acardius

noun

acar·​dius (ˈ)ā-ˈkärd-ē-əs 

: one of a pair of twin fetuses that develops without a heart and receives its blood supply from its twin through vascular connections on the placenta

… due to the parasitic relationship of the acardius, there is a high risk of mortality (50-75%) in the donor twin …

—Ona Marie Faye-Peterson et al., Handbook of Placental Pathology