Monotremes include the duck-billed platypus and the spiny echidna, both of which
ID: 271850 • Letter: M
Question
Monotremes include the duck-billed platypus and the spiny echidna, both of which are mammals, but lay eggs. Explain why these animals are considered mammals, and give a possible explanation of why there are not more egg-laying mammals.
Plants Animalia Bilateria Protostoma Deuterostoma Lophotrochozoa Ecdysozoa tation Pseudocoelom Radial Segmen symmetrySegmen- tation (in Pseudocoelom . tation Growth by molting Protostome development Deuterostome development Coelom Triploblasty (origin of mesoderm) Bilateral symmetry and cephalization Radial symmetry Diploblasty (ectoderm and endoderm) Epithelial tissue MulticellularityExplanation / Answer
Answer. Evolution progresses always from simpler to the complex side. Monotremes are considered as mammals because they have hair and have mammary glands (in their belly button) like that of mammals (mammary glands inside the chest nipples) .
As per evolutionary chart, they came just after the reptiles and birds, thus they retain their egg laying features with them and are oviparous in nature. There are only 5 living monotreme species.
As said evolution progresses towards the complex side thus most of the mammals are not egg laying as a better, much reliable process is them where a young one resembles the adult is developed and thus they are known as viviparous. Internal fertilisation is more safe for the growing embryo thus also higher mammals have evolved this process of vivipary.