Science of Water : Concepts and Applications

198 The Science of Water: Concepts and Applications
to their body surface. These worms are commonly red because their blood contains hemoglobin.
Tubifex worms may be very abundant in situations when other macroinvertebrates are absent; they
can survive in very low oxygen levels and can live with no oxygen at all for short periods. They are
commonly found in polluted streams, and they feed on sewage or detritus.
(2) HIRUDINEA (LEECHES)
There are many different families of leeches, but all have common characteristics. They are softbodied,
worm-like creatures that are fl attened when extended. Their bodies are dull in color, ranging
from black to brown and reddish to yellow, often with a brilliant pattern of stripes or diamonds
on the upper body. Their size varies within species but generally ranges from 5 mm to 45 cm
when extended. Leeches are very good swimmers, but they typically move in an inchworm fashion.
They are carnivorous and feed on other organisms ranging from snails to warm-blooded animals.
Leeches are found in warm protected shallows under rocks and other debris.
(3) GASTROPODA (LUNG-BREATHING SNAIL)
Lung-breathing snails (pulmonates) may be found in streams that are clean. However, their
dominance may indicate low DO levels. These snails are different from right-handed snails because
they do not breathe under water by use of gills but instead have a lung-like sac called a pulmonary
cavity, which they fi ll with air at the surface of the water. When the snail takes in air from the
surface, it makes a clicking sound. The air taken in can enable the snail to breathe under water for
long periods, sometimes hours.
Lung-breathing snails have two characteristics that help us identify them. First, they have no
operculum or hard cover over the opening to the body cavity. Second, snails are either “righthanded”
or “left-handed”; the lung-breathing snails are “left-handed.” We can tell the difference by
holding the shell so that its tip is upward and the opening toward us. If the opening is to the “left”
of the axis of the shell, the snail is termed sinistral—that is, it is left-handed. If the opening is to
the “right” of the axis of the shell, the snail is termed dextral—that is, it is right-handed, and it
breathes with gills. Snails are animals of the substrate and are often found creeping along all types
of submerged surfaces in water from 10 cm to 2 m deep.
Before the Industrial Revolution of the 1800s, metropolitan areas were small and sparsely
populated. Thus, river and stream systems within or close to early communities received insignifi cant
quantities of discarded waste. Early on, these river and stream systems were able to compensate for
the small amount of wastes they received; when wounded (polluted), nature has a way of fi ghting
back. In the case of rivers and streams, nature provides their fl owing waters with the ability to
restore themselves through their own self-purifi cation process. It was only when humans gathered
in great numbers to form great cities that the stream systems were not always able to recover from
having received great quantities of refuse and other wastes. What exactly is it that man does to
rivers and streams? Man upsets the delicate balance between pollution and the purifi cation process,
unbalancing the aquarium.
REFERENCES
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American Public Health Association, 1981. Standard Methods, 15th ed. Washington, DC: American Public
Health Association.
Asimov, L., 1989. How Did We Find Out About Photosynthesis? New York: Walker & Company.
ASTM, 1969. Man on Water. Philadelphia: American Society for Testing and Materials.
Carson, R., 1962. Silent Spring. New York: Houghton Miffl in.
Carson, R., 1998. Preface, in Lost Woods: The Discovered Writing of Rachel Carson, Linda, L. (ed.). Boston:
Beacon Press.