Not all animals have lungs. Many animals live in environments where lungs would be efficient enough for survival. Gills are another type of respiratory system, which are very efficient at removing oxygen from water: there is only 1/20 the amount of oxygen present in water as in the same volume of air. Gills greatly increase the surface area for gas exchange and they occur in a variety of animal groups including arthropods (including some terrestrial crustaceans), …showed first 75 words of 560 total…

…showed last 75 words of 560 total…