Animal phyla

This page is part of a series introducing various animal phyla. For the first day of animal phyla, you should look at these pages:

On other lab days, we'll look at some other animal phyla:


  • Identify members of the animal phyla covered in lab. 
  • Describe the animal body plan features on this page and identify them in the animal specimens you see in lab. You won't see all of these features in one lab day.


The specimens are listed on the pages for the specific phyla.

What is an animal phylum?

An animal phylum (plural: phyla) is a high-level taxonomic group, describing a group of species sharing a phylogenetic relationship based on common ancestry. Animal phyla are also traditionally defined by the fact that all members of the phylum share a number of distinct morphological features, which are known as the body plan of that phylum. Thus, by studying the defining features of various animal phyla, you are also studying the fundamental features that define animal diversity.

Defining features of animal phyla

The fundamental features that define animal phyla are based on early events in animal development, such as gastrulation. These early events change rarely in evolution, and in many cases set the stage for later developmental events. Features that appear later in development, such as feathers or hair in vertebrates, are more likely to be defining features of clades within a phylum.

Embryonic tissue layers
As you learned in the animal development lab, gastrulation forms three embryonic tissue layers, which later differentiate to become all the cell types of the mature animal. This type of development is known as triploblastic. This is true for almost all animals, but you'll two exceptions in Bio 6A: the sponges (phylum Porifera), which don't have well-defined tissues at all, and the phylum Cnidaria (jellyfish, etc.), which form only two embryonic tissue layers (diploblastic development). Without three embryonic tissue layers to form a foundation for later development, the sponges and cnidarians are limited to very simple body forms.
Humans are bilaterally symmetrical – our left and right sides look more or less like mirror images of one another, while our fronts look quite different from our backs. Most of the animals you're familiar with are also bilaterally symmetrical, but some animals aren't. Jellyfish (phylum Cnidaria) are more or less round and don't have a left or right side; they can move in any direction equally well. Cnidarians are radially symmetrical.
Sponges have no particular symmetry; they are not radially or bilaterally symmetrical.
Cephalization means having a head. For humans and many other animals, our head is the part of the body that has the brain and most of the sensory organs. Cnidarians such as jellyfish don't have any cephalization; their nerves and sensory organs are distributed all around their bodies. Jellyfish don’t have brains; they also don't have a front or a back. Cephalization only occurs in bilaterally symmetrical animals.
Body cavity
The body cavity is the place where digestive and other internal organs form. Many of your organs are hanging more or less freely in two large cavities in your body: the abdominal cavity (containing the intestinal tract) and the thoracic cavity (containing the lungs). The body cavity is formed early in embryonic development, and has important effects on later development. With respect to the body cavity (or the lack thereof), animal come in three types:
Coelomate: the body contains a cavity (the coelom), fully lined with mesodermal tissue, in which major organs develop. This type of structure is found in the most complex phyla of animals, including arthropods and chordates.
Pseudocoelomate: The body contains a cavity (the pseudocoelom) that is not fully lined with mesodermal tissue, and in which major organs develop. This type of structure is found in some less-complex animals, such as nematodes.
Acoelomate: No body cavity; the body is more or less a solid mass. Cnidaria and flatworms are acoelomate. Note that the gastrovascular cavity (or other type of gut) is not a body cavity. Acoelomate animals have simple body structures.
Many animals' bodies are divided into segments. Segmentation is obvious in a segmented worm such as an earthworm (phylum Annelida) or in an insect (phylum Arthropoda); segmentation is also visible in repeated body structures such as the backbones of vertebrates (phylum Chordata). On the other hand, some animals, such as planarians (phylum Platyhelminthes) or jellyfish (phylum Cnidaria) are not segmented at all.
Digestive tract
Aside from sponges, all animal phyla have a digestive tract, or gut. Cnidaria and flatworms have a gastrovascular cavity, a digestive tract with a single opening. This could be called a 2-way gut, since any indigestible waste must come out the same way it went in. For example, some flatworms eat snails by sucking the entire snail into the flatworm's gastrovascular cavity through the pharynx, digesting it, then spitting out the shell through the pharynx. Most other animals have an alimentary canal (also called a complete digestive tract, or simply a 1-way gut), with a separate mouth and anus. This type of digestive tract allows for much greater complexity and specialization.
Alimentary canals are only found in animals with triploblastic development and a coelom or pseudocoelom.
Circulatory system:
Animal circulatory systems vary widely. Animals with a gastrovascular cavity don't have a separate circulatory system; the gastrovascular cavity allows nutrients to be carried throughout the body, but does not perform other circulatory system functions such as delivering oxygen. Animals with this type of body plan must be fairly small and have relatively low metabolic rates.
Larger, more complex animals may have an open or closed circulatory system, as you learned in lecture.
Animals such as crabs (phylum Arthropoda) have a hard external skeleton that both protects and supports the animal's body. An animal with an exoskeleton must typically molt, or shed its exoskeleton, in order to grow.  The technical term for molting is ecdysis. Nematodes (phylum Nematoda) have a more flexible exoskeleton called a cuticle, but they must also molt in order to grow.

Animals come in an immense range of different styles, and animal diversity can't be described in a few short paragraphs. However, the features listed above provide a way of categorizing animal phyla in terms of their body plans. These features create both opportunities and limits for the evolution of a group. For example, there aren’t any large terrestrial animals that don’t have some sort of hard skeleton.

Body plan table

This table summarizes some of the fundamental body plan features of the animal phyla covered in lab.

Table of body plan features

Study questions for animal phyla

You don't need to turn in answers to these questions. However, you may want to think
about them to help you prepare for the next lab exam. Some of the answers can be found on this page, but others may be on other pages of this site or in Campbell.

  1. Why does it matter how many embryonic tissue layers are present in a phylum? (In this lab, you've seen animal phyla with either two or three embryonic tissue layers.)
  2. How is body plan with a true coelom different from one with no body cavity or with a pseudocoelom?
  3. Compare & contrast the skeletons (or lack of skeletons) in the three phyla from these labs.
  4. Which phyla have motility, and how does it work?
  5. Which phyla have muscles?
  6. Which phyla have circulatory systems? Why would an animal have a circulatory system? How can any animal survive without one?
  7. Which phyla have specialized excretory organs? Why would an animal have them? How can any animal survive without one?

Terms & Concepts to Remember

This list includes terms from all the phyla listed at the top of the page.

  • Annelida
  • Appendages: jointed, branched
  • Arthropoda
  • Bilateral symmetry
  • Cephalization
  • Colonial vs. individual polyps
  • Choanocyte
  • Cnidaria
  • Cnidocyte
  • Coelom
  • Cuticle
  • Digestion: Intracellular vs. Extracellular
  • Ecdysis (molting)
  • Epidermis
  • Exoskeleton vs. Endoskeleton
  • Flagellated cells
  • Free living vs. parasitic
  • Gastrovascular cavity
  • Hydrostatic skeleton
  • Larva
  • Mantle cavity
  • Medusa
  • Mesoglea
  • Metamorphosis
  • Mollusca
  • Nematocyst
  • Nematoda
  • Nerve cords
  • Neurulation
  • Notochord
  • Pharynx
  • Phylum (plural: phyla)
  • Platyhelminthes
  • Polyp
  • Porifera
  • Pseudocoelom
  • Radial symmetry
  • Segmentation; specialization of segments
  • Spicules
  • Suspension feeding
  • Symmetry: radial vs. bilateral vs. no symmetry
  • Tagma/tagmata (arthropods)
  • Tissues

Sample multiple-choice questions

On the lab exam you'll see slides of the various phyla listed on this page. You might see multiple-choice (or fill-in) questions like these:

1. What phylum is this?

2. This species has:

  1. No body cavity.
  2. Pseudocoelom.
  3. True coelom.

3. What is the structure indicated by the pointer? (Answers could be any of the structures labeled on the animal phyla pages.)

4. How many embryonic tissue layers does this species form?

References & further reading

Campbell Biology, Chapter 33: An introduction to the invertebrates.

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