Cross references: Amniote
"The anamniotes are an informal group comprising the fishes and the amphibians, the so-called "lower vertebrates", which lay their eggs in water. They are distinguished from the amniotes, the "higher vertebrates" (reptiles, birds and mammals), which lay their eggs on land or retain the fertilized egg within the mother.
The name refers to the amnion, an embryonic membrane produced during the fetal development of amniotes which serves to transport oxygen into the egg and expel carbon dioxide. As the name suggests, anamniotes do not develop an amnion during fetal life and are able to exchange oxygen, carbon dioxide and waste metabolites with the surrounding water. The water helps in the diffusion of waste products (particularly ammonia), allowing the embryo to complete embryonic development without being poisoned by their own waste products. During their development, all anamniote classes pass through a stage which resembles fish, thus indicating their close physiological relationship."
"The group is characterized by retaining the primitive vertebrate condition in several traits: