Animals belonging to Phylum Chordata are fundamentally characterised by the presence of a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits. Amnion on the other hand is one of the four extraembryonic membranes or foetal membranes viz. amnion, chorion, allantois and yolk sac. The space between amniotic membrane and embryo is filled with amniotic fluid which prevents the desiccation of embryo. Amnion . is not a universal feature of chordates. Chordates are divided into Amniota and Anamniota based on the presence and absence of the amniotic membrane respectively. Anamniotes include agnatha, fishes and amphibians while amniotes include reptiles, birds and mammals.