In case you were wondering: An animal is poisonous when its toxins are passively deployed. In contrast, a venomous animal directly injects you with a toxin.
Determining just how venomous a critter is isn't an exact science, but there is a type of standard: the LD50, or how much venom is needed to kill 50 percent of a test population of lab mice.
We've weighed a number of factors for this countdown, including that LD50; how much venom an animal inflicts at a time; and how much good old-fashioned damage is done to a victim.
The results? Lethal. Of course.
Yep. A shrew.
Using its grooved incisors, the shrew delivers a killing bite, with venom potent enough to kill 200 mice.
While that's not enough to kill a human, the pain, swelling and muscle problems the toxins would cause are not worth annoying a shrew to find out.
The Gila monster is one of the most venomous lizards to humans. A bite isn't fatal to us, but it can cause severe pain, dropped blood pressure and hemorrhaging.
Recovery can take up to five days.
The male version of this waddling Frankenstein of animals has serious venom in the pointy spurs of his hind legs.
For humans, the platypus's weird cocktail of toxins creates excruciating pain that lasts for weeks, and is somehow immune to the effects of morphine. That's some malevolent mojo right there.
It may not have fangs, but this lizard literally chews on its victims until its venomous saliva travels through its grooved teeth and into a wound. Ouch.
In humans, a bite causes pain, swelling, sweating, falling blood pressure and, very rarely, death by respiratory failure.
Yes. Like, the one you fry.
The majority of catfish are venomous and inflict incredibly painful wounds via strong, hollow ray fins.
The stinging proteins are potent enough to hospitalize an unlucky person, or, in the case of the Plotosus lineatus (striped eel catfish) even cause death.
Also known as the mulga, this snake delivers a large amount of venom -- 150 milligrams in one bite -- but fortunately, its venom isn't quite so potent as other ones.
Death is rare but possible if left untreated.
The hemotoxic venom from this critter causes nausea, vomiting, necrosis, muscle and joint pain, the bursting of red blood cells, organ damage, and possibly death in young children with weak immune systems.
Of all stingrays, the blue-spotted is the most venomous.
Death is very rare. But unfortunately, as seen in the case of TV personality and wildlife expert Steve Irwin, a sting to the abdomen, heart or other vital organ could be fatal.
The eastern diamondback is both the largest rattlesnake and most venomous snake in North America with a super-high venom yield: from 400-1,000 mg. It only takes 100-150 mg for a human lethal dose.
The lionfish announces its toxicity with its bold warning coloration. The venom, delivered through the fin rays, can cause a host of problems ranging from pain, vomiting and fever to convulsions, paralysis and even death.
The female black widow has large venom glands that deliver super-concentrated venom that interferes with the nerve signals that control muscles. The result is potentially fatal and causes severe pain and elevated blood pressure.
As ugly as the lionfish is attractive, the stonefish is the more toxic of the two.
The 13 spines on the fish's back can deliver a powerful neurotoxin that can cause excruciating pain and possibly death within a mere six hours if left untreated.
The aftermath of a bite from this Old World viper is is not pretty.
Some of the symptoms are bleeding from the gums and in urine, and pain for up to a month. The bite can also be lethal.
This critter's venom is so potent that it rivals the power of some snake venoms. The neurotoxins can cause intense pain, paralysis, asphyxiation and ultimately, death. Children are in particular susceptible.
The venom of mature cobras contains both nerve and heart toxins.
The mortality rate for untreated victims is estimated to be 20 to 30 percent.
The krait is particularly insidious because its bite may not cause any pain or even be noticed if a person is asleep. Its venom is full of powerful neurotoxins, though. A victim can literally suffocate to death four to eight hours after being bitten.
The Tunisian variety of this scorpion is responsible for approximately 90 percent of deaths from scorpion stings in North Africa, causing up to 400 deaths a year.
The neurotoxins in the venom can kill a person within six hours.
What makes this snake particularly dangerous is that it can deliver a lot of venom at a time.
The combination of neurotoxins and cardiotoxins can kill in as soon as seven hours.
The king cobra may not have the most potent venom on this list, but it still ranks, because the venom's effects are incredibly deadly.
First of all, it can spit its toxin, so it doesn't even need to bite you. Also, only 7 milliliters of the venom can kill 20 humans or one elephant.
This scorpion is tiny but mighty deadly. Its sting can impart a lethal venom with a median lethal dose of 0.25 mg/kg.
Thanks to its venom power, the saw-scaled viper is blamed for more human deaths than any other snake species combined in its region.
A tap from this arachnid, whose image is captured here by a National Geographic team, has a fatality rate of up to 40 percent.
The potent venom of this spider is reputedly twice as deadly as cyanide. The effects on a human include increased blood pressure, arrhythmia, coma and death. The venom acts swiftly and can kill a small child within 15 minutes.
The boomslang can open its jaws as wide as 170 degrees when biting. Its venom is highly potent and contains a hemotoxin that disrupts a human's blood coagulation. That said, the venom is slow-acting, which helps buy time to obtain and anti-venom.
The Dubois has the deadliest venom of all sea snakes and can kill a mouse with one bite. Sea snake venoms are heinous. They can cause paralysis, blurry vision, difficulty swallowing or speaking, and, in about 3 percent of victims, death.
This highly toxic snake's venom affects both the nervous system and blood coagulation. If left untreated, mortality is 100 percent. In cases of severe envenomation, death comes swiftly -- within a half hour.
The aquatic snails may move slowly, but that's precisely why their venom is extra potent. They have hollow modified teeth called "radulae" that are sharp enough to penetrate a wetsuit, and the estimated human lethal dose is incredibly small -- it's believed that the venom in one of these mollusks is enough to kill 20 adult humans.
This tiny jellyfish, which may only grow as big as a person's fingertip, packs a huge punch. Unfortunate victims contract Irukandji syndrome, excruciating pain that even morphine has little effect on.
Even worse, there is no anti-venom for the sting. The toxins can cause fatal brain hemorrhages.
Although it's relatively small -- only the size of a golf ball -- its venom is debilitating and deadly. It can cause respiratory failure within 10 minutes and death within 30. One bite can kill up to 26 men, and there is no antidote.
The inland taipan has one of the most lethal venoms; one drop can kill up to 100 adult men, and it works in as little as 45 minutes.
Although the Australian box jellyfish just misses having the most potent venom on this list, it is probably the most deadly. It is large in size yet almost transparent in the water, and its tentacles can sting you with its millions of nematocysts, injecting a hefty amount of venom while holding its victim in place.
The venom's toxins can cause extreme pain, paralysis, delirium, shock, cardiac arrest and even death within minutes. The jellyfish has enough venom to kill 60 adults.