"Few Setae"
Class Oligochaeta consist of earthworms and other
worms that live in terrestrial or freshwater environments. As shown by
the class name, these worms lack the bristles that allow movement and increase
surface area. There are some that are marine, but most live out of sea
water. Also, all oligochaetes are hermaphroditic, neither male nor female.
One oligochaeta, the 
earthworm,
extracts nutrients from the soil as it burrows, excreted from the anus.
Because the nutrients thatearthworms dig up are necessary for fertile soil,
these earthworms are an intricate part of farms, actually tilling the soil.
The common attricutes to oligochaetes are:
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Body color may be red, tan, brown or black.
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Cylindrical, thin (some are very thin), segmented body may be up to 5 inches.
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May have short bristles or hairs (setae) that help with movement (usually
not visible).
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Moves by stretching and pulling its body along in a worm-like fashion.
To get a good understanding of the oligochaeta worms,
the earthworm will be used as an example. The earthworm has more than 100
segments which divide the coelom into identical parts, except where parts
have been fused. Earthworms exhibit the
adaptation of cephalization. The anterior portion contains the sense organs
and is specialized for burrowing.
Earthworms move using circular and longitudinal
muscles that line the interior body wall. Setae anchors the worm and circular
muscles contract. This produces fluid pressure in the anterior coelom and
elongates the body while pushing the head forward. The longitudinal muscles
then take action by contracting and pulling the posterior along. Click
to see an example
picture of earthworm movement.
Earthworms feed on the soil and organic matter when
burrowing. The muscular pharynx sucks in the soil. It then passes through
the tube-like, elastic esophagus to the crop where the soil temporarily
stored. From there it travels to the gizzard where it is ground up, releasing
and breaking up organic matter. As it travels through the intestines, the
food is then digested and absorbed by the blood. Wastes are excreted through
the anus. Click to see an internal,
posterior portion of an earthworm.
Earthworms have a closed circulatory system. Blood
travels to the posterior through the ventral blood vessel and forward through
the the dorsal vessel. The major vessels are linked in the anterior by
the aortic arches. Click here for a detailed
cross section diagram.
All worms respire through the skin. Oxygen and carbon
dioxide diffuse across the moist skin. The skin must be moist for the exchange
to take place. Secretions of mucus and a thin cuticle keep the skin moist.
Earthworms
rid the body of waste using nephridia. Water can be reabsorbed by blood
vessels here. The remaining wastes are excreted through nephridia pores
on the ventral surface.
Earthworms are sensitive to light, touch, moisture,
chemicals, temperature, and vibrations. Many sense organs are present in
each segment. A pair of ganglia in each segment coordinates movements.
There is also a cerebral ganglion in the head to control total body actions.
The ganglia and the brain are connected via a ventral nerve cord.
All earthworms are hermaphrodites but self-fertilization
cannot occur. They mate by joining head to tail and forming a mucus coat
around part of their bodies. Sperm is injected into the coats and moves
to the seminal receptacle of the other worm. Eggs then move to the female
genital pore. A swelling around the sex organs called the clitellum secrets
mucus and a chitin sheath. As both worms wriggle to slip the sheath off,
the eggs and sperm are joined and fertilized. Click to see example
picture of earthworms mating. Cick here to see internal
reproductive organs.