Some sharks and rays give birth to live young but lack placentas or umbilical cords. So how do their embryos obtain oxygen?
Taketeru Tomita of the Hokkaido University Museum in Hakodate, Japan, and his team used ultrasound to study a pregnant manta ray (Manta alfredi) to find out. They found that the embryo rhythmically raises and lowers its jaw, pumping uterine fluid into its mouth and through holes behind its eyes called spiracles. After birth, the spiracles shrink as the animal becomes able to force water over its gills by swimming.
Biol. Lett. http://dx.doi.org/10.1098/rsbl.2012.0288 (2012)