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9.13: Arthropods

Difficulty Level: At Grade Created by: CK-12
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What does this lobster have in common with a wasp?

You might notice that their bodies have segments. And they both have a hard outer layer. Because they share these and other features, they are both classified as arthropods.

What are Arthropods?

How often do you think you see an arthropod? Well, have you ever looked up close at an ant? A spider? A fly? A moth? With over a million described species in the phylum containing arthropods, chances are, you encounter one of these organisms every day, without even leaving your house.

Types of Arthropods

Arthropods belong to the phylum Arthropoda, which means “jointed feet,” and includes four living subphyla. These are:

  • Chelicerata, which includes spiders ( Figure below ), mites, and scorpions.
  • Myriapoda, which includes centipedes and millipedes.
  • Hexapoda, which includes the insects.
  • Crustacea, which includes lobsters, crabs, barnacles, crayfish, and shrimp.

A species of spider in its web

Spiders are one type of arthropod.

Characteristics of Arthropods

Characteristics of arthropods include:

  1. A segmented body ( Figure below ) with a head, a thorax, and abdomen segments.
  2. Appendages on at least one segment. They can be used for feeding, sensory reception, defense, and locomotion.
  3. A nervous system.
  4. A hard exoskeleton made of chitin, which gives them physical protection and resistance to drying out. In order to grow, arthropods shed this covering in a process called molting .
  5. An open circulatory system with hemolymph , a blood-like fluid. A series of hearts move the hemolymph into the body cavity where it comes in direct contact with the tissues.
  6. A complete digestive system with a mouth and anus.
  7. Aquatic arthropods use gills to exchange gases. These gills have a large surface area in contact with the water, so they can absorb more oxygen.
  8. Land-living arthropods have internal surfaces that help exchange gasses. Insects and most other terrestrial species have a tracheal system, where air sacs lead into the body from pores in the exoskeleton. Others use book lungs, or gills modified for breathing air, as seen in species like the coconut crab. Some areas of the legs of soldier crabs are covered with an oxygen absorbing skin. Land crabs sometimes have two different structures: one used for breathing underwater, and another used to absorb oxygen from the air.

The blue American lobster illustrates the segmented body plan of the arthropods.


  • hemolymph : The blood-like circulatory fluid of some invertebrates.
  • molting : Shedding of the old shell to make way for a new growth.


  • The arthropods include four living subphyla: chelicerates, including spiders, mites, and scorpions; myriapods, including centipedes and millipedes; hexapods, including insects; and crustaceans.
  • Arthropods are characterized by a segmented body, a hard exoskeleton, and appendages used for feeding, sensory structures, defense, and locomotion.


Use the resource below to answer the questions that follow.

  1. Why are jointed limbs significant for Arthropods?
  2. How are appendage adaptations and segmentation key to the success of Arthropods as a group?
  3. Compare and contrast the skeleton of a lobster to the skeleton of a sea star.
  4. Soft shell crabs are delicacies in some restaurants. Where do soft shell crabs come from?
  5. What aspect of horseshoe crabs behavior do scientists feel give clues to why Arthropods first left the ocean?
  1. What are examples of arthropods.
  2. How do arthropods affect people?


  1. What are examples of arthropods?
  2. What are some distinguishing features of the arthropods?




Invertebrate animals with jointed limbs, a segmented body, and an exoskeleton made of chitin.


Blood-like circulatory fluid of some invertebrates.


Shedding of the old shell to make way for a new growth.

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Difficulty Level:

At Grade

Concept Nodes:


7 , 8

Date Created:

Nov 29, 2012

Last Modified:

Mar 05, 2015
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