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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