Re: Which came first--the lymph system or the blood vessels?

       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!