aspects of fish biology form and function

309a
opens to the exterior through its own gill slit (figure 10). Normally
there are five gill slits on each side of the head, but in one genus
(Heptranchias) there are seven gill slits and in three genera
(Chlamydoselaches, Hexaohus and Pliotrema) there are six= The skeletal
structure supporting the gills is cartilaginous . The spiracles of the
sharks and rays are vestigial gill clefts.
The majority of fish draw water in through the mouth- The cavity
of the mouth and the cavity in which the gills lie act as a double-
chambered pumpo . Water is sucked in through the mouth, the mouth is
closed and compressed to force water into the gill chamber and the out
to the exterior. A flap of skin on the jaw acts as a valve to prevent
the escape of water when the mouth cavity is compressed, while the closed
gill cover prevents an inflow of water from the rear of the system. The
two chambers work slightly out of phase to produce a continuous flow of
water over the gills.
In certain fast swimming fish such as the tuna and mackerel
(e.g. Scomber australasicus) , the pumping system is dispensed with except
when resting. These fish swim with their mouths open allowing water to
flow freely into the mouth and over the gills.