The primitive structure of animals of the order Marsupials (kangaroo, koala) is the absence or underdevelopment of the placenta during pregnancy. Therefore, after a short gestation period, young marsupials are born small and underdeveloped. For a long time they are in a leathery bag on the mother’s belly, attaching with their lips to the nipple and warming themselves with the warmth of her body.
Animals of the order Insectivores (moles, hedgehogs, shrews, desman) are the most primitive group among placental animals. They have a small brain with a smooth surface of the cerebral cortex; their teeth are not differentiated into incisors, canines and molars.
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