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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 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). Invertebrates live just about anywhere. There are so many invertebrates on this planet that it is impossible to count them all. There are probably billions of billions of invertebrates. They come in many shapes and sizes, live practically anywhere and provide many services that are vital for the survival of other organisms, including us. They have been observed in the upper reaches of the atmosphere, in the driest of the deserts and in the canopies of the wettest rainforests. They can even be found in the frozen Antarctic or on the deepest parts of the ocean floor.

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

All vertebrate organisms are in the phylum Chordata. Invertebrate, which make up about 95% (or more) of the animal kingdom, are divided into over 30 different phyla, some of which are listed below (Table below). Numerous invertebrate phylum have just a few species; some have only one described species, yet these are classified into separate phylum because of their unique characteristics.

Phylum Meaning Examples
Porifera Pore bearer Sponges
Cnidaria Stinging nettle Jellyfish, corals
Platyhelminthes Flat worms Flatworms, tapeworms
Nematoda Thread like Nematodes, heartworm
Mollusca Soft Snails, clams
Annelida Little ring Earthworms, leeches
Arthropoda Jointed foot Insects, crabs
Echinodermata Spiny skin 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 thought to be closely related because they are both segmented. They have since been separated into the Ecdysozoans and Lophotrochozoans. 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 ways in which 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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