MadSci Network: Evolution |
Heart and blood vessels evolved first to circulate blood in both invertebrates and vertebrates. As vertebrates evoved, when some "moved" from aquatic to teressestrial habitat lymphatic system evolved. It is evident from the fact that lymph nodes are present only in teresstrial vertebrates. Phylogenetic development of lymph node could be seen from tailless amphibians (Anurans). Ranid frogs and bufid toads possess rudimentary lymph nodes (Margaret & Rodney 1974; ISBN 0 216 90074 3). Presence of lymphatic system might have given the teressestrial animals an additional advantage to handle the "new" load of air borne pathogens. In invertebrates the heart with the "open circulation" handeled the mixed fluid - blood and lymph - hemolymph. It is not the lymphatic and blood vessels that were "mingled" once but the fluids - blood and lymph!
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