• skin -- some aquatic, or small terrestrial (some of the smallest spiders and mites, for example), animals can breathe simply by exchanging gas through the surface of their body
• gill -- many aquatic, and a few smaller terrestrial, animals use gills to breath. Yes, even land animals can do this, as with isopods like the woodlice you probably can find living under rocks in your yard. Gills are simply layers of tissue adapted specifically to gas exchange.
• book lung -- Some spiders, scorpions, and other arthropods still use primitive book lungs, essentially gills adapted for land use, in their respiration. These are simply tissue with many wrinkles to increase their surface area.
• Labyrinth organ -- A secondary breathing organ specific to the labyrinth fish, essentially an enclosed maze of tissue, evolved from a niche in their gill structure.
• Invertebrate trachea -- tubes evolved by many arthropods, possibly from book lungs, which simply lead directly into their bodies through holes called spiracles, where their internal organs generally absorb their own air. These can be very primitive, as with some spiders, or more complex, ending with specialized air sacs, as with many insects.
• lung -- The lung is made up of muscle tissues the cells inside the lung which collect the oxygen in the air pass it into the blood stream via vein and carbon dioxide passes out and that's breathing respiratory.

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