In a specimen-based viva, the examiner describes this case: a crab exchanges gases using feathery structures associated with appendages under the carapace. Choose the concept that fits it best.

Aquatic crustaceans generally respire by gills. These structures are adapted for extracting dissolved oxygen from water. This MCQ belongs to the Crustacea topic in Animal Diversity-I and is written to test concept understanding rather than memorized wording.