Final answer:
Crayfish adaptations for aquatic life include gills for respiration, a protective and supportive carapace, and specialized muscles for swimming efficiently.
Step-by-step explanation:
Three structures of the crayfish that are adaptations to life in water include the gills, the carapace, and the specialized musculature for swimming. Crayfish have gills which allow them to extract oxygen from the water, an essential adaptation for aquatic respiration. The carapace is part of the crayfish's exoskeleton that offers protection and support; it is also often infused with calcium carbonate, which strengthens it against physical damage and reduces water loss. Additionally, crayfish have a system of muscles arranged in a manner that facilitates efficient movement in water, allowing them to propel themselves quickly and maneuver with agility.