Aquatic arthropods are not like fish whose gills are in their mouths. The gills of crab, lobster, shrimp, prawn, and krill are at the base of their legs. Their gills develop as outgrowths of the first segment of the thoracic legs. For a prawn, the second maxilla in front of the first peripods have thin leaflike appendages that draw water into the gill chambers, and pull it out. All animals began breathing water. They say a fetus in the womb breathes amniotic fluid as its lungs develop and the contractions of birth push this fluid out to prepare for the first breath.