Which of these shapes does NOT have an internal skeleton?

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The choice indicating that the starfish does not have an internal skeleton is correct because starfish, which are echinoderms, possess an external skeleton made up of hard calcareous plates that form their structure. This external framework is not considered an internal skeleton since it does not reside within the body, unlike the internal skeletons found in fish, birds, and mammals.

In contrast, fish, birds, and mammals all have endoskeletons (internal skeletons) composed of bones and cartilage that provide support and structure to their bodies. The presence of an internal skeleton is a defining characteristic of vertebrates, which include fish, birds, and mammals. Therefore, the starfish stands out among these options as it relies on an external structure rather than an internal one for its bodily support, making it the correct answer to the question.