Practice question

Aquatic arthropods predominantly respire using gills, diverse filamentous or lamellate outgrowths with large surface area and thin diffusion barrier. Crustacean gills classified as phyllobranchiate or trichobranchiate, located in branchial chambers ventilated by bailer appendage scaphognathite. Horseshoe crabs possess book gills, leaf-like lamellae attached to opisthosomal appendages. Book lungs and internal tracheal tubes are terrestrial adaptations reducing evaporative loss, forming air-filled invaginations with hemolymph counterflow. Cutaneous skin diffusion insufficient for larger active arthropods. Hence gill retention reflects secondary aquatic lifestyle and requirement for efficient oxygen extraction from water.