Study Guide

Animal Evolution and Diversity - Common Mistakes

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Common Mistakes

What is an Animal?

When we think of life, we generally remember plants and animals. Try a mnemonic to get them all:

All Boy Pilots Fly Paper Airplanes (Archaea, Bacteria, Protista, Fungi, Plantae, Animalia).

Animal Evolution

From Fred Flintstone to Dr. Alan Grant, we have loved to think we could or did live with dinosaurs. Dinosaurs lived millions of years before the first even human-like creatures, let alone us. Everything that has ever existed has not existed at the same time. There were no cave men fighting off dinosaurs.

The Animal Family Tree

The answer is in the roots. Scientists don't just use big, strange words to sound impressive (though we're pretty sure they enjoy that). The words often come from multiple root words and are shorter ways to express a long concept. Check out the roots. Parazoa versus eumetazoa? "Zoa" means animal. "Para" means beside and "meta" means after. Look for words you might know that use some of the same roots for clues, like paranormal and metamorphosis. "Eu" is a little less common, but it means "very much". So parazoa = beside the animals versus eumetazoa = well after animals.

Remember that the picture we are using here is a branching tree, not a straight line. Being related does not mean one thing came from the other. You, your siblings, and your cousins all have a common ancestor. You are related, but one of you did come from the other.

Let the Food Come to Them

How is this different from a plant again?

Sponges do look a bit like a plant. Sponges are anchored to something and don't have familiar animal features, for instance, a mouth. The similarity to plants ends there, though. Plants have cell walls and sponges, like other animals, do not. Plus, sponges eat other organisms. Plants don't. Sponges have other characteristics of animals, as well, being eukaryotic, diploid and multi-cellular.

Polyps and Medusas

Sponges, jellies, anemones, hydras, comb jellie— there are so many tube and sack-like animals. Want to look cool at the aquarium (this is where everyone goes to be cool, right)? There are some quick things to look for when you want to identify them.

  • Radial symmetry (sponges are asymmetrical)
  • Presence of tentacles and how many there are (sponges have none, jellies and all the others have ten, expect sea jellies, which have only two)
  • No eyes (squid and octopus have tentacles, but also eyes and mouths)
  • Bioluminescence (only the comb jellies glow)
  • Cilia (only the comb jellies have loads of flickering cilia around their bodies)

Get A-head

That's a lot of worms. Names can tell you a lot. Flatworms are flat. Proboscis worms have the trunk-like thing that shoots out the front. Rotifers have the circle of tiny little whips (cilia) around their mouth. Nematodes—okay, that name may not help much in a quick pinch, but nematodes have none of those things, unlike the earthworms we will soon meet. Earthworms have ridges. These are all smooth.

Invertebrates Get More Complex

What do you call that thing in the ocean with a shell? There are a lot of animals that live in the ocean and have something like a shell. Here are some tips.

  • Sponges are asymmetrical tubes.
  • The outside of coral looks like it could be just an asymmetrical tube, but inside is a radially symmetrical animal with tentacles that come out of the top of the tube.
  • Mollusks have a shell, but no tentacles or legs that bend. Octopus and squid are tricky, because you can't see the shell and they do have arms that are like the tentacles of jellyfish and hydras, but remember the speed and the head. The other things with tentacles don't have heads or eyes.
  • Octopus and squid both have eight arms, but a squid has two additional, longer tentacles. An octopus has a round head, while a squid has a more pointy head with two fins on it.
  • Crustaceans have a shell, but also jointed legs and very obvious heads and body segments.

What's in a Worm?

How are these animals related to mollusks and arthropods? Remember the connections are not always obvious on the outside. These groups are all protostomes and that involves developmental pathways. Unlike the other protostomes, these three groups are also coelomate. The other coelomates are deuterostomes and have an endoskeleton.

Life Takes to the Land

  • Insect vs. spider:

    Which bug is which? Bug just means any creepy-crawly thing. Insects and spiders both get called bug, but they are not the same thing. Here's the quick scoop.






Body parts




Yes, most of the time





Makes web



  • mollusk vs. crustacean

Both live in water and have a hard outside. Mollusks have hard shells and soft appendages, if any at all. Crustaceans, however, have legs, many jointed legs. Crustaceans also have obvious body segments, with antennae on their heads and obvious eyes.

  • horseshoe crab vs. crab

The name is a problem here. Just like jellyfish not being fish, horseshoe crabs are not crabs. Horseshoe crabs are related to spiders, a different arthropod family. Crabs have claws on their front legs and antennae. Horseshoe crabs have neither.

  • centipede vs millipede: Both look sort of like worms with legs, but the exoskeleton gives them away as arthropods. The problem is they look a bit alike. The key is to check out their legs. Centipedes have one pair per segment and millipedes have two. For what it's worth, millipedes are also the safer ones, eating plants, while centipedes have the venomous claws for killing prey.

Controversial Cousins

More animals on the ocean floor. Great. What is unique about echinoderms:

  • Soft skin on the outside with prickly appearance because of endoskeleton underneath
  • No tentacles, no shells, no antennae, no head
  • Tube feet that act like little suckers
  • Radial symmetry with five or more spokes

What about those other ones? Phoronids, bryozoans and brachiopods all have lophophores – tentacles around the mouth that have cilia on them.

Get a Backbone

  • Chordate vs. vertebrate: All vertebrates are chordates, but not all chordates are vertebrates. Remember that a backbone is not the same thing as a nerve cord or notochord (even though the word "chord" sounds like it).
  • Is it a fish or a shark? We could put rays in here too, but they don't get mistaken for the others much.

Jawless fish


Bony fish

Scary teeth

Yes, but in a soft round mouth

Yes, very scary in stiff jaws

Sometimes and also always in jaws



Paired and stiff

Paired with visible rays; can swivel

Body shape

Sort of like a tube

Sleek with pointed ends

Sleek with pointed ends, but may be more flattened

Nothing Like a Place at the Beach







Soft and must stay in water

Hard shell, amniotic (fluid inside)








Through moist skin and weak lungs

Lungs only

Now You're Getting Hot

  • Look at the birdie. No, it's a bat. Yes, they both fly. Then again, both fish and Michael Phelps swim. Birds and bats are very different.




Aves – in other words, birds


Covered by




Hatch from eggs

Come straight from their mom

Babies eat

Worms, seeds, etc. – whatever mom and dad bring

Milk nursed right from mom

Fly with

Wings with their own unique bone structure

Wings that still have the same bones as mammals' front limbs




Cute and Furry

Mammals laying eggs? What makes a mammal? Look at the outside of the animal and look at its mouth. Mammals have fur or hair. Echindas look like they have a beak, but if they were birds, they'd have feathers. Look for teeth. Other animals, like big, smiley crocodiles, have teeth, but no fur and their teeth are all pretty uniform in shape and size. 

Mammals have complicated teeth. Want a trick one? Aquatic mammals like whales and dolphins look a bit like fish. So, here's another indicator. Fish breathe water. Mammals breathe air. They aren't any big, air-breathing reptiles in the water anymore, unless you want to consider the Loch Ness Monster. And, if you catch anything nursing a baby, it is a mammal.


It's easy to mistake the difference between having a common ancestor and being descended from one another. Here's another way to picture the difference. Let's say we have some apples. Then we make applesauce from some and an apple pie with the rest. Did the applesauce come from apple pie or the pie from the sauce? Neither. They had a common ancestor, the apple.

Primate-schmimate, what's the difference? We do all have those great primate hands, but there are other things to check. One quick difference is to look for a tail. Apes and humans (go ahead and turn around and look if you need to) do not have tails. The prosimians usually are small with big eyes and most are active at night. Monkeys are very busy all day, with long limbs and tails.

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