Animals are grouped by a handful of body-plan features: level of organisation, symmetry, number of germ layers, the nature of the body cavity, segmentation and the notochord.
How it's asked: Matching phyla to acoelomate/pseudocoelomate/coelomate, diploblastic vs triploblastic, radial vs bilateral symmetry, and open vs closed circulation.
Why this is true
Each feature records a step in building a more complex body. Cells working loosely together (sponges) came before cells organised into tissues (coelenterates), and tissues before organs (flatworms) and organ systems. Adding a third germ layer, the mesoderm, between ectoderm and endoderm gave material for muscles and organs, so every phylum from Platyhelminthes onwards is triploblastic. A body cavity lined fully by mesoderm (a true coelom) lets the gut and body wall move independently; where mesoderm forms only scattered pouches between ectoderm and endoderm the cavity is a pseudocoelom, and where there is no cavity the animal is acoelomate. Because these traits are structural, they let you place an unfamiliar animal in its phylum.
Key points
Body plan by phylum
| Phylum | Organisation | Symmetry | Germ layers | Coelom |
|---|---|---|---|---|
| Porifera | Cellular | Mostly asymmetrical | — | — |
| Coelenterata | Tissue | Radial | Diploblastic | — |
| Ctenophora | Tissue | Radial | Diploblastic | — |
| Platyhelminthes | Organ | Bilateral | Triploblastic | Acoelomate |
| Aschelminthes | Organ system | Bilateral | Triploblastic | Pseudocoelomate |
| Annelida | Organ system | Bilateral | Triploblastic | Coelomate |
| Echinodermata | Organ system | Radial (adult), bilateral (larva) | Triploblastic | Coelomate |
Common traps
- Echinoderms are radial only as adults; their larvae are bilateral — questions that say "echinoderms are radially symmetrical" mean the adult.
- Aschelminthes are pseudocoelomate, not acoelomate; the flatworms are the acoelomates.
Worked example
A student examines an animal that is triploblastic and bilaterally symmetrical, has a complete gut and a body cavity not lined completely by mesoderm, and is circular in cross-section.
High-yield
Acoelomate = Platyhelminthes; pseudocoelomate = Aschelminthes; diploblastic = Coelenterata and Ctenophora; open circulation = Arthropoda and Mollusca.
Quick check
Q1.Which group is pseudocoelomate?
Q2.Open circulatory system is found in: