Why are gills more effective for aquatic forms?

Question

why not gills for terrestrial beings like us?

Answer 1

The main use of gills in most aquatic animals is to extract oxygen from water, not from air so this would clearly not work for terrestrial animals.

Also because the structure of gill is adapted for use in an aquatic environment i.e. one that allows things to float and not act under the effects of gravity, if terrestrial animals were to have gills said gills would most likely collapse or fail. This could only really be addressed if a species were to evolve specially reinforced gills and this seems unlikely to me as there is is no real use for an intermediary between regular gills and some form of reinforced ones.

Its should however be noticed that certain species such as the Caribbean Hermit Crab do have modified gills to allow them to live in humid conditions but as they are invertebrates the initial form of gill was different from vertebrate gills which if we were to have retained our gills when becoming terrestriallised, would have been the type we would have.

Hope that was a bit of help. Sorry if I made it a bit difficult to read.