When you consider evolution, then you have to take into account where all life came from...the Ocean. Specifically micro-organisms in the ocean, they've existed for tens of Billions of years but land organisms around 5 billion years and that's to do with retreating tides etc. So from bacteria a single cell organism you make your way to minor complex organisms with the basic of physiological furniture and that's the 'digestive tract' and these guys are basically worms. Then quite a few million stages after that are the insects, as they improve upon just having only an advanced digestive tract to include other higher ordered functions as well adaptive mechanisms such as flight. But life is more diverse than just one track so it branched off before land dwelling and more than likely we as a species were developed in the ocean as multi-functioning organ-ed fish then onto land as res-pirating fish (such as lung fish, the first example of amphibians) then to higher order amphibians such as salamander types. Of course much of the stages proposed by Darwinian evolution are speculative so you decide :D ik this answer is long but its kinda hard to explain in one short sentence XD
as a result of accumulation of waste products in the body , these substances are removed from the body via the skin hence they undergo cellular respiration
