Fish Structure And Function

FISH have certain specialized function that enable them to survive them in water: gills instead of lungs, swim bladder to maintain buoyancy and the ''lateral line system'' to detect changes in the fish's surroundings by a form of echo location.


BREATHING

Fish ''breathe'' by drawing water in through the mouth and passing it over the gills.Oxygen in the water is absorbed by the gills filaments and then passed into the blood.Meanwhile, carbon dioxide and other waste are expelled.Some species have developed extra breathing organs for collecting oxygen in stagnating waters or in water with decaying plants, where oxygen levels are low.Members of the anabantid family , for example, have an auxilary organ near the gills that holds atmospheric gulped from the surface and extracts oxygen from it.The maze like organ has prompted the popular name of ''ladyrinth'' fishes.Some catfishes also gulp air and extract oxygen in a capillary-rich offshoot of the gut.

SWIM BLADDER 

Most fishes have gas-filled bladder that acts as a buoyancy compensation device, enabling them to maintain the position anywhere in the water.The bladder automatically infilates or deflates to give the fish neutral buoyancy ,equalizing the fish's weight with that if the surrounding water.Some fishes use their swim bladder to make or to amlify sounds.

THE GILLS

Gills absorb oxygen from water as it passes into the mouth and out through the gills cavity.

SIGHT AND SMELL

Sight is not important to fish as it is to humans, as many fish can navigate and locate in the darkest and murkiest waters by using their lateral line system to detect objects.A fish's eye do not needs eyelids because they are permenantly lubricated by the surrounding water.A fish's sense of smell is much more sensitive than a human, and a fish has taste buds , usually on barbels and fins.

LATERAL LINE SYSTEM

A fish's nervous system is linked to the outside world through perforations in single row of scales known as the lateral line.The single row runs horizontally along the length of the fish.Vibration caused by fish's own movement are reflected back from obstacles or by other fish and then detected by nerve endings deep inside the 'porholes' in the lateral line scales.

OSMOSIS

A fish's skin acts as a semipermeable membrane, or a one-way transfer system for water.Osmosis  causes the fluid to defuse through the membrane until there is unequal concentration on both sides of the membrane.The fluid of  freshwater fish's body is more concentrated than the liquid in which it lives.Thus, water constantly passes into the fish.To avoid bursting , freshwater fishes excrete as much watter as posssible and drink little.Conversely marine fishes loose water to the more concentrated sea water outside, and must drink constantly but excrete little.Few fish can pass from one type of water to other without distress.

FOOD-FINDING ORGANS

A fish's ability to taste food is aided by taste buds on the end of the barbels, such as those of the catfishes, and on the hair like cirri carried by blennies.Other fish ,notably the gouramies ,carry sensory cells on the tips of their pelvic  fins.