How Animals Are Classified: Quick Revision

A brief note on How Animals Are Classified. Read it before a test. Try each question at the end before you open its answer.

Levels and systems

• Cellular (Porifera), tissue (coelenterates, ctenophores), organ (Platyhelminthes), organ system (annelids to chordates).
• Gut: incomplete, one opening (flatworms); complete, a mouth and an anus.
• Circulation: open, blood bathes the tissues (arthropods, molluscs); closed, blood in vessels (annelids, chordates).

Symmetry

• Asymmetrical: no plane gives equal halves (most sponges).
• Radial: any plane through the central axis (coelenterates, ctenophores, adult echinoderms).
• Bilateral: one plane only, left and right (most other animals).

Germ layers and the body cavity

• Diploblastic: ectoderm and endoderm, with mesoglea between (coelenterates).
• Triploblastic: ectoderm, mesoderm and endoderm (flatworms to chordates).
• Coelomate: a cavity lined by mesoderm (annelids, molluscs, arthropods, echinoderms, hemichordates, chordates).
• Pseudocoelomate: mesoderm in scattered pouches (Aschelminthes). Acoelomate: no cavity (Platyhelminthes).

Segments and the notochord

• Metameric segmentation: segments outside and inside, organs repeated (annelids, arthropods, chordates).
• Notochord: a rod from the mesoderm on the dorsal side of the embryo; chordates have it, non-chordates do not.

A worked example

A marine animal has true tissues, bilateral symmetry, three germ layers, a true coelom and no notochord.

1. Tissues. It is not Porifera.
2. Bilateral and triploblastic. It is not a coelenterate or a ctenophore.
3. A true coelom. It is not a flatworm or a roundworm.
4. No notochord. It is a coelomate non-chordate: Annelida, Arthropoda, Mollusca, Echinodermata or Hemichordata.

Practice with new numbers

A program to try

The program prints the basic features of six groups as a table.

Mistakes to avoid

• Calling the mesoglea a germ layer. Coelenterates stay diploblastic.
• Giving flatworms a pseudocoelom. They have no cavity; roundworms have the pseudocoelom.
• Thinking every coelomate is a chordate. Only animals with a notochord are.
• Calling a gut with one opening complete. It needs a mouth and an anus.

Check yourself

1. Which level of organisation do flatworms reach?

2. Name the kind of circulation of a mollusc.

3. What does a pseudocoelomate lack that a coelomate has?

4. Which structure separates chordates from all other animals?