Download Crayfish Dissection 2016

Crayfish Dissection
Names:
Place the crayfish in the dissecting pan ventral side down.
Examine the exoskeleton. Name two important functions of the
exoskeleton.
Locate the major body regions, the Cephalothorax and the
Abdomen. What two major body regions make up the cephalothorax?
Locate the antennae and the antennules.
Which are longer than the others?
How many antennae are present on your crayfish?
How many antennules?
Locate the eyes.
What type of eye does the crayfish have?
Find the carapace.
protect?
What important organ does the carapace
Count the number of segments on the abdomen.
there?
How many are
Locate the telson. What is its function?
Turning the crayfish over, locate and examine the appendages.
Identify the following appendages and the number of each:
Mandible:
Maxillae:
Maxillipeds:
Chelipeds:
Walking Legs:
Swimmerets:
The maxillipeds are sensory and also
function to shred food.
The maxillae pass food to the mouth,
where the mandibles grind it.
Which of the appendages does the crayfish use to capture prey
and defend himself with?
Examine the walking legs. Does anything about their structure
indicate that they have functions in addition to locomotion?
Explain.
On the abdomen are five pairs of swimmerets. The structure of
these appendages differs between the sexes. In females all the
swimmerets are similar and, as their name implies, function in
swimming. In males the first two pairs are modified and
function in transferring sperm.
What sex is your crayfish?
sex to examine.
Be sure you find an example of each
On the end of the abdomen is a tail fan, formed from the last
pair of abdominal appendages (called uropods) and a tail-like
extension of the last segment. The tail fan functions in rapid
backward locomotion. The tail-like extension of the last
segment is called the:
You are now ready to start the internal investigation of the
crayfish. Using your scissors, carefully remove the carapace.
If some material sticks to the carapace, use the teasing needle
to break it loose.
Circulatory system: within a cavity located dorsally, near the
center of the thoracic region, is the irregularly shaped heart.
It is tiny and looks like a bit of extra tissue, so remember to
look for it and not tear it up or throw it out. Oxygenated
blood from the gills enters the heart cavity, passes through the
openings in the wall of the heart, and is then pumped through
arteries to the tissues. The blood is returned to the heart
through the body cavity. There are no capillaries or veins.
What kind of circulatory system is this?
Reproductive system: the sexes are separate in the crayfish.
In most specimens the Y-shaped gonads (testes or ovaries) are
very difficult to see. They are embedded in the large white or
yellow digestive glands located just ventral to the heart. In
the male, sperm ducts lead from the testes to the base of the
special swimming legs; in the female, oviducts lead from the
ovaries to the base of the third pair of legs. During mating,
the male gives the sperm to the female in a packet. She holds
it until she is ready to release her eggs. The eggs are
fertilized as they move past the sperm. Crayfish do not spawn
because there is not enough plankton in fresh water to support
the larvae. The fertilized eggs stick to the swimmerets, where
embryonic development takes place. Tiny young crayfish may
cling to the swimmerets for about two months before becoming
independent. When a female holds her eggs, it is called:
Digestive and Excretory systems: anterior and ventral to the
heart is the large, saclike, two-chambered stomach. A short
esophagus connects it to the mouth. The foregut or cardiac
stomach is a large angular organ. If you tap on it with your
needle it feels hard. Why is this?
Posterior to the cardiac stomach is the midgut or pyloric
stomach. The large digestive gland is between the stomach and
the intestine. Why does it secrete enzymes or digestive juices
into the pyloric stomach?
Nutrients are absorbed from the rear portion of the stomach and
undigested particles pass through the intestine to the anus
(located ventrally on the last segment) where they are expelled.
If you like, you can remove the cardiac stomach, wash it out,
and see the chitinous grinding teeth that it uses to chew. You
may even see the bristles that are used to sort food.
Ammonia or urinary wastes are removed from the blood and body
fluids by a pair of green glands, which are found slightly
anterior and ventral to the stomach. They expel waste products
through small ventral pores, which are quite difficult to find.
What is this process of removing ammonia waste called?
Respiratory system: to see the gills, remove the side wall of
the carapace on one side. Note that some gills are attached to
appendages and others to the body wall. In the typical
crustacean, where are the gills attached?
Water is moved over the gills by the action of a scoop-shaped
structure, called a gill bailer, at the base of the second
maxilla. What part of the typical crustacean appendage is the
gill bailer?
Nervous system: carefully remove the stomach and note the
threadlike nerves, one on each side of the esophagus. Trace
them anteriorly; they fuse with the small, bulbous cerebral
ganglion or brain. In the thoracic region, the nerves fuse to
form the ventral nerve cord, which runs posteriorly to the
telson. Remove the exoskeleton on the abdomen. The enlarged
portion of the cord in each segment is called a ganglion.
Attached to the ganglia are small nerve branches, which control
the abdominal muscles. The abdomen is straightened by the
dorsal extensor muscle and flexed by the ventral flexor muscle.
The nerves are often hard to see, so do not worry if you cannot
find them.