High Quality Content by WIKIPEDIA articles! The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother`s blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and lizards with varying levels of development up to mammalian levels. Note, however, that the homology of such structures in various viviparous organisms is debatable at best and, in invertebrates such as Arthropoda, is definitely analogous at best. However, a recent publication describes what amounts to a phylogenetically analogous, but physiologically and functionally almost identical structure in a skink. In some senses it is not particularly surprising, because many species are ovoviviparous and some are known as examples of various degrees of viviparous matrotrophy. However, the latest example is the most extreme to date, of a purely reptilian placenta directly comparable to a eutherian placenta.