Origin theory of fins

a) Gill-arch Theory

• According to this theory, the paired fins are the modified gill-structures and the girdles represent the gill-arches.
• The position of the pelvic fins can be explained on the assumption that some of the posterior gill-archs have been shifted posteriorly.

b) Fin-fold Theory

• According to this theory, the paired fins have originated from a paired lateral fin-folds running down each side of the body behind the gillopenings up to the end of the tail.
• These lateral fin-folds are separated at the anterior part, but become fused posteriorly as a median ventral finfold.