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9.1: Invertebrates

Difficulty Level: At Grade Created by: CK-12
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How are these jellyfish like an insect?

Jellyfish and insects don't seem to have much in common. They look much different. They live in very different environments. But both of these animals are classified as invertebrates.

What Are Invertebrates?

Animals are also often identified as being either invertebrates or vertebrates. These are terms based on the skeletons of the animals. Vertebrates have a backbone made of bone or cartilage (Cartilage is a flexible supportive tissue. You have cartilage in your ear lobes.). Invertebrates, on the other hand, have no backbone (Figure below).

Snails are an example of invertebrates, animals without a backbone.

All vertebrate organisms are in the phylum Chordata. Invertebrates are divided into several phyla, some of which are listed in Table below.

Phylum Examples
Porifera Sponges
Cnidaria Jellyfish, corals
Platyhelminthes Flatworms, tapeworms
Nematoda Nematodes, heartworm
Mollusca Snails, clams
Annelida Earthworms, leeches
Arthropoda Insects, crabs
Echinodermata Sea stars, sea urchins


  • cartilage: Firm tissue that provides flexible support in animal skeletons.
  • invertebrates: Animals that lack backbones.
  • vertebrates: Animals that have backbones of bones or cartilage.


  • Invertebrates are animals without a backbone.
  • Invertebrates include insects, earthworms, jellyfish, and many other animals.


Use the resource below to answer the questions that follow.

  1. What is the difference between a Protostome and a Deuterostome? What does this suggest about the relationships between animals? What group of animals is closest to Chordates?
  2. Arthropods and Annelids were once though to be closely related because they are both segmented. They have since been separated into the Ecdysozoans and Lophotrochozoans respectfully. What is the basis of the new grouping? What does this teach you about the assumption that similar characteristics have common origins?
  3. Given the fundamental difference between Protostomes and Deuterosomes and the importance of eating to animals, do you think digestive systems are good places to look to for determining relationships between organism? Do you think all digestive systems can be traced back to a common ancestor?
  4. Why are insects justifiably called terrestrial crustaceans? Do you think calling all insects crustaceans would help or hinder people's understanding of the relationships between animals?
  5. Do you think the increased use of molecular analysis will change the way we view animals? Why or why not?


  1. What are some examples of invertebrates?
  2. How do you determine if an animal is an invertebrate?

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cartilage Firm tissue that provides flexible support in animal skeletons.
invertebrates Animals that lack backbones.
vertebrates Animals that have backbones of bones or cartilage.

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Difficulty Level:
At Grade
7 , 8
Date Created:
Nov 29, 2012
Last Modified:
Aug 30, 2016
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