Evolution and control mechanisms of air respiration organs in invertebrates

The conquest of the terrestrial environment required numerous adaptations, although aerial respiration was what allowed animals to become completely independent of aquatic environments. Here, we analyze the specialized organs for air respiration, describing lungs, tracheas and some modifications of these organs that have occurred in certain gastropods and arthropods. These organs have appeared independently on numerous occasions, even in closely related taxa, and have experienced frequent regressions. In some cases, as in the case of the snail's palaeal cavity, the organs evolved from homologous structures, but independently in different lineages, with specific areas of the nervous system always being responsible for their control. Although the neurons that perform these tasks are ancient and may have a common evolutionary history, homologous visceral control structures do not seem to exist in vertebrates, nor among invertebrate phyla. This situation is clearly a consequence of the fact that respiration has evolved independently on numerous occasions, even within the same phylum, so that specific organs may be lost, or equivalent organs may arise independently and secondarily. However, as they arise from common ancestral structures, there could be cases of deep homology.