endothelia

from The Collaborative International Dictionary of English v.0.48

Endothelium \En`do*the"li*um\, n.; pl. {Endothelia}. [NL., fr.
   Gr. 'e`ndon within + ? nipple.] (Anat.)
   The thin epithelium lining the blood vessels, lymphatics, and
   serous cavities. See {Epithelium}.
   [1913 Webster]