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Lab 15 - continued Arthropods - Crayfish (Part 3 of 5)

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40 questions

Last Name, First Name

Class Period:

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On the top of page 190, there is a short answer question in the middle of the paragraph under "Arthropods". "Arthropods are segmented like the annelids, but specialization of segments has occurred. Explain."

Specimen provided:
garden spider

Subphylum

Appendages (attached to body):

Specimen given:
crayfish

Subphylum:

Appendages

Specimen given:
Grasshopper

Subphylum:

Appendages:

Specimen provided:
centipede

Subphylum:

Appendages:

Specimen provided:
millipede

Subphylum:

Appendages:

Crayfish (or "crawdads" or "crawfish" as they are sometimes called) are freshwater crustaceans that are very similar to lobsters. You can readily find these in freswater ponds and streams in much of the United Stataes. Watch for their chimney like mud mounds in the shallow waters. If you try to catch them, you will soon discover their amazing ability to make a hasty restreat-backwards!

The crayfish is an extraordinary example of the arthoropod characteristic of jointed legs. The crayfish has twenty pairs of appendages that have been modified for specific functions. Crayfish have the ability to regenerate lost body parts, so you may In this investigation you will locate and examine the main external structures and respiratory system.

Name the phylum to which the crayfish belongs.

The major characteristics of members of this phylum are

Describe the main evolutionary advantage of having jointed appendages and bodies.

Crayfish belong to class

The group of organisms to which the crayfish belongs is the largest of the plant and animal phyla. Indeed, of the millions of known species of animals, 3/4s are arthropods. In addition to crayfish, this group contains millipedes, centipedes, spiders, scorpions, horseshoe crabs, insects, and a little known animal called Peripatus which is considered to be the "missing link" between the annelids (segmented worms) and the arthropods.

The crayfish is a cannibalistic scavenger that lives on the muddy bottoms of streams and ponds. It emerges at night to feed on dead and decaying matter, insect larvae, and worms.

As an arthropod, the crayfish has a body protected by a hard exoskeleton made of chitin. Periodically, the crayfish sheds, or molts, its outer skeleton and grows a new one. After molting, the crayfish takes in large amounts of water to increase its body size before the new skeleton hardens. The crayfish body has five pairs of jointed legs and two segments, a fused cephalothorax, and a jointed abdomen. The carapace, a part of the exoskeleton, covers the cephalothorax. Crayfish breathe by means of gills. Tubular structures, called green glands, remove excess water and wastes from the blood. The "brain" of the crayfish consists of a pair of ganglia connected to a ventral nerve cord.

Describe an exoskeleton and where it is located on an organism.

The crayfish exoskeleton is made of ___________.

When a crayfish sheds its exoskeleton, the process is called molting.

Describe how a crayfish increases its body size after a molt.

Crayfish use gills for breathing.

Describe what the green gland does.

1) Obtain a specimen
2) Be sure to rinse your crayfish specimen before beginning work on it
3) Place the crayfish in the dissecting pan with its dorsal (back) side up
4) Feel the hard exoskeleton

5) The body is divided into two major regions: the abdomen and the cephalothorax which includes the head and thorax (chest). The cephalothorax is covered by a piece of exoskeleton called the carapace. Note the curved cervical groove that marks the division between the head and thorax. The pointed anterior end of the carapace is the rostrum. Beneath it are the eyes. Note that each compound eye is at the end of a stalk.

6) The segmented abdomen ends in a segment called the telson. Unlike the cephalothorax, the abdomen can be flexed.

Try to bend the cephalothorax and abdomen. Describe your results.

Compare and contrast the head-body relationship of the crayfish and humans. What do humans have in place of the cervical grove?

7) Locate the five pairs of appendages on the head region. First, locate the antennules in the most anterior segment. Behind the antennules, observe the much longer pair of antennae. The two shorter branced antenules are located between the antennae. The crayfish uses these structures for taste, touch, and smell.

8. Locate the mouth. Surrounding the mouth are jagged jaws called mandibles, sued for biting and chewing. Observe the mandibles, or true jaws, behind the antennae. Locate the two pairs of maxillae, which are the last appendages on the head region.

9. Observe the three pairs of maxillipeds on the thoracic portion of the cephalothorax. The mandibles are two pairs of maxillae and three pairs of leg-like maxillipeds; these structures are used to hold food.

10. Observe the walking legs with the large chelipeds, or claws or pinching legs, which the crayfish uses for defense and capturing prey. Behind these walking legs, locate the other four pairs of walking legs, one pair on each segment.

11. Use the walking legs to determine the sex of your specimen. Locate the base segment of each pair of walking legs. The base segment is where the leg attaches to the body. Females will have a feathery, flexible pair of appendages compared to the males that have hard and long appendages used to transfer sperm to the female. Use a hand lens to study the inside surface of the base segment of the third pair of walking legs. If you observe a crescent-shaped slit, you have located a genital pore of a female. In a male, the sperm duct openings are on the base segment of the fifth pair of walking legs. Use a hand lens to observe the opening of a male genital pore. Exchange your specimen with a classmate who has the opposite sex to observe the different appendages. (It is possible we may not have a male and a female with our specimens.)

Based on your observations, do you have a male or female?

13. Observe the six distinct segments on the abdomen. On the abdomen, note the small appendages called swimmerets. These are used in swimming and reproduction. In a female crayfish the first pair of swimmerets are small and feathery. In a male the first two pairs, which transfer sperm to the female, are larger. longer, ridgid and folded forward. Observe a pair of swimmerets on each of the first five segments. The swimmerets, on the first five segments also aid in respiration by moving water over the gills.
>>Grab the edge of the carapace on one side or the other of the crayfish and lift it gently. The gills are located under the carapace. The carapace is actually not "glued-down" to most of the cephalothorax.

14. Observe a pair of paddlelike appendages, the uropods, on the last abdominal segment. Locate the triangular-shaped telson in the middle of the uropods. The uropods and telson form a tail fin that is used to propel the crayfish backward through the water.

15. Turn the crayfish ventral side up. Observe the location of each pair of appendages from the ventral side.

From which view—dorsal or ventral—can you see the location of the appendages on the segments more clearly?

Skills Assessment - 10 pts After you have identified all the external features, show them to your teacher.

dorsal vs ventral side
cephalothrorax, head, thorax, abdomen, carapace, rostrum, cervical groove, telson, uropod, compound eyes, swimmerets, claspers, walking legs, antenna, antennule, 3rd maxilliped, cheliped (claws or pincers), mouth, mandibles, maxillae, sex appendage

Next, you will study the internal anatomy of a crayfish. If you must store your specimen until the next lab period, cover it with a paper towel dampened with water. Then place the specimen on the tray in a plastic bag. Close the bag.

Internal Anatomy of a Crayfish

1) Put on safety goggles, gloves, and a lab apron.
2) Use one hand to hold the crayfish dorsal side up in the dissecting tray.
3) With your other hand, use scissors to carefully cut through the back of the carapace along dissection cut line 1 shown in Figure 2. Cut along the indentations that separate the thoracic portion of the carapace into three regions. Start the cut at the posterior edges of the carapace, and extend the cut along both sides in the head region. CAUTION: Use sharp instruments with extreme care. Never cut objects while holding them in your hand. Place objects on a suitable work surface for cutting.
4) Use forceps to carefully loosen the carapace from the body. Be careful not to pull on the carapace too hard or too quickly. You could disturb or tear the underlying structures.

5) Place the specimen on its side, with the head facing left, as shown in Figure 3. Using scissors, start cutting at the base of cut line 1. Cut along the side of the crayfish, as illustrated by cut line 2. Extend the cut line forward toward the rostrum (at the top of the head).
6) Use forceps to carefully lift away the remaining parts of the carapace, exposing the underlying gills and other organs. The epidermis that adheres to the exoskeleton secretes the exoskeleton. Carefully remove any epidermis adhering to the internal organs.

7) Refer to Figure 4 to locate and identify the organs of the digestive system. Food travels from the mouth down the short esophagus into the stomach. Locate the digestive gland, which produces digestive substances and from which the absorption of nutrients occurs. Undigested material passes into the intestine. Observe that the intestine is attached to the lobed stomach. The undigested material is eliminated from the anus.
8) Refer to Figure 5 to locate and identify the organs of the respiratory system. Locate the gills, which are featherlike structures found underneath the carapace and attached to the chelipeds and walking legs. Observe that the gills occur in distinct, longitudinal rows.
>>The outer row of gills is attached to the walking legs. The outer row of gills are the podobrachia (foot gills). A constant flow of blood to the gills releases carbon dioxide and picks up oxygen.
>>Carefully separate the gills with a probe or dissecting needle, and locate the inner row(s) of gills. These inner gills are the arthrobranchia (joint gills) and are attached to the chitinous membrane that joins the appendages to the thorax.
>> cut off one of the gills and place in a petri dish with water in it to observe how it fans out; observe the numerous gill filaments arranged along a central axis.

The podobrachia gills are appropriately named because they are attached to the walking legs which act to expand and push water in and out of the gills.

How many podobranchia do you find in your specimen?

How many rows of arthrobranchia do you find in your specimen?

9) Refer to Figure 4 and identify the organs of the circulatory system. Locate the dorsal tubular heart and several arteries. The crayfish has an open circulatory system in which the blood flows from the arteries into sinuses, or spaces, in tissues. The blood flows over the gills before returning to the heart. The heart is within the pericardial sinus, or cavity, at the posterior edge of he cephalothorax. With the hand lens, observe the three pairs of ostia, or small openings, through which blood enters the heart. Observe as many arteries as you can without damaging the internal organs.

10) Refer to Figure 4 to locate and identify the organs of the nervous system. Find the ventral nerve cord that lies along the central midline of the crayfish. The thin plates of shell that partly cover the nerve cord in the cephalothorax make the cord diffficult to find in this area. Locate a ganglion, one of the enlargements, located on the ventral nerve cord.
>> Find the brain just above the esophagus. Carefully cut away the rostrum and any remaining carapace. Between the eyestalks is the brain, a small white mass. Note the nerves traveling from the brain to the eyes and antennae.

11) Locate the dorsal brain, which is located just behind the compound eyes. Note the two large nerves that lead from the brain, go around the esophagus, and join the ventral nerve cord. The compound eyes are made up of long visual rods. The outer surface of each rod is called a facet. Light is focused through each facet onto the retina, producing a fuzzy but wide-ranging image.
>> Remove an eye by clipping it at its base and examine it with the dissecting microscope. Note the numerous facets in the eye. Because the eyes are on movable stalks, the crayfish has a very wide field of view.

12) Refer to Figure 4 to locate and identify the organs of the digestive and excretory system. The blood carries cellular wastes to the disklike green glands. Locate these organs just in front of the stomach by carefully removing some of the muscle tissue in the anterior part of the cephalothorax. Locate the stomach, a large, thin-walled, two-chambered sac that is just above the mouth and esophagus. The large, yellowish-green digestive glands on either side of the stomach and part of the intestine excretes waste through pores at the base of each antenna.
>> Cut through the stomach, and notice in the anterior region of the stomach wall the heavy, toothlike projections, called the gastric mill, that grins up food. You should be able to see some grinding stones injested by the crayfish.
>> Using a probe, separate the digestive glands from the stomach to observe where the stomach joins the intestine. Follow the intestine along the length of the abdomen to the anus. The anus is located on the ventral surface of the telson.

13) Refer to Figure 4 to locate and identify the organs of the reproductive system. The gonad, either testis or ovary, is beneath and slightly anterior to the heart. The animal shown in the diagram is a male crayfish. If your specimen is a male, locate the testis. The testis is the long, white organ under the heart and a bit forward. The sperm ducts that carry sperm from the testis open at the fifth walking leg. If your specimen is a female, locate the bi-lobed ovary. It is in the same relative position as the testis, but the ovary appears as a large, reddish/pinkish/orange mass under the heart. Then locate the short oviducts that extend from near the center of each side of the ovary and open at the third walking leg.
>> Mating takes place in autumn. Sperm pass from the a male's testes through the ducts to the outside. Using the modified swimmerets, the male transfer his sperm to the female's seminal receptacle, where the sperm are stored over the winter. The eggs are not fertilized until the female lays them in April.

14) Exchange your specimen with a nearby classmate who has a crayfish of the opposite sex, if available. Then study its reproductive system.

Identifying Relationships: The feathery quality of the gills gives them a very large surface area. Why is this important?

Drawing conclusions: Is the crayfish most vulnerable to its enemies from the dorsal or ventral side? Why?

Research and Communication: The crayfish, like some other animals, can regenerate lost appendages. It can also undergo autotomy. Investigate autotomy in crayfish and report what it is and how it is helpful to crayfish.