Question

1. although echinoderms lack backbones, which structure do they use for protection?

a hard plate
an exoskeleton
a radial canal
an endoskeleton

Explanation:

Echinoderms (like starfish, sea urchins) have an endoskeleton made of calcareous plates or ossicles beneath their skin, which provides protection. A hard plate is too vague, exoskeleton is for arthropods, radial canal is for water vascular system (not protection). So the correct structure for protection in echinoderms is an endoskeleton.

Answer:

D. an endoskeleton (Note: Assuming the options are labeled as A: a hard plate, B: an exoskeleton, C: a radial canal, D: an endoskeleton. If the original labels differ, adjust the identifier but the text is "an endoskeleton".)