5.5 Animal Diversity and Body Plans
Key Takeaways
- Animal body plans are characterized by **symmetry** (asymmetric, radial, or bilateral), **tissue layers** (diploblastic = 2 layers, triploblastic = 3 layers), **body cavity** (acoelomate, pseudocoelomate, true coelomate), and **embryonic development** (protostome vs. deuterostome).
- **Protostomes** (annelids, mollusks, arthropods) form the **mouth first** from the blastopore; **deuterostomes** (echinoderms, chordates) form the **anus first** - this developmental split is one of the deepest in the animal tree.
- **Arthropoda** is the most diverse phylum (insects, crustaceans, arachnids, myriapods), characterized by a **chitinous exoskeleton** that requires periodic molting (ecdysis); insects have six legs and three body regions while arachnids have eight legs and two regions.
- **Chordata** is defined by four embryonic traits: a **notochord**, a **dorsal hollow nerve cord**, **pharyngeal slits**, and a **post-anal tail**; vertebrates evolved a backbone, jaws, paired limbs, the amniotic egg, and (in mammals) hair and mammary glands.
- The **amniotic egg** (amnion, chorion, allantois, yolk sac) was the key tetrapod innovation that freed reptiles, birds, and mammals from needing water for reproduction.
Four Body-Plan Features
The Praxis tests four big-picture body-plan features. Memorize all four together because they recur across every animal phylum.
1. Symmetry
- Asymmetric - no plane of symmetry. Example: most sponges (Porifera).
- Radial symmetry - body parts arranged around a central axis (like a wheel); any cut through the center gives mirror halves. Example: Cnidarians (jellyfish, sea anemones), adult echinoderms (pentaradial).
- Bilateral symmetry - left and right halves mirror each other along a single sagittal plane; correlates with cephalization (concentration of sensory and nervous tissue at one end). Example: all other animal phyla, from flatworms to vertebrates.
2. Tissue Layers (Germ Layers)
- No true tissues: sponges (Porifera).
- Diploblastic - two germ layers (ectoderm + endoderm). Example: cnidarians.
- Triploblastic - three germ layers (ectoderm, mesoderm, endoderm). All bilaterians, from flatworms onward.
Mesoderm gives rise to muscle, the circulatory system, the reproductive system, and connective tissues - so triploblastic animals can build complex internal organs.
3. Body Cavity (Coelom)
- Acoelomate - no body cavity between gut and body wall. Example: flatworms (Platyhelminthes).
- Pseudocoelomate - cavity present but only partially lined with mesoderm. Example: roundworms (Nematoda).
- Coelomate (eucoelomate) - true body cavity fully lined with mesoderm. Example: annelids, mollusks, arthropods, echinoderms, chordates.
A true coelom cushions internal organs, allows independent muscle action, and provides space for organ growth.
4. Protostome vs. Deuterostome Development
A decisive split in bilaterian embryology:
| Feature | Protostomes | Deuterostomes |
|---|---|---|
| Blastopore becomes... | Mouth | Anus |
| Cleavage pattern | Spiral and determinate | Radial and indeterminate |
| Coelom formation | Splits within mesoderm (schizocoely) | Outpockets of archenteron (enterocoely) |
| Example phyla | Annelida, Mollusca, Arthropoda, Platyhelminthes | Echinodermata, Chordata |
Indeterminate cleavage in deuterostomes is why a single mammalian blastomere can develop into a complete organism (identical twins).
Major Animal Phyla
The following phyla appear repeatedly on the Praxis. Learn the diagnostic feature for each.
| Phylum | Common Name | Diagnostic Features |
|---|---|---|
| Porifera | Sponges | Asymmetric, no true tissues; water flow through pores and choanocytes (collar cells) |
| Cnidaria | Jellyfish, hydra, coral, anemones | Radial symmetry, diploblastic, cnidocytes (stinging cells with nematocysts); polyp + medusa stages |
| Platyhelminthes | Flatworms (planarians, tapeworms, flukes) | Bilateral, triploblastic, acoelomate, gastrovascular cavity with one opening |
| Nematoda | Roundworms (Ascaris, C. elegans) | Bilateral, pseudocoelomate, complete digestive tract, molting cuticle |
| Mollusca | Snails, clams, octopuses, squids | Soft body with three regions: head-foot, visceral mass, mantle; many secrete a shell; radula in many; squids and octopuses have closed circulation and complex brains |
| Annelida | Segmented worms (earthworms, leeches, polychaetes) | Bilateral, triploblastic, coelomate, segmented body (metamerism), closed circulatory system |
| Arthropoda | Insects, crustaceans, arachnids, myriapods | Chitin exoskeleton + jointed appendages + periodic molting (ecdysis); most species-rich phylum (~80% of described animal species) |
| Echinodermata | Sea stars, urchins, cucumbers | Deuterostome, adult pentaradial symmetry (larvae bilateral), water vascular system with tube feet, calcareous endoskeleton |
| Chordata | Tunicates, lancelets, vertebrates | Notochord + dorsal hollow nerve cord + pharyngeal slits + post-anal tail (at some life stage) |
Arthropod Subgroups
| Subgroup | Legs | Body Regions | Examples |
|---|---|---|---|
| Insecta (insects) | 6 (3 pairs) | Head, thorax, abdomen | Ant, butterfly, beetle |
| Arachnida | 8 (4 pairs) | Cephalothorax, abdomen | Spider, scorpion, tick |
| Crustacea | 10+ (varies) | Cephalothorax + abdomen, often | Crab, lobster, shrimp |
| Myriapoda | Many (one or two pairs per segment) | Head + many trunk segments | Centipede, millipede |
All arthropods share the chitin exoskeleton and molt (ecdysis); the number of legs and body regions is how the Praxis distinguishes subgroups.
The Chordate Body Plan
Four diagnostic features appear in chordates at some point during development (even if lost in adults):
- Notochord - flexible rod between gut and nerve cord (replaced by the vertebral column in vertebrates).
- Dorsal hollow nerve cord (becomes brain and spinal cord in vertebrates - contrasted with the ventral solid nerve cord of arthropods/annelids).
- Pharyngeal slits (gill slits) - become gills in fish, jaw and ear bones in tetrapods.
- Post-anal tail - a muscular tail extending past the anus.
Major Vertebrate Innovations
The vertebrate lineage shows a clear sequence of body-plan upgrades:
- Jaws (gnathostomes - cartilaginous and bony fishes) → predation
- Bony skeletons (Osteichthyes)
- Lungs and limbs (tetrapods)
- Amniotic egg (amniotes: reptiles, birds, mammals) → fully terrestrial reproduction
- Endothermy (independently evolved in birds and mammals)
- Hair, mammary glands, live birth (mammals)
The amniotic egg (amnion + chorion + allantois + yolk sac) is the most-tested vertebrate innovation - it provides a self-contained aquatic environment so embryos can develop on dry land without returning to water.
Convergent Quick-Hits
- Endothermy ("warm-blooded") is found in mammals AND birds (independently), so it does not uniquely define mammals.
- Four-chambered hearts also occur in both mammals and birds.
- The truly unique mammalian features are hair/fur and mammary glands.
- Pseudocoelom is shared by nematodes and rotifers - both molting ecdysozoans.
A marine biologist examines two animals: Animal X is a sea star (echinoderm); Animal Y is a clam (mollusk). Which of the following statements correctly compares their development?
A student is told that an unknown invertebrate has six legs, three body regions (head, thorax, abdomen), and a chitinous exoskeleton that the animal periodically sheds. To which group does it belong?