * On Ben 10, Osmosians look externally like humans, but have the ability to absorb matter and energy, as well as minor degrees of super strength and durability, which increases when they do. Though to them, pure energy is a G-Rated Drug that drives them insane if they absorb too much of it (though this is reversed if the energy is discharged). Also, Osmosians can live much longer than humans and those who live for hundreds of years grow horns.
 * This seems to only apply to Kevin (it's also a rule of thumb for half-breeds unless the alien side of them is ''really' inhuman like a four-arms), season two Big Bad Aggregor was a full-blooded osmosian and he looked like a generic christian devil (though it may have just been Rule of Symbolism, he also wielded a pitchfork).