CRAYFISH DISSECTION
OBJECTIVE:
To examine the crayfish
CLASSIFICATION:
Kingdom - Animalia
Phylum - Arthropoda
Class
- Crustacea
MATERIAL:
Preserved crayfish, dissecting pan, scissors,
forceps, scalpel
PROCEDURE:
External Anatomy
With the specimen, locate the following structures and
label them on a drawing of the crayfish: rostrum,
cephalothorax, abdomen, telson, antennule, claw, walking
leg, compound eye, telson.
The crayfish serves as an excellent example to show
just what is meant by homologous organs. The appendages of
the crayfish have a similar structure and similar origin,
but they have a different function. To help you understand
this principle, carefully remove an antenna, walking leg,
maxilliped, and swimmeret with your forceps. Straighten
these out on a sheet of paper and draw them on a separate
sheet of paper.
Internal Anatomy
Read the following instructions carefully before
starting your dissection. Each of the main organs which
you are to find is underlined in the directions. You need
only locate the organs on your specimen, and then draw and
label them on a separate drawing. Label the heart,
esophagus, stomach, intestine, reproductive glands, and
ducts. In a dorsal diagram, label the digestive organs,
brain, nerve cord, ganglia, and kidneys.
Place the animal in the dissecting pan dorsal side up.
Carefully insert the point of the scissors under the top of
the shell at the back of the cephalothorax and cut up the
midline to the rostrum. Cut across the carapace just back
of the eyes and remove the two pieces of the carapace. Not
the exposed gills and study their structure. Remove the
gills and legs attached to the thorax. Carefully separate
the dorsal layer of muscles in the thorax and locate the
light-colored heart just underneath. Remove the heart and
cut along the sides of the thorax to expose the organs
underneath. the two light-colored masses extending on each
side of the body into the head are the digestive glands.
Between these will be found the small pair of white
reproductive organs and ducts in the male animal. The
female will probably have a dark colored mass of eggs in
the body which will have to be cleaned out before
proceeding further. This will destroy the reproductive
organs of the female.
To locate the intestine, insert the point of the
scissors under the dorsal side of the shell of the abdomen
and cut back to the telson. Spread the shell and the
intestine will be found as a tube on the top side of the
muscles of the abdomen. Trace it forward to the point
where the intestine joins the large, thin-walled stomach in
front of the cephalothorax.
Now remove all the organs in the thorax by cutting the
short esophagus below the stomach and the bands of muscle
holding the stomach just back of the eyes. You should be
able to lift out most of the internal organs in one piece.
Clean out the remaining tissue in the head so that the
green glands,or kidneys, just in back of and below the
antennules are exposed. In the front part of the head
cavity, between the eyes, note the small mass of white
tissue, the brain. Trace the nerves that go from the brain
to the antennae and the eyes. Cut the hard tissue on the
floor of the thorax with your scalpel so that you can
follow the nerve cord back from the brain to the abdomen.
Spread the shell of the abdomen apart and pull out the
large muscle. (This is the part of the body eaten in
shrimp and lobster). The nerve cord should now be exposed
on the floor of the abdomen. The
enlargements of the nerve cord in each segment of the
abdomen are the ganglia.
REVIEWING PRINCIPLES
To illustrate homologous organs, give the functions of
the following appendages:
1. antenna
2. mandible
3. maxilla
4. maxilliped
5. first walking leg
6. walking legs
7. first abdominal cavity
8. swimmeret