In most vertebrates the organs of external respiration are thin-walled structures well supplied with blood vessels. Such structures bring blood into close association with the external medium so that the exchange of gases takes place across relatively small distances. There are three major types of respiratory structures in the vertebrates: gills, integumentary exchange areas, and lungs
most vertebrates develop an outpocketing of pharynx or esophagus that becomes one or a pair of sacs (swim bladders or lungs) filled with gases derived directly or indirectly from the atmosphere. Similarities between swim bladders & lungs indicate they are the same organs.
Invertebrates are animals without a backbone. Invertebrates include organisms from the following phyla: Porifera, Cnidaria, Nematoda, Mollusca, Annelida, Arthropoda, Echinodermata, etc. 
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