- Occurrence of heparin in the invertebrate styela plicata (Tunicata) is restricted to cell layers facing the outside environment. An ancient role in defense? Cavalcante, M.C., Allodi, S., Valente, A.P., Straus, A.H., Takahashi, H.K., Mourão, P.A., Pavão, M.S. J. Biol. Chem. (2000)