Basis for Classisfication
6 Minutes
Basis of animal classification for students explores cellular levels, symmetry, germ layers, and coelom types. A science lesson in English.
- Unicellular bacteria and multicellular sponges in phylum Porifera
- Bilateral and radial symmetry shown using a leaf and an orange
- 3D germ layers and coelom types from acoelomate to coelomate
What this lesson teaches
Animal classification divides living organisms from kingdom down to phylum based on four main structural criteria: cellular arrangement, body symmetry, germ layers, and the nature of the coelom. Scientists observe these similarities and differences to group organisms systematically within the animal kingdom.
Cellular arrangement
- Living things are classified in seven stages from kingdom, phylum, class, order, family, genus, to species.
- Organisms are unicellular when a single cell performs every life process on its own, as seen in bacteria.
- Organisms are multicellular when they are composed of many cells, such as sponges.
- Sponges have multiple cells but lack distinct organs, placing them in the phylum Porifera.
Body symmetry
Symmetry describes how an animal's body is balanced across a central axis. Bilateral symmetry occurs when an organism with a head and a tail can be divided down its central line into two identical halves, similar to a folded leaf. Radial symmetry occurs when cutting through the central axis produces multiple identical parts, similar to slicing an orange. Animals showing radial symmetry, such as jellyfish, lack a distinct head and tail.
Germ layers in development
Germ layers are early layers of cells that appear during embryonic development and build body structures. Animals with only two germ layers, the outer ectoderm and inner endoderm, are known as diploblastic, with tapeworms serving as an example. Animals that develop all three layers, including a middle mesoderm, are triploblastic, such as leeches and butterflies. The ectoderm eventually forms the brain, nervous system, and skin, the mesoderm forms the bones and muscles, and the endoderm develops into the liver and kidneys.
The coelom and body cavities
- The coelom is a fluid-filled body cavity, or empty space, found in organisms with bilateral symmetry that allows organs like the brain and lungs to fit and grow properly.
- Acoelomates, such as flatworms in the phylum Platyhelminthes, have no body cavity between the mesoderm and endoderm.
- Pseudocoelomates, such as roundworms in the phylum Nematoda, have an uneven mesoderm that forms an incomplete or false cavity layer.
- Coelomates have a true body cavity that is fully enclosed by a layer, which includes organisms in the phyla Annelida, Arthropoda, and Chordata.