Mr and Mrs Smith's Life Science
Earthworms
Earthworms are invertebrates. That is, they do not have a backbone. Insects,
sea stars, spiders, jellyfish, and millipedes are other examples of invertebrate animals.
Segments
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Study the illustration of the earthworm. You will notice that
earthworms have long, cylindrical body that is divided into similar segments. The grooves that extend around the body of the worm show the arrangement of the segments. Some species of earthworms have a body composed of over 100 segments. |
Symmetry
Earthworms have bilateral symmetry. This means that if you cut the earthworm
down the centerline, the left side of the body would be identical to the right side.
Body Colors
One of the key features of an earthworm is the colour of its body. Some species of
earthworms
have a dark-red or red-violet body while other species are muddy-green.
However, there are
species that do not have these colors.
Prostomium
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Some species of earthworms have a tongue-like lobe above the mouth called prostomium. The prostomium is the first segment at the anterior (head end) of the animal. The prostomium is actually a sensory device. Earthworms do not have a nose, eyes, ears, or hands to gather sensory information about their environment. Instead, they depend on their prostomium and sensory receptors in their skin to "feel" their way through the soil. |
 Egg Sac |
The top-side of an animal is called the dorsal surface. For example, the fin you see in all shark movies shows the dorsal fin of a shark just before it attacks. The dorsal surface of some species of earthworms is darker than its ventral surface. When looking from the Dorsal side of the worm you can see the shape of the prostomium. Earthworms have different shaped prostomiums. |
Adult (sexually mature) earthworms have a distinct swelling called a egg
sac. The egg sac is located about one-third of the way down the earthworm. The egg sac
is often white or orange in colour. It produces most of
the material secreted to form earthworm cocoons. The egg sac forms a band that can be
flared, non-flared, saddle-shaped, or annular. It is generally found between segments 26
and 33.
 |
The egg sac is only found on adult worms. Young or juvenile worms do not have a egg sac. The egg sac of each species of earthworm has a distinct color, size, and shape. The diagram shows the shape
and structure of the egg sac. |
Setae
Each segment, except the first and last, have tiny
bristle-like structures
called setae. These structures help the earthworm to move and act to sense the
environment.
The number and arrangement of setae are important clues to
the
identification of earthworms.
Check out the patterns in the diagram.
 |
Each segment, except the first and last, have tiny bristle-like
structures called setae. These structures help the earthworm to move and act to sense the environment. The number and arrangement of setae are
important clues to the Check out the patterns in the diagram. |
Periproct
The periproct is the last segment of an earthworm. It contains the
opening for the elimination of waste called the anus.