COMPARATIVE INVERTEBRATE DISSECTION LAB
The unique structural adaptations that are used to help classify organisms are best understood with a
dissection of those organisms. Dissections provide a unique 3-D glimpse at the wonderful organization of a
living organism and provide a unique opportunity to correlate structure and function. You will be dissecting
three organisms – a grasshopper, an earthworm and a starfish. Each one of these organisms is a
representative of a larger group that has unique characteristics in terms of the phylogeny of animal groups.
For each organism:
1. classify that organism as fully as possible, from domain to species
2. which of the following terms apply to your organism:
parazoa vs. eumetazoa
radiata vs. bilateria
acoelomate vs. pseuodocoelomate vs. coelomate
protostomia vs. deuterostomia
3. explain the unique characteristics of your organism’s phylum and class
4. briefly explain how each organism:
a. obtains, digests, and distributes food to its cells
b. obtains and distributes oxygen and removes carbon dioxide
c. reproduces through sexual reproduction
Earthworm Dissection: (http://sps.k12.ar.us/massengale/earthworm_dissection.htm)
Phylum
Annelida
Class
Oligochaeta
Family
Lumbricidae
Genus species
Lumbricu terrestris
Background:
Among the most familiar invertebrate animals are the earthworms, members of the phylum Annelida. The
word annelida means "ringed" and refers to a series of rings or segments that make up the bodies of the
members of this phylum. Internally, septa, or dividing walls, are located between the segments. There may
be more than 100 segments in an adult worm. The clitellum is a swelling of the body found in sexually
mature worms and is active in the formation of an egg capsule, or cocoon. Eggs are produced in the ovaries
and pass out of the body through female genital pores. Sperm are produced in the testes and pass out
through tiny male genital pores. During mating, sperm from one worm travel along the sperm grooves to
the seminal receptacles of another worm. Fertilization of the eggs takes place outside the body as the
cocoon moves forward over the body, picking up the eggs of one worm and the sperm of its mate. The
pumping organs of the circulatory system are five aortic arches. Circulatory fluids travel from the arches
through the ventral blood vessel to capillary beds in the body through a closed circulatory system. The
fluids then collect in the dorsal blood vessel and reenter the aortic arches. The earthworm takes in a mixture
of soil and organic matter through its mouth, which is the beginning of the digestive tract. The mixture enters
the pharynx, which is located in segments 1–6. The esophagus, in segments 6–13, acts as a passageway
between the pharynx and the crop. The crop stores food temporarily. The mixture that the earthworm ingests
is ground up in the gizzard. In the intestine, which extends over two-thirds of the body length, digestion and
absorption take place. Soil particles and undigested organic matter pass out of the worm through the rectum
and anus. The nervous system consists of the ventral nerve cord, which travels the length of the worm on
the ventral side, and a series of ganglia, which are masses of tissue containing many nerve cells. Each
segment contains an enlargement, or ganglion, along the ventral nerve cord. Sense organs are concentrated
at the anterior end – cephalization. Excretory functions are carried on by nephridia, which are found in
pairs in each body segment. They appear as tiny white fibers on the dorsal body wall. The earthworm has no
gills or lungs. Gases are exchanged between the circulatory system and the environment through the moist
skin.
Procedure:
1. Use the following websites to help guide your dissection:
http://biog-101-104.bio.cornell.edu/BioG101_104/tutorials/animals/earthworm.html
http://www.staff.brookings.k12.sd.us/Reidell/Earthworm%20Dissection%20show_files/frame.htm
2. Place earthworm in the dissecting tray & rinse off the excess preservative. Identify the dorsal side, which
is the worm’s rounded top, and the ventral side, which is its flattened bottom. Turn the worm ventral side
up, as shown in the diagram below.
3. Use a hand lens as you observe all parts of the worm, externally and internally. Find the anterior end by
locating the prostomium, which is a fleshy lobe that extends over the mouth. The other end of the worm’s
body is the posterior end, where the anus is located.
4. Locate the clitellum, which extends from segment 33 to segment 37. Look for the worm’s setae, which
are the minute bristle-like spines located on every segment except the first and last one.
5. Refer again to the diagram of the ventral view of the worm to locate and identify the external parts of its
reproductive system. Find the pair of sperm grooves that extend from the clitellum to about segment 15,
where one pair of male genital pores is located. Look also for one pair of female genital pores on segment
14. There is another pair of male genital pores on about segment 26. Try to find the two pairs of openings of
the seminal receptacles on segment 10. Note: These openings are not easy to see. The seminal
receptacles store sperm from other worms (even though worms are hermaphrodites) Also present are large ,
cream colored seminal vesicles, that store sperm made in the testes (10/11). The ovaries are located at 13.
6. Turn the worm dorsal side up. Using a scalpel and scissors, make a shallow incision in the dorsal side of
the clitellum at segment 33. Using the forceps and scalpel, spread the incision open, little by little. Separate
each septum from the central tube using a dissecting needle, and pin down each loosened bit of skin.
Continue the incision forward to segment 1.
7. Use the diagram below to locate and identify the five pairs of aortic arches, or hearts. Then find the
dorsal blood vessel. Look for smaller blood vessels that branch from the dorsal blood vessel.
seminal vesicles and
seminal receptacles
8. Locate the digestive tract, which lies below the dorsal blood vessel. Refer to the diagram above to locate
the pharynx, esophagus, crop (stores food temporarily), gizzard (grinds food using soil particles), and
intestine (digestion and absorption).
9. To find organs of the nervous system, push aside the digestive and circulatory system organs. Use the
diagram below to locate the ventral nerve cord. Trace the nerve cord forward to the nerve collar, which
circles the pharynx. Find one pair of ganglia under the pharynx and another pair of ganglia above the
pharynx. The ganglia above the pharynx serve as the brain of the earthworm.
10. The worm’s excretory organs are tiny coiled, white nephridia. There are two in every segment except
for the first one and last three. Use the preceding diagram to locate some nephridia.
11. Use the diagram below to locate and identify a pair of ovaries in segment 13. Look for two pairs of tiny
testes in segments 10 and 11. To find these organs, you will again have to push aside some parts already
dissected.
12. Dispose of your materials according to the directions from your teacher.
13. Clean up your work area and wash your hands before leaving the lab.
Earthworm Questions:
If you need additional resources, try:
http://www.ucmp.berkeley.edu/annelida/annelida.html
1. Among the earthworm’s structural adaptations are its setae. How do you think the earthworm’s setae make
it well adapted to its habitat?
2. How is the earthworm’s digestive system adapted for extracting relatively small amounts of food from
large amounts of ingested soil?
3. What advantage does hermaphroditism have for organisms such as the earthworm?
4. In what ways does the earthworm show cephalization?
5. In terms of what you actually observed in your dissection, describe the “tube-within-a-tube” coelomic
body structure of the earthworm.
6. Your dissection of the earthworm did not go beyond segment 32.What will you observe if you dissect the
remainder of the worm to its posterior end?
7. On a separate piece of paper, draw and label the parts of the earthworm you observed, and color code the
systems. Use green for the reproductive system, yellow for the digestive system, blue for the excretory
system, and red for the nervous system.
Starfish Dissection
Starfish are unique among invertebrates in that they are deuterostomes. They have radial symmetry, a
calcareous endoskeleton with projecting spines and a water vascular system used in food manupulation and
locomotion. Starfish are interesting in being able to regenerate a whole organism from only one fifth of the
central disk that has at least one arm remaining attached!
Use the images at the site below to help guide you during your dissection
http://www.k-state.edu/organismic/echinoderms_and_protochordates.htm
1. Central disk
2. Arm
3. Madreporite
4. Ambulacral
ossicles
5. Endoskeletal
plates
6. Ampullae
7. Coelom
8. Anus
9. Pyloric
stomach
10. Cardiac
stomach
11. Digestive
gland
12. Gonad
13. Stone canal
http://sps.k12.ar.us/massengale/starfish_dissection2.htm
Procedure (Aboral Surface):
1.Obtain a preserved starfish and rinse off any preservative with water.
2.Place the starfish in the dissecting pan with its dorsal or aboral (top) surface upward.
3.Observe the starfish and determine its symmetry.
4.Locate the central disc in the center of the starfish. Count and record the number of arms or rays the
starfish has.
5.Locate the small, round hard plate called the madreporite on top of the central disc. Water enters through
this into the water vascular system. Label the central disc, arms, and madreporite on Figure 1.
6.Feel the upper surface of the starfish for spines. These spines protect the starfish and are part of their
internal skeleton. Label these on figure 1.
7.Look at the tip of each arm and find the eyespot. Label this on Figure 1.
Figure 1 -Aboral Surface
Procedure (Oral Surface):
Turn the starfish over to its ventral or oral surface (underside).
8. Locate the mouth in the center of the central disc. Find the ring of oral spines surrounding the mouth.
Label these on figure 2.
9. Find the groove that extends down the underside of each arm. This is called the ambulacral groove. Label
this on figure 2.
10. Feel the numerous, soft tube feet inside each groove. these are part of the water vascular system & aid in
movement and feeding. Label these on Figure 2.
At the end of each arm/ray is an eyespot containing a red pigment, that allows the starfish to detect and
respond to light. Spread the tube feet at the tip of the arm and observe the eyespot using the compound
microscope.
Figure 2 - Oral Surface
Procedure (Internal anatomy):
11. With the starfish's aboral surface facing you, cut off the tip of a ray. Cut along lines a, b, and c (Figure 3)
and carefully remove the aboral surface of each arm, loosening the delicate mesenteries beneath by which the
soft organs are attached.
Figure 3 - Cuts in Arm
Also cut transversely, at mid-length, through one arm of the bivium (2 arms closest to madreporite) to
provide a cross section. Cut a circular flap of skin from the central disc. (You will have to also cut around the
madreporite in order to remove this flap.)
12. Inside each arm, locate two long, greenish digestive glands called the pyloric/hepatic caeca. These
secrete digestive enzymes through hepatic duct to the pyloric stomach. Above (dorsal) the pyloric stomach
is a small pair of lobed rectal ceca, which function in waste consolidation. Ventral to the pyloric stomach is
the large bag-like cardiac stomach, which protrudes through the mouth while feeding. Digestion can begin
outside the body until the stomach is retracted by five pairs of retractor muscles, on in each arm.
13. Label these on Figure 4. Stomach - disc, thin, sac-like, and 5-lobed; cardiac portion= larger, with
pleated walls and retractor muscles; pyloric portion = aboral, smaller, 5-sided and smoother. Also - Intestine
- very slender, short, from pyloric stomach to anus. (hard to locate)
Also - Locate: Coelom or body cavity - space containing internal organs; lined with thin ciliated peritoneum
that aid in distribution of coelomic fluids.
14. Remove the pyloric caeca from the dissected ray. Find the gonads (testes or ovaries) underneath. These
may be small if the starfish is NOT in breeding season. Each is bilobed and attached by duct opening
aborally; sexes separate. Label these on figure 4. Remove these to see the rest of the water vascular system.
Figure 4 - Starfish Digestive & Reproductive Systems
15. Embedded in the soft body wall epidermis are endoskeleton plates called ossicles. Locate these and label
them in Figure 4. The largest ossicles support the ambulacral groove. Around the base of these spines are
jawlike structures called pedicellariae, which capture small animals and clean/protect the epidermis from
small larva that might settle on the starfish. Examine a small area on the aboral surface under binocular
microscope and distinguish the following:
1. Papulae or dermal branchiae - thin hollow soft projections from internal cavity providing a large
surface area, which function as gills.
2. Pedicellariae - minute pincers with two jaws; in circles around spines and elsewhere
WATER VASCULAR SYSTEM
16. Cut off the tip of a ray to observe the parts of the tube feet. Find the zipper-like ridge that extends the
length of the ray. The tube feet are attached to these.
17. Locate the bulb-like top of a tube foot called the ampulla. This sac works like the top of an eyedropper
to create suction. The bottom of the tube foot is a sucker. Label these in Figure 4.
18. Running down the center of each arm is a lateral/radial canal to which tube feet/ampullae are attached.
Label this in Figure 5.
19. In the central disc the five lateral canals connect to a circular canal called the ring canal, which is hard
and circular, around the mouth region. Find this canal & label it on figure 5. Also look for Tiedemann
bodies - nine, small swellings in the ring canal that are the source of ameboid cells that patrol the water
vascular system.
20. A short, canal called the stone canal leads from the ring canal to the madreporite where water enters.
Find this canal & label the stone canal & madreporite on Figure 5.
Remove the side of the stomach near the madreporite; then starting from the latter, trace the parts of the
system.
21. Draw an arrow on Figure 5 tracing the path that water takes when it enters & moves through the starfish.
Figure 5 - Water Vascular System
http://wwwbio200.nsm.buffalo.edu/labs/tutor/Starfish/
Discussion Questions:
For more info on starfish:
http://www.ucmp.berkeley.edu/echinodermata/asteroidea.html
1. What is the mode of action of the water vascular system?
2. How do the ampullae and tube feet act to affect locomotion? food taking? adhering to solid objects?
3. Describe the nature of the starfish “skeleton”.
4. How does the starfish exchange gases?
5. What structures are involved in digestion? Describe how they interact.
Grasshopper Dissection:
Introduction:
Insects are arthropods with jointed appendages, segmented bodies, and an exoskeleton composed of chitin.
They are bilaterally symmetrical and have a ventral nerve cord and brain. Insects are in the class Insecta, &
are the largest and most diverse group of animals on earth. The genus Romalea is a large grasshopper
common in the southeastern United States. Insects have three body regions (head, thorax, & abdomen), 6
pairs of legs attached to the thorax, a single pair of antenna attached to the head, mouthparts adapted for
chewing or sucking, and two pairs of wings. Some insects may have a single pair of wings or be wingless.
Insect legs are often adapted for digging, crawling, jumping, or swimming. The insects are mostly terrestrial,
they breathe air which enters small lateral openings on the body called spiracles and circulates in a system of
ducts to all organs and tissues. Their chewing or sucking mouth parts are adapted for feeding on plant or
animal materials.
Classification:
Kingdom – Animalia
Phylum – Arthropoda
Class – Insecta
Order - Orthoptera
Procedure (External Anatomy): Examine the entire grasshopper and identify the major subdivisions and
parts of the body.
1. Obtain a preserved grasshopper & rinse off any preservative with water. Place grasshopper in the
dissecting pan.
2. Observe that the body of the grasshopper is divided into 3 regions --- the head, the thorax, and abdomen.
Label these on Figure 2. Then look at the wings. The front wings are thickened into a tegmen and function to
protect the membranous hind wings. Note how the wings are attached so that they can pivot and fold over the
back.
3.Examine the head, which is composed of several fused plates called sclerites including the frons, clypeus,
and gena that compose the head capsule, and locate the following parts:
Antennae (two, slender appendages)
Compound eyes (2, large lateral)
Ocelli (or simple eyes) - 3, small, between compound eyes
Mouth parts - Labrum (upper lip), mandibles (jaws) below the labrum, maxillae located behind the
mandibles to help cut & hold food, and the lower lip or labium
1. Labrum
2. Mandibles
3. Labial Palps
4. Labium
5. Maxillary Palps
6. Maxillae
7. compound eye
8. ocelli
7. Label the mouthparts, eyes, and antenna on Figure 1.
8. Using forceps, remove each of the appendages from the head, and attached them to table 1. Cut open the
tough head capsule and look for the brain and examine the internal support structures.
9. Examine the following appendages on the thorax (middle section of the grasshopper's body):
THORAX
Legs (first 2 pairs are for walking & the last pair are for jumping)
Wings (forewings have a leathery appearance & protect the hind wings)
10. Using forceps, remove one of the walking legs and identify these parts --- the coxa connects the femur
(the thickest part of the leg) to the grasshopper's body; a slender, spiny tibia connects the femur to the tarsal
segments (lowest part of the leg). Label these on Figure 2.
11. Remove a jumping leg and attach the walking leg & jumping leg to Table 1.
12. Raise both pairs of wings and locate the first abdominal segment.
13. Locate the tympanic membrane or eardrum on the first abdominal segment. Label this on Figure 2.
14. Using a magnifying glass, locate the spiracles or tiny pores for respiration on each side of the thoracic
and abdominal segments. Label these on Figure 2.
15. Determine if your grasshopper is a male or female by looking at the end of the abdomen. Females have a
tapered abdomen that ends in a pointed egg laying tube called the ovipositor. Male have a more rounded
abdomen that turns upward.
16. Label the ovipositor on Figure 2.
ABDOMEN
Spiracles (small openings on the side of somites or body segments)
Auditory Organs (two located laterally on the 1st body somite or segment)
Ovipositor (on female)
Figure 1 - Grasshopper Head
Figure 2 - External Grasshopper anatomy
Table 1 - External Appendages of the Grasshopper
Antenna
Labrum
Mandible
Maxilla
Labium
Forewing
Hindwing
Walking Leg
Jumping Leg
Sex of Grasshopper
http://entomology.unl.edu/charts/refchrts.htm
The numbered structures are referred to in bold-faced type in the descriptions below.
1. head
2. thorax
3. abdomen
4. antenna
5. ocellus
6. compound eye
7. hindwing
8. tympanum
9. forewing
10. jumping leg
11. ovipositor
12. spiracle
13. segment
14. walking leg
15. coxa
16. trochanter
17. femur
18. tibia
19. tarsus
20. maxilla
21. mandible
Internal Anatomy:
8. Place the grasshopper right side up and carefully cut through the exoskeleton with your
scissors. Place some water in your dissecting tray, which will help float the organs, particularly the trachea
which will look like a mass of branched silvery tubes. These tubes extend inward from the spiracles and
distribute gases directly to cells.
If you are careful you will see the heart, which is a dorsal tube, often it gets removed with the exoskeleton.
The heart and aorta are part of an open circulatory system that bathes cells in hemolymph. Notice the many
muscles that are attached to the exoskeleton, try to follow some and discern their purpose.
9. Locate the digestive system. If your grasshopper is female everything may be covered with a large yellow
mass of ovaries, follow them back to the ovipositor. Then push the ovaries aside and look for the gut. The
esophagus leads from the mouth to the crop which functions to store food. Under the esophagus you may be
able to see very small grayish grape-like clusters of the salivary glands. This leads to the gizzard and to the
stomach. Just posterior to the gizzard, find the fingerlike projections called the gastric caeca. They are
attached at the junction between the gizzard and the stomach.
Follow the gut back to the hindgut where the very slender malpighian tubules, are attached. These act as a
kind of kidney for removing nitrogenous wastes in insects.
10. Remove the digestive system and look for the ventral nerve chord and the ganglia this will appear as two
white lines along the inside of the sternum which are periodically connected with larger white masses.
Discussion Questions:
http://animaldiversity.ummz.umich.edu/site/accounts/information/Insecta.html
http://www.ucmp.berkeley.edu/arthropoda/arthropoda.html
1. Which regions of the insect's body are specialized for sensory functions?
2. Describe the location and mechanism of the muscles used for movement.
3. Briefly compare compound eyes vs simple eyes.
4. Describe the relation of structure and function of the grasshopper's mouthparts.
5. How are the ends of the legs adapted for holding onto plants?
6. Describe the structural and functional adaptations of the insect digestive system.
7. Describe the differences between the two pairs of wings (appearance & function).
8. How might the adaptation of wings that fold backwards be explained from an evolutionary perspective?
9. Describe the location, structure and function of spiracles?
10. How did you determine the sex of your grasshopper?
11. What characteristics make the grasshopper and other insects well adapted for life on land?