** Usually have a structure analogous to the placenta seen in mammals connecting the mother's blood supply with the that of the embryo.
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* The embryos of some viviparous fishes exhibit a behaviour known as '''oophagy''' where the developing embryos eat eggs produced by the mother.
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* '''Intrauterine cannibalism''' is a rarely seen form of viviparity where the largest embryos in the uterus will eat their weaker and smaller siblings.