Editing Chondrichthyes (section)

===Skeleton===
The skeleton is cartilaginous. The [[notochord]] is gradually replaced by a vertebral column during development, except in [[Holocephali]], where the notochord stays intact. In some deepwater sharks, the column is reduced.<ref>{{Cite book|url=https://books.google.com/books?id=cxxSN4YA2i8C&dq=Notochord:+Chimaeroids+%22Some+deepwater+squaloid,+hexanchoid,+and+lamnoid+sharks+%22&pg=PA23|title=Sharks of the World: An Annotated and Illustrated Catalogue of Shark Species Known to Date|first1=Leonard J. V.|last1=Compagno|first2=Food and Agriculture Organization of the United|last2=Nations|date=20 November 2001|publisher=Food & Agriculture Org.|isbn=9789251045435 |via=Google Books}}</ref>

As they do not have [[bone marrow]], [[red blood cell]]s are produced in the [[spleen]] and the epigonal organ (special tissue around the [[gonad]]s, which is also thought to play a role in the immune system). They are also produced in the [[Leydig's organ]], which is only found in certain cartilaginous fishes. The subclass [[Holocephali]], which is a very specialized group, lacks both the Leydig's and epigonal organs.