SALLY BENSUSEN / SCIENCE PHOTO LIBRARY SALLY BENSUSEN / SCIENCE PHOTO LIBRARY
Skate embryo. Artwork of the embryo of a hedgehog skate (Raja erinacea) in its egg case showing it creating a flow of water to help it breathe. It has inserted a muscular appendage on its tail into one of the case's horns (upper left), which it beats rapidly. This creates a flow of oxygenated water (blue arrows) 100 times greater than the natural flow through the case, allowing the embryo to breathe even in slow-moving water. During its development, the embryo uses all four horns for this purpose. The muscular appendage is lost when the young skate hatches. The tough black outer layer of the case (cut away here) protects the developing embryo from predators.
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