Animal Kingdom · Lesson 2 of 11
Body Plans: Symmetry, Coelom and Notochord
“Compare symmetry, germ layers, cavities, segmentation and chordate structural features.”
• Distinguish asymmetry, radial symmetry and bilateral symmetry using body axes. • Explain diploblastic and triploblastic embryonic organisation. • Identify acoelomate, pseudocoelomate and true coelomate cross-sections. • Explain metameric segmentation and relate it to organ organisation. • Define the notochord and distinguish chordate features from unrelated look-alike structures.
Why body architecture reveals animal relationships
Two animals can have very different colours and habitats yet share a body plan. Body axes, tissue layers, body cavities and repeated segments provide evidence of how an animal develops and how its organs are arranged. These characters are especially valuable when identifying a phylum from a drawing or cross-section. They are not interchangeable: symmetry describes geometric arrangement; germ layers describe embryonic origins; and coelom describes a specific internal cavity relationship.
Asymmetry, radial symmetry and bilateral symmetry
An arrangement in which the body generally cannot be divided into corresponding equal halves along a plane through its centre.
A body arrangement in which multiple planes passing through a central axis can give comparable halves.
A body arrangement that has one principal plane dividing it into approximately corresponding left and right sides.
Sponges are often irregular and asymmetrical; a cut through one sponge may not give matching halves. A sea anemone is organised around an oral-aboral axis, with tentacles surrounding a mouth, so several planes can show repeated sectors. An earthworm or crab has a more defined front and back and a left-right plane. Bilateral organisation commonly supports directional movement and concentration of sensory structures towards an anterior region.
Adult echinoderms such as sea stars show secondary radial organisation, usually approximately fivefold, yet their larvae are bilaterally symmetrical. That combination is essential: the same lineage can exhibit different symmetry at different life stages. Comb jellies are often described as radial in introductory comparisons, though their arrangement can be more precisely described as biradial. Do not assume radial symmetry always means every possible plane gives identical anatomical halves.
The embryonic germ layers
The outer embryonic germ layer that contributes to the external body covering and nervous structures in triploblastic animals.
The inner embryonic germ layer contributing to much of the digestive lining and associated organs in triploblastic animals.
The third, intervening embryonic germ layer of triploblastic animals, giving rise to many muscles, connective tissues and internal structures.
Cnidarians develop from two principal germ layers: the outer ectoderm and inner endoderm, with an intervening mostly noncellular mesoglea. They are described as diploblastic. Flatworms, annelids, arthropods and chordates are triploblastic because an embryonic mesoderm lies between ectoderm and endoderm. The distinction concerns embryonic tissue origins, not simply the number of adult body coverings.
A mesoglea is not the same as a mesoderm. It is a gelatinous region between the tissue layers of cnidarians, whereas mesoderm is a cellular embryonic layer involved in formation of many internal structures. This distinction prevents an incorrect conclusion that a jellyfish becomes triploblastic merely because three visually different bands can be drawn across its body.
Body cavities and the precise meaning of a coelom
A body cavity between the body wall and gut that is completely lined by mesoderm-derived tissue.
A body cavity in which the space is not completely lined by mesoderm.
A triploblastic body organisation without a fluid-filled cavity between the digestive tract and body wall.
The critical test is not merely whether an animal contains an empty-looking internal region. A true coelom is enclosed by mesodermally derived linings; in annelids it forms spaces around internal organs. A pseudocoelom, characteristic of the nematodes discussed as Aschelminthes in the source, remains only partially bounded by mesodermal tissues. Flatworms are acoelomate, with tissue between the body wall and digestive structures rather than a large open body cavity.
Coelomic spaces provide room for organs to grow and move, can contribute to hydrostatic support, and may aid internal transport or cushioning. However, coelom classification is not a universal ranking of biological complexity. Animals have specialised their body cavities in different ways, and some coelomate groups possess a large haemocoel in addition to a much reduced true coelom.
| Coelom type | Characteristic | Representative |
|---|---|---|
| Acoelomate | No true body cavity separating gut and body wall | Flatworms (Platyhelminthes) |
| Pseudocoelomate | Body cavity not completely lined by mesoderm | Roundworms / nematodes |
| Coelomate | Body cavity lined by mesoderm-derived tissue | Annelids, molluscs, chordates |
Metamerism: repeated units along the body
A body plan in which a series of segments repeat along the length, often with repetition of internal structures.
An earthworm's body shows repeated external rings and internal compartmentalisation. Some excretory, muscular and neural structures recur segment by segment. This repetition enables coordinated movement and regional specialisation. Arthropods also display segmented ancestry, with segments often fused into functional regions such as head, thorax and abdomen. A segmented shape should be supported by internal developmental evidence rather than confused with incidental surface folds.
Notochord and the chordate distinction
A flexible supportive rod derived from embryonic mesoderm and located along the dorsal region in chordates.
A notochord develops in chordates at least during a life stage, providing axial support and developmental signals. In many vertebrates it is largely replaced by a vertebral column as the embryo develops, although its remnants may persist. Chordates also exhibit a dorsal hollow nerve cord, pharyngeal slits and a post-anal tail at some stage. Non-chordates do not share this full diagnostic combination.
A hemichordate has a stomochord, historically compared with a notochord, but these structures are not regarded as the same anatomical feature. The chapter treats hemichordates separately from chordates. An adult tunicate can lack an obvious tail and still be a chordate because the larva has chordate traits. Always examine the life cycle when adult appearance seems contradictory.
Problem
An unknown animal has ectoderm, mesoderm and endoderm, but the body cavity is not completely lined by mesoderm. Its body is unsegmented. Which cavity category fits?
- 1.Three embryonic germ layers make the organism triploblastic.
- 2.A cavity not completely lined by mesoderm indicates pseudocoelom.
- 3.A nematode-like body is a likely match, but other diagnostic characters are needed before deciding on a particular species.
Quick Quiz
Quiz
Which body symmetry is typical of an earthworm?
Which structure lies between ectoderm and endoderm as a true embryonic layer in triploblasts?
What makes a body cavity a true coelom?
A roundworm is commonly which cavity category?
Why are adult sea stars and their larvae important for symmetry?
Which character supports placement in Chordata?
Practice Problems
- Explain asymmetry, radial symmetry and bilateral symmetry with examples.
- Distinguish diploblastic and triploblastic animals.
- Use a cross-section to identify acoelomate, pseudocoelomate and coelomate bodies.
- Explain metameric segmentation and its functional importance.
- Explain why a tunicate is a chordate while a hemichordate is not.
An irregular sponge is often asymmetric. Sea anemones show radial organisation around an oral-aboral axis. An earthworm shows a principal plane separating left and right sides. Adult echinoderms can be radial although larvae are bilateral.
Key Takeaways
Symmetry compares planes of correspondence: many sponges are asymmetrical, cnidarians radial and annelids bilateral. Diploblasts have ectoderm and endoderm; triploblasts have a true mesoderm. A true coelom is fully mesoderm-lined; pseudocoeloms are not; acoelomates lack that cavity. Metamerism involves repeated body segments, as in earthworms. The notochord and additional chordate traits can be present during development even if less obvious in adults.