Types of Body Cavity (Coelom)
Acoelomate
Gut
Body wall
Mesoderm
(solid)
e.g., Planaria (flatworm)
No body cavity;
mesoderm solid
Pseudocoelomate
Gut
Body wall
Mesoderm
(outer only)
Pseudocoelom
e.g., Ascaris (roundworm)
False cavity; mesoderm
only on body wall
Coelomate
Gut
Parietal
peritoneum
Visceral
peritoneum
True Coelom
e.g., Earthworm, Humans
True cavity; mesoderm
lines both sides
*Source: Wikimedia Commons — Body cavity types: acoelomate, pseudocoelomate, coelomate*
Interpretation of Diagram
Acoelomate (e.g., Platyhelminthes):
- No fluid-filled body cavity
- Space between body wall and gut filled with MESENCHYME (parenchyma)
- Layers from outside to inside: Epidermis → Muscle (with mesoderm) → Mesenchyme → Gut (endoderm)
- Limitations: No room for complex organ development; limited support for organs
Pseudocoelomate (e.g., Aschelminthes):
- Fluid-filled cavity IS present
- Cavity lies between MESODERM (body wall) and ENDODERM (gut wall)
- Mesoderm lines ONLY the body wall side (not the gut side)
- This is "false coelom" — not fully peritoneum-lined
Coelomate (e.g., Annelida, Arthropoda, Chordata):
- Fluid-filled cavity lined by PERITONEUM (mesoderm) on BOTH sides
- Parietal peritoneum lines the body wall; Visceral peritoneum lines the gut
- Organs suspended in cavity by mesenteries (folds of peritoneum)
- Provides: organ support, hydrostatic skeleton, independent gut movement
Key Rule for identification:
- If mesoderm lines BOTH sides → True coelom (coelomate)
- If mesoderm lines ONLY body wall → Pseudocoelom
- If NO fluid cavity (solid mesenchyme) → Acoelomate
- If NO mesoderm at all → Check if diploblastic