Arthropods show a stepwise growth curve because they:

1. An arthropod is enclosed in a rigid exoskeleton that cannot stretch. 2. Between moults the animal gains mass but its external dimensions barely change. 3. At each moult the old exoskeleton is shed and the body expands quickly before the new one hardens. 4. Plotting size against time therefore gives a series of flat sections and sudden rises, a stepwise curve. 5. Fish grow continuously throughout life, and humans follow a sigmoid curve with growth spurts, so neither is stepwise. _Source: NECTA ACSEE 2023 Biology 133/2, Question 3: Growth and Development_