| THE
BIOLOGY OF WORMS
The
night crawler and common redworm, have a distinct, darker
colored "head" end which does contain the primitive
"brain" of the animal, and this tends to be the end
of the worm that travels "forward" the most. The
"tail" end of the worm tends to be more flattened
than the head and lighter in color. Red worms (Eisenia
fetida), a very common composting worm, are smaller, bright
red and don't have a dark colored head end. Worms do have a
proper top (dorsal) and bottom (ventral) surface, they are not
just symmetrical tube-like organisms. The surface of the
worm's skin is smooth and slimy, but also has many tiny
bristles or "setae" protruding from it. These help
the worm move and serve to anchor it in its burrows for self
defense. The setae are part of the reason that robins have
such a hard time pulling worms out of the ground. If you place
a big night crawler on a piece of cardboard or paper, you can
hear the setae scraping as the worm crawls!
Basic
Worm Biology
| Without
going into a lot of details, here's a quick overview
of worm biology. The body plan of an earth worm
is a segmented tube. Each segment is a separate
fluid-filled compartment surrounding a central
digestive tract, or gut, which runs the length of the
worm's body. Many of the worm's internal organs are
also segmented, occurring as separate units in each
segment, but there is also considerable specialization
in the head end of the worm. The "brain",
"hearts" and other major organ systems are
clustered in the head end. Earth worms have no eyes,
but they do possess cells which are sensitive to
light. This is why when worms are brought out into the
light, they squirm and thrash around. Worms also don't
have ears, but they can feel vibrations in the ground.
While earth worms don't have lungs, they do need
oxygen to survive. Instead of lungs, worms have
developed the ability to absorb oxygen directly
through their moist skin, which is kept moist by
mucous secreting cells. If a worm dries out, it will
suffocate. |
It is
commonly believed that worms come out during a rain storm to
avoid drowning, but that's simply not true. Since earthworms
absorb oxygen directly through their skin, they can survive
for a long time in well oxygenated water. But why should
earthworms crawl around on the ground surface, exposing
themselves to predators? Well, precisely what they are doing
crawling around on the soil surface at night or after it rains
isn't entirely clear. Some researchers suggest that earthworms
may be looking for different kinds of food (lots of organic
matter gets knocked loose during a rain storm), while others
believe that they may be looking for potential mates. The
"mating theory" has been documented by several
scientists, although the results were not conclusive. It
appears that earthworms can find and mate more easily in the
moist 2-dimensional plane represented by the soil surface,
than while plowing through the more confining 3-dimensional
world beneath the soil. Several researchers have also noted
that most of the worms crawling around the surface tend to be
large, and presumably sexually mature, members of their
respective species. They noted that if there were
non-reproductive benefits to being above ground when it rains,
worms of all ages would be present. So, rather than being a
problem to overcome, the abundant surface moisture after a
rain may actually represent an opportunity for worms.
Worms move by a process known as "peristaltic
contraction". A worm's body is a fluid filled tube
divided into separate segments. There are circular muscles
that surround each segment and longitudinal muscles running
from segment to segment for the length of the worm.
Contraction of the longitudinal muscles shortens and widens
the segments of worms body. Circular muscle contraction
lengthens and narrows the segments. By alternating these
processes in waves down it's entire body length the worms
crawls forward or backward. Inside its tunnel, the widening of
several segments serves to anchor that part of the body
against the tunnel walls. The "leading end" segments
are then elongated by circular muscle contraction (squeezing),
pushing that end forward, and the "trailing end" is
drawn up by longitudinal muscle contraction.
Populations
The populations of earthworms vary dramatically with soil
conditions. Usually, about 70 earthworms can live per square
yard of habitat. However, most researchers estimate normal
populations at a more modest 10 - 15 per square yard.
Normally, earthworm populations increase from spring until
late fall. During the winter, hardships take their toll and
earthworm populations generally decrease, although this effect
is not as severe in Southern California owing to our
relatively mild winters. How each species of earthworm relates
to other species is uncertain. Each species probably has
optimal soil conditions, food types and climatic conditions
that will allow it to dominate the overall worm population in
its preferred habitat. In addition, earthworms can modify
their habitat, creating conditions which favor that species
and help it to maintain a competitive advantage over other
worm species.
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