Gill
The primary respiratory apparatus in fish, the
gills evolved to maximize extraction of oxygen
from water. The gills are also the sites for carbon
dioxide and ammonia removal from the body.
In most teleosts (bony fishes), there are four gills
on each side. Fine structure is comprised of
primary and secondary lamellae. (Fig 1) Primary
lamellae contain a specific type of cartilage (gill
cartilage) for structural support while secondary
lamellae branch off the primary lamellae. A
grouping of primary and secondary lamella (gill
filaments) can be thought of as having a "Christ-mas tree" (three-dimensional) structure. Because
the epithelium on secondary lamellae is
one cell thick and the surface area of the gills is
large, gill compromise is a constant threat.