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Invertebrate Classification

Invertebrates represent 97% of animal species and are organized into 30 phyla.

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Invertebrate Classification

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.

Differences between protosomes and deutrostomes

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?

Science Friday: Isn't this Octopus Adorabilis?

What do you call an tiny octopus with big eyes, gelatinous skin and is cute as a button? Nobody knows quite yet! In this video by Science Friday, Stephanie Bush aims to classify and name this presently undescribed deep-sea cephalopod.


  • 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.


  1. What are protostomes?
  2. Describe evidence showing that echinoderms are more closely related to chordates than are other invertebrate phyla.
  3. 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?

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Annelida Invertebrate phylum of segmented worms, such as earthworms.
Arthropoda Invertebrate phylum of animals characterized by a segmented body, jointed appendages, exoskeleton, and brain.
Cnidaria Invertebrate phylum that includes animals such as jellyfish and corals; characterized by radial symmetry, tissues, and a stinger called a nematocyst.
deuterostomes Animals in which the coelom forms from a pouch of endoderm.
Echinodermata Invertebrate phylum of sea-dwelling animals characterized by complete a digestive system, coelom, spiny internal skeleton.
Mollusca Phylum of invertebrates that are generally characterized by a hard outer shell, a mantle, and a feeding organ called a radula.
Nematoda Phylum of invertebrates called roundworms, which have a pseudocoelom and complete digestive system.
Platyhelminthes Invertebrate phylum of flatworms; characterized by a flat body because they lack a coelom or pseudocoelom.
Porifera Invertebrate phylum of sponges; have a non-bony endoskeleton and are sessile as adults.
protostomes Animals in which the coelom forms within the mesoderm.

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