The statement that all deuterostomes exhibit radial symmetry in their bodies is incorrect. This assertion is false because deuterostomes fundamentally possess bilateral symmetry, not radial symmetry.
Understanding Deuterostome Symmetry
While radial symmetry, where body parts are arranged concentrically around a central axis, is a characteristic feature of certain invertebrates such as jellyfish and sea anemones, it is not a defining trait of deuterostomes. Instead, deuterostomes primarily exhibit bilateral symmetry. This means their bodies can be divided into two mirror-image halves along a central plane, typically displaying distinct anterior (head) and posterior (tail) ends, as well as dorsal (back) and ventral (belly) surfaces. This body plan facilitates directed movement and the development of specialized sensory structures.
Key Characteristics of Deuterostomes
Beyond their predominant bilateral symmetry, deuterostomes are defined by several other distinctive embryonic developmental features:
- Blastopore Forms Anus: During early embryonic development, the first opening that forms, known as the blastopore, develops into the anus. The mouth then forms subsequently.
- Radial Cleavage: The cells divide in a pattern where daughter cells are positioned directly above the parent cells, forming tiers. This is characteristic of their early embryonic cell division.
- Indeterminate Cleavage: The developmental fate of individual cells in the early embryo is not rigidly determined from the outset. If an early cell is separated from the embryo, it still has the potential to develop into a complete organism.
- Enterocoelous Coelom Formation: The coelom, or main body cavity, develops from out-pocketings of the archenteron (the primitive gut) during embryonic development.
Representative Deuterostome Groups
The superphylum Deuterostomia includes some of the most complex and well-known animal phyla:
- Chordates: This vast phylum encompasses all vertebrates (mammals, birds, reptiles, amphibians, and fish), as well as certain invertebrate groups like tunicates and lancelets. All chordates exhibit bilateral symmetry.
- Echinoderms: While adult echinoderms (e.g., starfish, sea urchins, sea cucumbers) often display secondary radial symmetry (typically five-fold), their larval stages are distinctly bilaterally symmetrical, confirming their deuterostome lineage.
- Hemichordates: This group includes marine worms such as acorn worms and pterobranchs, which also share the fundamental deuterostome developmental patterns.
Symmetry Comparison
The following table highlights the key distinction in body symmetry:
| Feature | Deuterostomes (General) | Other Invertebrates (e.g., Cnidarians) |
|---|---|---|
| Primary Symmetry | Bilateral Symmetry | Radial Symmetry |
| Body Plan | Two mirrored halves | Arranged around a central axis |
This fundamental difference in symmetry, along with other embryonic development patterns, is crucial for classifying and understanding the evolutionary relationships among animal phyla.
