Phylum Nematoda — Roundworms
The rise of the body cavity and the one-way digestive tube
🔬 1. What Are Roundworms?
The name Nematoda comes from the Greek word nemos, meaning "thread" — and that's exactly what many of these worms look like: tiny, smooth threads.
- Shape: Nematodes are unsegmented worms that are round in cross-section (imagine slicing one like a tiny sausage — you'd see a circle, not a flat ribbon).
- Diversity: Phylum Nematoda includes more than 28,000 known species. About 16,000 are parasitic, while roughly 12,000 are free-living in soil and water — so not all roundworms are parasites!
- Size range: From microscopic soil nematodes (< 1 mm) to Ascaris lumbricoides, which can reach 20–35 cm in length.
🫧 2. The Pseudocoelom — A Primary Body Cavity
One of the biggest evolutionary leaps from flatworms to roundworms is the appearance of a body cavity.
Definition: A pseudocoelom ("false cavity") is a fluid-filled space between the body wall and the digestive tract. It is called "pseudo" because it is only partially lined by mesoderm (the middle tissue layer), unlike a true coelom which is fully lined by mesoderm.
Think of the roundworm body plan as two concentric tubes (a tube within a tube) separated by this fluid-filled space:
| Layer | What It Is |
|---|---|
| Outer tube | Body wall (epidermis + muscles) |
| Fluid space | Pseudocoelom (filled with fluid) |
| Inner tube | Digestive tract (gut) |
Why does the pseudocoelom matter?
💪 Support
The pressurized fluid acts like a hydrostatic skeleton, giving the worm its round, firm shape.
The pressurized fluid acts like a hydrostatic skeleton, giving the worm its round, firm shape.
🔄 Transport
Nutrients and gases diffuse through the fluid — it works like a simple circulatory system.
Nutrients and gases diffuse through the fluid — it works like a simple circulatory system.
🛡️ Cushioning
The fluid cushions and lubricates internal organs, protecting them during movement.
The fluid cushions and lubricates internal organs, protecting them during movement.