Lesson Topics

1 | Invertebrates

2 | Arthropods

3 | Molluscs

4 | Echinoderms

5 | Annelids

6 | Nematodes

7 | Platyhelminths

8 | Cnidarians

9 | Porifera

10 | Summary

Lesson Summary

• Invertebrates are animals without a backbone.

• Invertebrates may have an exoskeleton, an endoskeleton or no skeleton.

• Major invertebrate groups include:

• Arthropods

• Molluscs

• Echinoderms

• Annelids

• Nematodes

• Platyhelminths

• Cnidarians

• Porifera

• Arthropods:

• Have an exoskeleton made of chitin.

• Have a segmented, bilaterally symmetrical body.

• Have paired, jointed legs; often also have antennae.

• Have an open circulatory system, an excretory system and a nervous system with a primitive brain.

• Arthropods include:

• Insects

• Arachnids

• Crustaceans

• Chilopods

• Diplopods

• Molluscs:

• Have a soft, non-segmented body, usually covered in a hard shell.

• Lack legs, but may have tentacles.

• Have a strong, muscular ‘foot’.

• Have a specialised feeding structure containing teeth.

• Echinoderms:

• Have an internal skeleton made of calcium carbonate.

• Have a soft body, often with spiny ‘skin’.

• Are bilaterally symmetrical in the larval stage and radially symmetrical in the adult stage.

• Have a vascular system that pumps water instead of blood.

• Annelids:

• Have a soft, tubular, bilaterally symmetrical body.

• Have a segmented body, with some repeated organs.

• Have a closed circulatory system.

• Have a specialised digestive system, excretory system and nervous system, including a primitive brain.

• Nematodes:

• Have a soft, tubular, bilaterally symmetrical body.

• Have a non-segmented body.

• Have a complete digestive tract.

• Platyhelminths:

• Have a soft, flat, bilaterally symmetrical body.

• Have a non-segmented body.

• Have an incomplete digestive tract.

• Cnidarians:

• Have a soft, radially symmetrical body, lacking organs.

• Have a single opening for food and waste.

• Have stinging tentacles.

• Porifera:

• Have a spongy, asymmetrical body, lacking tissues and organs.

• Have a body full of pores that allow water to flow through.

• Have an endoskeleton or exoskeleton containing needle-like spicules.

• 
  

  Invertebrate Group	Symmetry	Segmentation	Body Cavity	Body Tissues	Other Features
  Arthropods	Bilateral	Yes	Yes	Yes	Exoskeleton; legs; most insects have wings
  Molluscs	Bilateral	No	Yes	Yes	Usually have a hard shell; may have tentacles
  Echinoderms	Radial	No	Yes	Yes	Endoskeleton; often spiny
  Annelids	Bilateral	Yes	Yes	Yes	Tubular body
  Nematodes	Bilateral	No	Partial	Yes	Tubular body
  Platyhelminths	Bilateral	No	No	Yes	Flat body
  Cnidarians	Radial	No	No	Yes	Stinging tentacles
  Porifera	None	No	No	No	Porous, spongy body; spicules

Summary of the invertebrate animal groups

(Header image: maneph9, Pixabay)