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Natural Science - Year I

Unit 12: Biological Classification, Invertebrates, Organ Systems

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Science Web Assignment for Unit 12

This Unit's Homework Page History Lecture Science Lecture Lab Parents' Notes

Science Lecture for Unit 12:
Invertebrate Organisms

For Class

Reading Notes

It is usually easier to compare and contrast groups of characteristics than to try to memorize isolated facts about new material. Look through the groupings below (systems, functions, similar anatomies) and make your own chart of the information. What details do you note that you would have missed otherwise?

Invertebrate Biology

Organ Systems

We have already looked at the modern classification of groups of vertebrate animals (starting at the top of Aristotle's chart). Now we turn to the invertebrate animals, which are generally simpler in form and number of systems than vertebrates. The invertebrates include all the other multi-celled life forms which cannot make their own food. (Plants make their own food, and the protists and bacteria, which we look at later in the course, are single-celled life forms).

Most animals have systems of organs for different purposes. The most complex organisms (mammals) have up to ten such systems, which perform the following functions:

The modern classification systems group animals according to which possess these systems and by how complex the systems must be to support the life of the organism.


Study each of the invertebrate classes below, either through the associated web link or a biology text. Make notes of the organism's

For example, the sponge, which is the simplest animal, reproduces both by budding (which does not require another sponge), and by a form of mating (which does require another sponge). It has no system of organs, so it has no stomach, mouth, or intestines, and no circulatory system. Each cell gets its nutrition directly from the water surrounding the sponge, or from the cell next to it. The sponge's life cycle is very simple: the new bud or ... swims freely for a short time, then fastens itself to a rock and procedes to grow and make little sponges until it dies. The sponge has no sense organs as such; it does expand and contract somewhat to force water through its internal cavity, so that it can filter the water for food.

Invertebrate Classes

The links below take you to a biologist's list of characteristics, and some of the terms are pretty technical. Don't worry about that. Concentrate on the major concepts -- what systems exist or are missing, what kind of symmetry occurs in the phylum, does the animal change shape during different phases of the life cycle, and so on.

Read through J. Kimball's summary of invertebrate animals (one page).

  • The term duetostomes and protostomes is used do distinguish between animals with vertebrates or at least a notocord (echinoderms) and invertebrates. Which invertebrates belong to ecdysozoans? What characteristics do they share? How are they different from lophotrochozoans? Why are sponges and cnidarians not in either protostome group?
  • Which is the most diverse group? (which has the most species)? Why do you think this group has so many different forms?
  • Many invertebrates are "segmented". What advantages does this give an animal?
Invertebrates Groups
For each group, click on the left column name for more information.



Jellyfish, Hydras, Corals, Anemones


Flukes, Tapeworms, Planaria

 Round Worms

Hookworms, eels


Clams, Mussels, Snails, Slugs, Squid, Octupus

 Segmented worms

Earthworms, Clams, Worms, Leeches

  • Crustaceans: crayfish, lobster
  • Insects: butterflies, ants
  • Arachnids: spiders, ticks, scorpions
  • Horseshoe Crabs

Sea stars, Brittle Stars, Sea Urchins, Sea Cucumbers

Study/Discussion Questions

Further Study On your Own (Optional)