ectomesoderm
From ZooTerms (Dictionary of Invertebrate Zoology)
ectomesoderm (noun; Greek ektos, outside; mesos, middle; derma, skin): Mesoderm derived chiefly from the ectoderm during early embryology of animals; forming mesenchyme predominently in Porifera, Ctenophora and in certain mollusks and annelids.
