Question

Which of the following are amniotes?
A. Chondrichthyes, Osteichthyes, Amphibia
B. Reptilia, Aves, Amphibia
C. Amphibia, Aves, Mammals
D. Reptilia, Aves, Mammals

Answer 1

Amnion is a protective membrane that covers the embryo. It forms a part of the extra embryonic membranes. Amnion is present in certain vertebrate animals who are known as the amniotes. Eggs of Amniotic animals are adapted for terrestrial survival.

Complete answer:
The amnion is believed to have evolved from the amphibian ancestors who started laying eggs on land, rather than in water. Modern day amphibians and fishes lack the amnion since they lay eggs in water. Reptiles, birds and mammals have developed amnion as an adaptation for terrestrial survival.
Chondrichthyes and Osteichthyes are a class of fishes. The difference in these two groups lie in their endoskeleton. The Chondrichthyes have a cartilaginous endoskeleton like the sharks and rays, while the Osteichthyes have a bony endoskeleton as seen in Rohu, clown fishes, etc. Both these groups of fishes lack the amniotic membrane.
Amphibians include animals like frogs, toads and salamanders. They are generally found in freshwater habitats. Amphibians are modified for terrestrial living as well as aquatic living. However, with a few exceptions, amphibians lay their eggs in water. Therefore, amphibians lack the amniotic membrane.
The birds and reptiles lay eggs covered with a hard shell, while in mammals the young ones develop in their mother’s uterus. These animals have developed the amnion as a means of protecting their embryos from dehydration and desiccation.
Therefore, we can conclude that Reptiles, birds (Aves) and mammals have amnion while Chondrichthyes, Osteichthyes and Amphibians lack the amnion and are known as anamniotes.

Hence, the correct option is (D).

Note: Amnion forms an integral part of the extra embryonic membranes. It serves to protect the embryo. The amnion is present in certain vertebrates that either lay eggs on land or the development of the embryo takes place in the female uterus. The amnion is filled with a fluid called the amniotic fluid. As the embryo grows covered with the amnion, it inflates to form the amniotic sac. The embryo freely floats in the amniotic fluid inside the amniotic sac. The fluid serves as a shock absorber, protecting the embryo from desiccation and dehydration.