All chordates studied to date, except tunicates, share a set of 13 Hox genes.
A gill is a respiratory organ found in many aquatic species that allows them to collect dissolved oxygen from water and expel carbon dioxide. Some species' gills, such as hermit crabs, have developed to allow them to breathe on land as long as they are maintained moist. A gill's microscopic structure exposes a huge surface area to the external environment.
The filaments and lamellae, with the exception of some aquatic insects, contain blood or coelomic fluid from which gases are transferred through the thin walls. The blood transports oxygen to the rest of the body. Carbon dioxide enters the water from the blood via the thin gill tissue.
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