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11.3: Invertebrate Classification

Difficulty Level: At Grade Created by: CK-12
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Invertebrate, arthropod or insect?

This Monarch caterpillar is an invertebrate. It is also an insect and an arthropod. Of all the animal species, it is estimated that well over 90% are invertebrates. Of all invertebrates, the insects are by far the most numerous. There are so many species of insects that scientists have yet to discover them all, let alone name or count them. Estimates of the total number of insect species fall in the range of 1 to 30 million. So, it helps if there are methods to classify not just the insects, but all invertebrates.

Classification of Invertebrates

Eight major phyla contain the majority of invertebrate species.

Major Invertebrate Phyla

Table below gives an overview of the eight invertebrate phyla with the greatest number of species.

Phylum (includes) Notable Characteristics Example
Porifera (sponges) multicellularity, specialized cells but no tissues, asymmetry, incomplete digestive system sponges
Cnidaria (jellyfish, corals) radial symmetry, true tissues, incomplete digestive system jellyfish
Platyhelminthes (flatworms, tapeworms, flukes) cephalization, bilateral symmetry, mesoderm, complete digestive system flatworm
Nematoda (roundworms) pseudocoelom, complete digestive system roundworm
Mollusca (snails, clams, squids) true coelom, organ systems, some with primitive brain snail
Annelida (earthworms, leeches, marine worms) segmented body, primitive brain earthworm
Arthropoda (insects, spiders, crustaceans, centipedes) segmented body, jointed appendages, exoskeleton, brain insect (dragonfly)
Echinodermata (sea stars, sea urchins, sand dollars, sea cucumbers) complete digestive system, coelom, spiny internal skeleton sea urchin

Protostomes and Deuterostomes

Most invertebrates (and higher animals) can be placed in one of two groups based on how they develop as embryos. The two groups are called protostomes and deuterostomes . As shown in Figure below , organisms in the two groups have different ways of forming the coelom and mouth, among other differences.

Mollusks, annelids, and arthropods are protostomes. Echinoderms and chordates are deuterostomes. This distinction is important. Why does it matter? It shows that echinoderms are more closely related to chordates than are the other invertebrate phyla. This is not apparent based on other, more obvious traits.

Protostomes vs. Deuterostomes. In protostomes such as mollusks, the coelom forms within the mesoderm. In deuterostomes such as echinoderms, the coelom forms from a pouch of endoderm. How does the formation of the mouth differ in these two groups of animals?


  • Eight invertebrate phyla contain most invertebrate species.
  • Invertebrates (and higher animals) can also be placed in one of two groups based on how they develop as embryos.


Use this resource to answer the questions that follow.

  1. What is the main difference between protostomes and deuterostomes?
  2. What is meant by "mouth first or second"?
  3. Compare the location of the mesoderm in protostomes and deuterostomes.
  4. Which develops first in annelids, the mouth or anus?
  5. Give three examples of annelids.


1. Describe evidence showing that echinoderms are more closely related to chordates than are other invertebrate phyla.

2. Assume you have discovered a new invertebrate. It has a segmented body, a brain, and jointed appendages. In which phylum would you place it? Why?




Invertebrate phylum of segmented worms, such as earthworms.


Invertebrate phylum of animals characterized by a segmented body, jointed appendages, exoskeleton, and brain.


Invertebrate phylum that includes animals such as jellyfish and corals; characterized by radial symmetry, tissues, and a stinger called a nematocyst.


Animals in which the coelom forms from a pouch of endoderm.


Invertebrate phylum of sea-dwelling animals characterized by complete a digestive system, coelom, spiny internal skeleton.


Phylum of invertebrates that are generally characterized by a hard outer shell, a mantle, and a feeding organ called a radula.


Phylum of invertebrates called roundworms, which have a pseudocoelom and complete digestive system.


Invertebrate phylum of flatworms; characterized by a flat body because they lack a coelom or pseudocoelom.


Invertebrate phylum of sponges; have a non-bony endoskeleton and are sessile as adults.


Animals in which the coelom forms within the mesoderm.

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Difficulty Level:

At Grade


Date Created:

Feb 24, 2012

Last Modified:

Sep 01, 2015
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