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

ExternalEarthworm.GIF (9765 bytes) 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

Anterior.GIF (3835 bytes)



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.

Prostomium.GIF (4009 bytes)



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.

Clitellum.GIF (4018 bytes)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.

 

ClitellumShape.GIF (8172 bytes)



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.
They may have any combination of shapes.

 

 

Setae

Setae.GIF - 4968 Bytes

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.

 

 

Setae.GIF (4968 bytes)



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.

 

Periproct
The periproct is the last segment of an earthworm.  It contains the opening for the elimination of waste called the anus.