Many people find it difficult to grasp life in all its diversity: not only the birds, reptiles and mammals that everyone knows and love, but also viruses, bacteria, protists, invertebrates, and trees and fungi. On the following images, you'll go on a guided tour of the biggest organisms on Earth, ranging from a giant (by microscopic standards) virus, to a gigantic (by anyone's standards) clonal colony of trees — with all your favorite whales, elephants, and anacondas in between.

01 of 25 Biggest Virus - The Pithovirus (1.5 Micrometers Long) Pithovirus, the world's biggest virus. Wikimedia Commons We can quibble about whether or not viruses are really living organisms — some biologists say yes, some aren't so sure – but there's no question that the Pithovirus is a true giant, 50 percent bigger than the previous record-holder, Pandoravirus, and (at 1.5 millionths of a meter) slightly larger than the smallest identified eukaryotic cell. You might think a pathogen as big as the Pithovirus would make a habit of infecting elephants, hippopotamuses or even human beings, but don't worry: it actually preys on amoebas only a little bit bigger than itself.

02 of 25 Biggest Bacterium - Thiomargarita (0.5 Millimeters Wide) Thiomargarita, the world's biggest bacterium. Wikimedia Commons It sounds like a mixed drink, but thiomargarita is actually Greek for "sulfur pearl," a reference to the granules of sulfur incorporated in this bacterium's cytoplasm (which give it a lustrous appearance) and the fact that the roundish thiomargarita tends to link up in long, pearl-like chains as it divides. Completely harmless to humans and other animals — it's a "lithotroph," meaning it subsists on inert chemicals on the ocean floor — the half-millimeter wide thiomargarita may be the world's only bacterium that's visible to the naked eye.

03 of 25 Biggest Amoeba - The Giant Amoeba (3 Millimeters Long) The Giant Amoeba, the world's biggest amoeba. Wikimedia Commons You can't beat the genus name attached to the giant amoeba: "Chaos," which presumably refers to this single-celled organism's constant undulations, as well as the fact that it harbors literally hundreds of separate nuclei in its cytoplasm. While falling well short of the monstrous amoebas that populate comic books and science-fiction movies, at up to 3 millimeters long, the giant amoeba is not only visible to the naked eye, but capable of (slowly) engulfing and digesting smaller multicellular organisms in addition to its usual diet of bacteria and protists.

04 of 25 Biggest Insect - The Goliath Beetle (3-4 Ounces) The Goliath Beetle, the world's biggest insect. Getty Images The appropriately named Goliath beetle, genus name Goliathus, is never seen in the wild outside the tropical forests of Africa — which is a good thing, since this insect weighs as much as a full-grown gerbil. However, there's a big asterisk attached to the Goliath beetle's "world biggest bug" title: this insect is twice as large as a larva than it is as a full-grown adult. If you're feeling adventurous, you can raise your very own Goliath beetle; experts advise (seriously) a diet of packaged dog or cat food, either wet or dry will do just fine.

05 of 25 Biggest Spider - The Goliath Birdeater (5 Ounces) The Goliath Birdeater, the world's biggest spider. Getty Images Only distantly related to the Goliath beetle, the Goliath birdeater of South America is the world's heaviest arachnid, weighing about a third of a pound fully grown. Amazingly, it takes female Goliaths at least three years to mature, and they have a life span in the wild of up to 25 years, about the same as your average house cat. (Males are less fortunate; even though they're not eaten by females after the act of mating, as in other spider species, they have an attenuated life span of only three to six years.)

06 of 25 Biggest Worm - The African Giant Earthworm (2-3 Pounds) The Giant Earthworm, the world's biggest worm. Getty Images If you hate worms, you may be dismayed to learn that there are not one, but over half a dozen, species of giant earthworm — the largest of which is the African giant earthworm, Microchaetus rappi, which measures up to 6 feet long from head to tail and weighs as much as an average-sized snake. As big as they are, though, giant earthworms are every bit as harmless as their more petite relatives; they like to burrow deep in the mud, keep their distance from humans (and other animals), and quietly eat rotten leaves and other decaying organic matter.

07 of 25 Biggest Amphibian - The Goliath Frog (5 Pounds) The Goliath Frog, the world's biggest amphibian. Wikimedia Commons "Goliath" is a popular name for plus-sized animals; not only do we have the Goliath beetle and the Goliath birdeater, but there's also the Goliath frog of west-central Africa. As big as it is, the Goliath frog is a strict vegetarian, feeding exclusively on an obscure aquatic plant, Dicraeia warmingii, that only grows on the banks of rapids and waterfalls. Impressively, at an average of five pounds, the Goliath frog isn't all that much smaller than the largest frog that ever lived, the 10-pound "devil frog" beelzebufo of late Cretaceous Madagascar.

08 of 25 Biggest Arthropod - The Japanese Spider Crab (25 Pounds) The Japanese Spider Crab, the world's biggest arthropod. Wikimedia Commons Looking a bit like a face-hugger from the "Alien" movies, the Japanese spider crab is a truly enormous, and enormously long-legged, arthropod. The legs of this invertebrate can attain lengths of over 6 feet, dwarfing its foot-long trunk, and its speckled, orange-and-white exoskeleton helps to camouflage it from the larger marine predators that would like to turn it into a nice undersea salad. Like many bizarre creatures, the Japanese spider crab is a prized delicacy in Japan, but has lately migrated from the menus of sushi restaurants as a response to pressure by conservationists.

09 of 25 Biggest Flowering Plant - Rafflesia (25 Pounds) Rafflesia, the world's biggest flowering plant. Getty Images Not something you want to plant in your backyard garden, rafflesia is known as the "corpse flower" — its huge, three-foot-wide blooms smell like rotting flesh, attracting the insects that help to spread its pollen. And that's not even the creepiest thing about rafflesia: this flower lacks stems, leaves and even roots, and instead grows by parasitizing the vines of another genus of plant, tetrastigma. Fortunately for the rest of us, rafflesia is restricted to Indonesia, Malaysia, Thailand, and the Philippines; you definitely won't encounter it in the wilds of New Jersey.

10 of 25 Biggest Sponge - The Giant Barrel Sponge (6 Feet High) The Giant Barrel Sponge, the world's biggest sponge. Wikimedia Commons Not only is the giant barrel sponge the largest sponge alive today; it's also one of the longest-lived invertebrate animals on earth, some individuals persisting for up to 1,000 years. Like other sponges, Xestospongia muta is a filter feeder, pumping sea water in through its sides, extracting the tasty microorganisms, and expelling waste out of its capacious top. The red coloration of this giant sponge derives from symbiotic cyanobacteria; like the corals with which it shares its reef habitat, it can be periodically "bleached" by ecological disruptions.

11 of 25 Biggest Jellyfish - The Lion's Mane (100 Feet Long) The Lion's Mane, the world's biggest jellyfish. Getty Images With its six-foot-diameter bell (in the largest individuals) and tentacles that can exceed 100 feet, the lion's mane jellyfish is to other jellyfish as the blue whale is to other cetaceans. Considering its size, though, the lion's mane jellyfish isn't all that poisonous (a healthy human can easily survive a sting), and it also serves an important ecological function, as various fish and crustaceans cluster under its huge bell. Fittingly enough, the lion's mane jellyfish is a favorite food source of another plus-sized animal on this list, the leatherback turtle.

12 of 25 Biggest Flying Bird - The Kori Bustard (40 Pounds) The Kori Bustard, the world's biggest flying bird. Getty Images At up to 40 pounds for the biggest males, the kori bustard pushes right against the limits of aerodynamics — this isn't the most graceful bird in the world when it takes off, and it can't flap its wings for more than a few minutes at a time. In fact, while it will briefly take flight when threatened, the kori bustard spends most of its time on the ground of its southern African habitat, squawking loudly and eating pretty much anything that moves. In this respect, the Kori is not dissimilar to the even heavier pterosaurs (flying reptiles) of the Mesozoic Era, such as the truly enormous Quetzalcoatlus.

13 of 25 Biggest Protist - The Giant Kelp (100 Feet Long) The Giant Kelp, the world's biggest protist. Getty Images Many people mistakenly believe that there are only four categories of life — bacteria, plants, fungi and animals — but let's not forget the protists, primitive eukaryotic organisms that tend to join in extended structures. Somewhat surprisingly, all seaweeds are protists, and the biggest seaweed of them all is the giant kelp, which can grow up to 2 feet per day and attain lengths of more than 100 feet. As you can imagine, kelp forests, which incorporate numerous giant kelp "individuals," are gigantic, tangled affairs that provide safe havens for numerous unrelated marine organisms.

14 of 25 Biggest Flightless Bird - The Ostrich (300 Pounds) The Ostrich, the world's biggest flightless bird. Getty Images At over 300 pounds for the largest subspecies, you might be forgiven for thinking that the ostrich (Struthio camelus) is about as big as a flightless bird can get. So you might be surprised to learn about the recently extinct Elephant Bird of Madagascar, which could attain weights of half a ton, or the comparably sized Thunder Bird, which vanished off the face of the earth a couple million years ago. Compared to these enormous ratites, the ostrich is a mere chick — albeit one with a much gentler disposition, subsisting on plants rather than small animals.

15 of 25 Biggest Snake - The Green Anaconda (500 Pounds) The Green Anaconda, the world's biggest snake. Getty Images Compared to the other organisms on this list, snakes are notoriously difficult to classify according to size: even trained naturalists have a tendency to overestimate the size of the snakes they observe in the wild, and it's nearly impossible to transport a dead (much less living) giant python to civilization to perform detailed measurements. That said, most authorities agree that the green anaconda of South America is the current title-holder; this snake can attain lengths of more than 15 feet, and well-attested individuals have been known to hit the 500-pound mark.

16 of 25 Biggest Bivalve - The Giant Clam (500 Pounds) The Giant Clam, the world's biggest bivalve. Getty Images A mainstay of "Spongebob Squarepants," "The Little Mermaid," and just about every animated movie set in the deep blue sea, the giant clam is a truly impressive mollusk. The twin shells of this bivalve can measure over 4 feet in diameter, and as you can imagine, these calcareous components make up most of the giant clam's weight (the soft tissues of a quarter-ton specimen only account for about 40 pounds). Despite its fearsome reputation, the giant clam will only close its shell when threatened, and simply isn't big enough to swallow a full-grown human whole.

17 of 25 Biggest Turtle - The Leatherback (1,000 Pounds) The Leatherback, the world's biggest turtle. Getty Images As testudines (turtles and tortoises) go, the leatherback is a true outlier. This sea turtle lacks a hard shell — rather, its carapace is tough and leathery — and is also incredibly fast, capable of swimming at close to 20 miles per hour. But of course, what really sets the leatherback apart from others of its kind is its half-ton weight, which puts it slightly above the Galapagos tortoise in the world's size rankings. (Even still, neither of these testudines approach the heft of prehistoric turtles like Archelon and Stupendemys, which tipped the scales at up to 2 tons apiece).

18 of 25 Biggest Reptile - The Saltwater Crocodile (2,000 Pounds) The Saltwater Crocodile, the world's biggest reptile. Getty Images Remember how things were 65 million years ago, when the biggest reptiles on earth weighed 100 tons? Well, the stock of these vertebrate animals has fallen ever since: today, the largest living reptile is the saltwater crocodile of the Pacific basin, the males of which can attain lengths of close to 20 feet, but weights of only a little more than a ton. The saltwater crocodile isn't even the biggest croc that ever lived; that honor belongs to two truly enormous crocodiles that terrorized the world's rivers tens of millions of years ago, Sarcosuchus and Deinosuchus.

19 of 25 Biggest Fish - The Ocean Sunfish (2 Tons) The Ocean Sunfish, the world's biggest fish. Getty Images Looking a bit like a giant head attached to the comb of a turkey, the ocean sunfish (Mola mola) is one of the ocean's most bizarre denizens. This six-foot-long, two-ton fish feeds exclusively on jellyfish (which have extremely poor nutritional value, so we're talking lots and lots of jellyfish), and the females lay hundreds of millions of eggs at a time, more than any other vertebrate animal. If you've never heard of Mola mola, there's a good reason: this fish is extremely difficult to sustain in aquariums, thriving only in the temperate regions of the Atlantic and Pacific Oceans.

20 of 25 Biggest Terrestrial Mammal - The African Bush Elephant (5 Tons) The African Elephant, the world's biggest terrestrial animal. Wikimedia Commons How much sustenance does a five-ton pachyderm need? Well, the typical African bush elephant eats about 500 pounds of vegetation every day, and drinks about 50 gallons of water. This elephant also (let's not be overly delicate) poops a lot in the course of the day, dispersing the seeds of many plants that otherwise would not get to visit different parts of Africa. Like other elephants, the African bush elephant isn't quite endangered, but it isn't quite thriving, either, as males succumb to human poachers who then sell their ivory tusks on the black market.

21 of 25 Biggest Shark - The Whale Shark (10 Tons) The Whale Shark, the world's biggest shark. Getty Images In the world's oceans, paradoxically, large sizes tend to go hand-in-hand with microscopic diets. Like the order-of-magnitude bigger blue whale, the whale shark subsists almost exclusively on plankton, with occasional side portions of small squids and fish. Ten tons is a conservative estimate for this shark; one deceased specimen floating off the coast of Pakistan was estimated to weigh 15 tons, and another dredged up near Taiwan was said to weigh 40 tons. Given how fishermen tend to exaggerate the size of their catches, we'll stick with the more conservative estimate!

22 of 25 Biggest Marine Animal - The Blue Whale (200 Tons) The Blue Whale, the world's biggest marine animal. Getty Images Not only is the blue whale the largest living animal; it may be the largest animal in the history of life on Earth, pending the unlikely discovery of any 200-ton dinosaurs or marine reptiles. Like the whale shark, the blue whale feeds on microscopic plankton, filtering countless gallons of seawater through the tightly meshed baleen plates in its jaws. Granted that it's difficult to persuade this enormous cetacean to step on a scale, naturalists estimate that a full-grown blue whale consumes anywhere from three to four tons of krill every day.

23 of 25 Biggest Fungus - The Honey Fungus (600 Tons) The Honey Fungus, the world's biggest fungus. Getty Images The last three items on our list aren't animals, but plants and fungi, which raises a difficult technical issue: how can you differentiate the "average" biggest plant and fungus from massive agglomerations, which can be said to constitute a single organism? We'll split the difference and nominate the honey fungus, Armillaria ostoyae, for this list; one Oregon colony encompasses an area of over 2,000 acres and weighs an estimated 600 tons. Botanists estimate that this huge honey fungal mass is at least 2,400 years old!

24 of 25 Biggest Individual Tree - The Giant Sequoia (1,000 Tons) The Giant Sequoia, the world's biggest tree. Getty Images There aren't many trees you can literally drive a car through (assuming you could bore a hole through the trunk without killing it). The giant sequoia is one of those trees: its trunk measures over 25 feet in diameter, its canopy towers over 300 feet into the sky, and the largest individuals have an estimated weight of up to a thousand tons. Giant sequoias are also some of the oldest organisms on Earth; the ring count of one tree in the Pacific Northwest has yielded an estimated age of 3,500 years, around the same time the Bablylonians were inventing civilization.