Leydig´s Organ (named after the German histologist who first described it in 1857) is a unique structure that is only found in elasmobranchs (sharks and rays) but with some exceptions. Along with the spleen and special tissue around the gonads, this structure produces red blood cells and it is nestled along the top and bottom of the esophagus.
ources
http://www.elasmo-research.org/education/topics/p_blood_cells.htm
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