Must Know High School Biology - Kellie Ploeger Cox 2019

PART FIVE Forms of Life
Introduction to Animals


Image Each branch along the animal phylogenic tree introduces new traits.

Image Invertebrates (animals without spinal columns) vastly outnumber all vertebrates (animals with spinal cords).

Animals are defined as heterotrophic (they need to eat to obtain energy), multicellular organisms that, for the most part, are able to move about freely using their nervous and muscular systems. The first animals evolved about 700 million years ago from an ancestral protist. Like animals, protists can be heterotrophic multicellular organisms made from non-photosynthetic eukaryotic cells, so it’s a logical common ancestor. There are numerous ways to classify animals, but below is one hypothesis of animal phylogeny. One of our must know concepts is to notice that each branch in the tree introduces a new trait to that animal group. One thing to marvel about—the only phylum that includes critters with backbones is Phylum Chordata. All other animals are invertebrates, or organisms without a spinal cord. It’s pretty amazing how outnumbered we are.


Notice that the outlier is the sponge, a critter that most people wouldn’t automatically think of as an animal (but it is!). Sponges are the weirdos in the group because they don’t have true tissues, which is more similar to the ancestral protist than to the rest of the animal phyla. The remainder are in the clade Eumetazoa, defined as animals with true tissues (groups of similar cells that work together as a unit).



If you have ever loofahed yourself in the shower with an expensive bath sponge (not the plastic variety) … it may literally be the skeletal remains of a sponge animal. Just FYI.

The next division is based on an animal’s overall body plan. If it’s radially symmetrical, that means it’s shaped like a donut or a cup—there’s a difference between the top and the bottom, but all the way around it’s the same. Bilateral symmetry, on the other hand, means that there’s only one way you could cut the shape in half for it to produce mirror images (for us, it would be a line drawn between the eyes, right down the body.) The vast majority of critters have bilateral symmetry, and for good reason. This type of symmetry correlates with sensory input being focused in one area: eyes, ears, nose, taste, and the brain to make sense of it all.


This is a good review of our previous evolution unit, and the importance of common ancestry. We learned that species are often defined as having a common ancestor if their embryos look similar. The different species categorized as lophotrochozoans are being grouped in a similar way, based on their developmental structures (trochophore larva), suggesting a common ancestor and a close relationship!

The Phylum Cnidaria includes jellies (can’t call them jellyfish because they aren’t fish) and are from an earlier lineage that had not adopted this more “advanced” bilateral body style. Cnidarians do not have a brain, but instead possess a “nerve net,” which can detect and respond to stimuli from all directions at once (the benefit of being radially symmetrical!). Everyone else is of the bilateral clade, which can be divided up further based on differences in development. The lophotrochozoans have a specific type of larval stage called a trochophore, but don’t stress about what, exactly, a trochophore looks like. It’s more important to simply understand that this branch of the phylogenetic tree is based on what the animals look like when they’re developing.

The lophotrochozoa clade includes flatworms, mollusks, and annelids. Flatworms are … well … flat. This is actually significant because their bodies have a very high surface-area-to-volume ratio and can rely on diffusion to move oxygen and nutrients throughout their tissues (no need for a circulatory system!). Flatworms you may be familiar with are the cute planaria and the arguably not-so-cute parasitic tapeworms. Another phylum is the mollusks, an amazing variety of critters, including clams, snails, slugs, cuttlefish, and the smartest of the invertebrates—the octopi. The final crew in the lophotrochozoa clade is the segmented worms (annelids), including our beloved and underappreciated earthworm.



There is an animal called a sea slug, an ocean-dwelling mollusk that feeds on algae and comes in an impressive variety of colors, shapes, and sizes. One species in particular—Elysia chlorotica—is particularly fascinating because it breaks our very definition of an animal … this special slug can photosynthesize! E. chlorotica feeds on algae, steals their chloroplasts, and embeds them in their skin, turning the slug a beautiful green. The slug is even leaf-shaped! Animals are defined as heterotrophs, eating to obtain nutrients, but this guy can go for months without food and photosynthesizes instead. Sadly, these amazing little invertebrates are becoming harder and harder to find.

The next group of bilateral critters reside within the ecdysozoa clade, meaning their life cycles all include a step of ecdysis (molting) of their external covering. This includes nematode worms, tiny guys who have adapted for life in almost every ecosystem (including living a parasitic existence in vertebrate bodies). There is only one other phylum in this clade, but it is the most numerous phylum of animals by far: the arthropods. Arthropods all have exoskeletons, a segmented body, and jointed legs—this includes crustaceans (crabs and lobster) and all the insects. There are approximately a billion billion arthropods living on Earth!

The deuterostome clade includes organisms that have a common pattern of development during early embryo formation. There are only two phyla included here, but one of them is the only vertebrate group in the entire animal phylogenic tree: chordates! It is also interesting to note that our closest invertebrate relative are the residents of Phylum Echinodermata, including the sea stars, sea urchins, sea slugs, and sea cucumbers. I would like to think that we are more like the intelligent and clever octopus, but nope, our close cousin is the glorious sea cucumber. I guess you don’t get to pick your relatives.

As we move forward in our investigation of animals, we will focus on the most well studied of the vertebrates: humans.


1. Choose the correct term from the following pair and then fill in the blank: Animals are defined as autotrophic/heterotrophic organisms, meaning that they can only obtain their nutrients by ______________________.

2. Based on the phylogenic tree from the beginning of the chapter, which animals are members of the clade that undergoes ecdysis (molting) during their life cycles?

3. Why don’t flatworms need a circulatory system?

4. Based on the phylogenetic tree on page 304, the phylum that includes ____________________ is the closest relative of our phylum, the ____________.

5. True or False: Animals are classified as ONLY being multicellular (and NOT unicellular).

6. Which two special tissue types are unique to the animal phylum?

7. List two characteristics that define members of Phylum Cnidaria (jellies).

8. What is the most numerous phylum of animals, and why?