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Biology for Advanced Level Secondary Schools

which is a modified gill slit on top of the head that provides oxygenated blood directly
to the eye and brain through a separate blood vessel. The intestine is short but with
absorptive features increased by spiral valves.
Lateral line

Dorsal fin
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Eye

Caudal fin
Mouth

Gills openings Anal fin
Clasper (males)
Pectoral fin Pelvic fin

Figure 3.60: Structure of a shark
Class Osteichthyes (e) They have four pairs of visceral clefts
The class Osteochythyes includes all as separate gill openings covered by
bony fishes, such as tuna, tilapia, lungfish, opercula.
herring and catfish. They are all aquatic (f) They produce eggs and undergo external
organisms found in marine and freshwater fertilisation.
bodies. (g) They have swim bladder, which is

Distinctive features of class Osteichthyes in the abdominal cavity next to the
The following features differentiate members backbone. This helps them to rise and
sink (buoyancy) in water at particular
of the class Osteichthyes from other fishes. depth without using much energy.

(a) Most have distinctive scales that are (h) Most Osteichthyes have a terminal
either cycloid or ctenoid. These scales mouth.
are characterised by overlapping
arrangement. Cycloid scales are smooth- Structure of Tilapia
edged and rounded. Ctenoid scales have The body is laterally flattened and tapered
small comb like projections on their at both ends. It has a streamlined shape
edges. to overcome water resistance during
swimming. Its body surface is covered by
(b) They are bony fish, as they have bony cycloid scales, which point backwards in
skeletons.
order to reduce resistance during swimming.
(c) Most have paired pectoral and pelvic fins, The fish has a lateral line that runs along
supported by bony rays for swimming. the side of its body. The lateral line is a
(d) Most have homocercal (symmetrical) series of sensory organs called neuromasts
tail fins. that helps the fish to sense vibrations and
water pressure for navigating and locating

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