Question: Segmentation in the body is first observed in which of the following?
A Platyhelminthes
B Aschelminthes
C Annelida
D Arthropoda
Answer: Annelida
| Phylum | Characteristics of Body Segmentation | Time of appearance | Examples |
| Annelida | · Repeated body segments with similar structures and functions. · Each segment typically has its own set of muscles and bristles (chaetae). | Cambrian period (541-485 million years ago) | · Earthworms · Leeches |
| Arthropoda | · Segmentation often fused into distinct regions such as head, thorax, and abdomen. · Appendages are jointed and specialized. | Cambrian period (541-485 million years ago) | · Insects · Arachnids · Crustaceans |
| Chordata | · Segmentation occurs during embryonic development, especially in the vertebral column (vertebrae). | Cambrian period (541-485 million years ago) | · Vertebrates · Invertebrate · chordates |
| Mollusca | · Some mollusks exhibit limited segmentation, particularly in the foot region. | Cambrian period (541-485 million years ago) | · Snails · Octopuses · Clams |
| Nematoda | · Pseudosegmentation: Internally, the body is divided into repeated units, but external segments are not visible. | Cambrian period (541-485 million years ago) | · Roundworms |
| Cnidaria | · Lack true body segmentation, but some exhibit radial symmetry with repeated parts (tentacles, segments). | Ediacaran period (635-541 million years ago) | · Jellyfish · Corals |
| Porifera | · Lack body segmentation; exhibit a simple, asymmetrical body plan. | Ediacaran period (635-541 million years ago) | · Sponges |
| Platyhelminthes | · Lack external segmentation; have a flattened body shape. | Ediacaran period (635-541 million years ago) | · Flatworms |