One of the key criteria for classifying animals is the type of body cavity (coelom) present between the digestive tract and the outer body wall. On this basis, triploblastic animals are divided into three groups.
A coelom is a fluid-filled body cavity located between the digestive tract and the outer body wall. It is derived from the mesoderm (the middle embryonic germ layer). The coelom cushions internal organs, allows organ movement, and acts as a hydrostatic skeleton in some animals.
- Animals that lack a body cavity entirely.
- The space between the ectoderm and endoderm is packed with solid mesodermal tissue (mesenchyme).
- Example: Phylum Platyhelminthes (flatworms, e.g., Planaria, tapeworms).
- Animals with a false body cavity called a pseudocoelom.
- The pseudocoelom is located between the mesoderm and endoderm and is not completely lined by mesoderm.
- It is a persistent blastocoel (remnant of the embryonic cavity).
- Example: Phylum Aschelminthes / Nematoda (roundworms, e.g., Ascaris).
- Animals with a true coelom that forms within the mesoderm and is completely lined by mesoderm-derived tissue called the peritoneum.
- The true coelom provides greater freedom of movement for internal organs.
- Examples: Phylum Annelida (earthworms), Phylum Mollusca, Phylum Arthropoda, Phylum Chordata.
| Feature | Acoelomate | Pseudocoelomate | Coelomate |
|---|
| Body cavity | Absent | Present (false) | Present (true) |
| Lining | Solid mesoderm | Partially lined by mesoderm | Completely lined by mesoderm |
| Example | Platyhelminthes | Aschelminthes | Annelida, Chordata |