Amniotes (from Greek ἀμνίον "amnion", "membrane surrounding the fetus", earlier "bowl in which the blood of sacrificed animals was caught", from ἀμνός "amnos", "lamb") are a clade of tetrapod vertebrates comprising reptiles (class Reptilia), birds (class Aves) and mammals (class Mammalia). Amniotes lay their eggs on land or retain the fertilized egg within the mother, and are distinguished from the anamniotes (fishes and amphibians), which typically lay their eggs in water. Older sources, particularly prior to the 20th century, may refer to amniotes as "higher vertebrates" and anamniotes as "lower vertebrates", based on the discredited idea of the evolutionary great chain of being.