A functional explanation for a sequence of developmental events in the carp. The absence of gills in early larvae.

The animal body is composed of structural elements which can be characterized by their size, position, external form, internal structure and their participation in functions. Their presence is taken for granted. The ontogenetic sequence of development of functional systems in fish shows an early absence of gills although effective and fast suction feeding movements are already found in 6 mm larvae. Calculations of the oxygen diffusion boundary layer in these larvae which would be present when oro buccal ventilation would occur show that no effective gill respiration is possible. The early absence and later development and use of gills for gas exchange reflects the changing demands of functions.