HOW AFISH BREATHES
Materials: A few gills from the head of a freshlycaughtfish, a dish of
water, a medicinedropper, yourhandlens.
Follow thisprocedure: Put the gills in a dish of water. Add adropper
full of water from time to time to keep the gills moist while you are observing
them. Observe the feather-like structureof the innercurve of the fish gills
with your handlens. Examine the tooth-like structureoftheouterarchof
the gills.
You will observe: Gills from freshly caughtfish willappearbrightred.
There is usually a double rowoffilaments (feather-likethreads)on theinner
curve. Theouter,more solidpartof the gills,appearsto have a rowoffine
tooth-likestructures.
The feathery filaments ofthe gills contain tiny blood vessels called
capillaries..Whenthe fish opens hismouth,water whichcontainsfluid oxygen
(and usually some food in the form of tinyplantsandanimals), is drawn in.
The water passes back over the gills and out through the two flaps
(opercula)on each sideofthe fish's head. As the water passes over the gills,
oxygen enters the blood capillaries in the filaments. Thus, the gills are the
respiratory(breathing) organs ofthe fish. Unlike human beings and other
mammals,most fish have no lungs.
The tooth-likestructureson theoutercurve of the gills are calledgill-
rakers. Theystrainoutthe tiny plants and animals in the waterthatthe fish
has swallowed. These tiny formsoflife are then directed, as food, into the
fish's digestive system.
STUDYING THE SKELETON OF A FISH
Materials:Ask a clerk at a fishmarketfor thebackboneofa codfish.
Examine it with yourhandlens.
You will observe: Separate bones with long projections on each side
fitting like a chain, one into theother.
Unlike the sea animals you collected (snails, oysters, clams and the like),
the fish is an animal with abackbone. Itis called avertebrateto distinguish it
from animals that do not have backbones. The backbone is made up of
separatesmall bones(vertebrae)which areseparatedby cushions ofcartilage,
soft bone-likematerialfound, for example, in the human nose. (You may
know cartilage asgristle.)