Levels of organisation and symmetry

Over a million kinds of animals

Zoologists have described more than a million species of animals. They differ in shape, size and way of life. Nobody can learn them one by one, so we sort them into groups.

The sorting uses a few fundamental features that many animals share. These include:

• how the cells are arranged in the body;
• the symmetry of the body;
• the kind of body cavity, if any;
• the patterns of the digestive, circulatory and reproductive systems.

Each feature splits the animals into a few groups. Put the features together, and each big group, a phylum, gets its own set of answers.

Levels of organisation

Every animal is made of many cells. But the cells are organised in different ways.

• Cellular level. The cells are loose aggregates. There is some division of labour among them, but no true tissues. Sponges, the Porifera, work like this.
• Tissue level. Cells that do the same job are arranged into tissues. The coelenterates, such as Hydra, have this level.
• Organ level. Tissues are grouped into organs, each with a special job. The flatworms, Platyhelminthes, have it.
• Organ system level. Organs work together in systems, each in charge of a body function. Annelids, arthropods, molluscs, echinoderms and chordates have it.

Organ systems differ too

Even animals at the organ system level differ in how their systems are built.

The digestive system may be:

• incomplete, with a single opening that is both mouth and anus, as in flatworms;
• complete, with two openings: a mouth and an anus.

The circulatory system may be:

• open, where the heart pumps blood out into spaces, and the cells and tissues are bathed directly in it, as in arthropods and molluscs;
• closed, where the blood stays inside vessels of different widths: arteries, veins and capillaries, as in annelids and chordates.

Symmetry

Imagine cutting an animal in two through the middle. Do you get two equal halves?

• Asymmetrical. No plane through the centre gives equal halves. Most sponges are like this.
• Radial symmetry. Any plane through the central axis gives two identical halves. Coelenterates, ctenophores and adult echinoderms are radial.
• Bilateral symmetry. Only one plane gives identical halves: the left and the right. Annelids, arthropods and most other animals are bilateral.

A radial animal meets its world from every side, so it suits a life fixed in one place or drifting. A bilateral animal has a front end that goes first, and a head often forms there.

You should find that the jellyfish gives equal halves at every angle. The bilateral animal gives mirror halves only at $90°$, along the line from head to tail.

Try it in Python

The program stores six animals with their level of organisation, prints them, and counts how many are at each level.

Practise with new numbers

Common mistakes

• Saying sponges have tissues. Their cells are loose aggregates, at the cellular level.
• Calling a gut with one opening complete. A complete gut needs both a mouth and an anus.
• Mixing up open and closed circulation. In open circulation the blood leaves the vessels and bathes the tissues.
• Thinking a starfish is bilateral as an adult. Adult echinoderms are radial.

Remember these

• Levels: cellular (sponges), tissue (coelenterates), organ (flatworms), organ system (higher animals).
• Gut: incomplete with one opening, or complete with a mouth and an anus.
• Circulation: open (arthropods, molluscs) or closed (annelids, chordates).
• Symmetry: asymmetrical (most sponges), radial (coelenterates, ctenophores, adult echinoderms), bilateral (most others).

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. At which level of organisation do sponges work?

2. What makes a digestive system complete?

3. In closed circulation, where does the blood flow?

4. How many planes give identical halves in a bilateral animal?

5. Name a group with radial symmetry.