Final answer:
The coelom in brittle stars is small because their arms do not contain any organs. Brittle stars have long, thin, flexible arms for movement and prey capture. Their small coelom is a result of their body structure and adaptation.
Step-by-step explanation:
Brittle stars have small coeloms because their arms do not contain any organs. As opposed to sea stars that have plump arms, brittle stars have long, thin, flexible arms that are sharply demarcated from their central disk. Brittle stars primarily use their arms for movement and grasping prey, rather than for housing organs. The small coelom in brittle stars is a result of their body structure and adaptation to their environment.