Which phylum is characterised by the presence of undifferentiated mesoglea between ectoderm and endoderm?
Cnidarians have a mesoglea between ectoderm and endoderm. They are diploblastic organisms.
Platyhelminthes and Aschelminthes are triploblastic organisms (with ectoderm, mesoderm and endoderm).
Poriferans have mesohyl between pinacoderm and choanoderm.