Q1.
Their bodies are cylindrical and are divided into segments. What advantages segmentation might offer to an organism? Observing how an earthworm moves may help you form a hypothesis.
Answer
Segmentation allows greater flexibility and more precise control of movement — because each segment can be operated separately instead of the whole body having to act as one block.
Watch an earthworm and you can see the mechanism. A wave of shortening-and-thickening passes down the body from front to back. At any instant some segments are long and thin (gripping the burrow ahead), others short and fat (anchoring). If the whole body had to contract together it could only shorten in one lump and would go nowhere. Because the body is divided, each segment has its own circular and longitudinal muscles and its own fluid-filled compartment of the body cavity (Fig. 12.15) to push against — so a wave can travel along the animal and drive it forward. The nerve cord runs the length of the body with a branch to each segment, which is what makes the timing precise.
The advantages, listed:
- Flexible, wave-like movement — essential for burrowing through soil.
- Precise local control, because muscles and nerves are repeated segment by segment.
- A safety margin — damage to one segment does not disable the whole animal.
- Room to specialise. Repeated units can later take on different jobs — which is exactly what arthropods do, where different segments become head, thorax and abdomen with legs, wings, antennae and mouthparts.
Note: annelids also show organ system level of organisation and possess a body cavity. The cavity matters as much as the segments: it separates the gut from the body wall, so the animal can move without squeezing its own intestine, and the fluid in it acts as a skeleton for the muscles to work against.