Download Phyla Annelida and Mollusca

Phyla Annelida and Mollusca
Phylum Annelida: Segmented Worms
Phylum Mollusca: Bivalves, Chitons, Snails, and Octopuses
Objectives
1. Understand the unique characteristics of Protostomes
2. Understand the unique characteristics of Phyla Annelida and Mollusca
3. Identify examples of each of the major classes of mollusks and annelids
4. Label both external and internal anatomy of earthworms and squid after in-lab dissections
Phylum Annelida
Figure 1. Examples of a polychaete, oligochaete, and leech (Class Hirudinea).
Class Polychaeta – Examples include clam worms and marine worms. They have parapodia, fleshy
appendages used for movement and respiration. Parapodia may have many setae, bristles or hairs.
Class Oligochaeta – An example includes the earthworm, Lumbricus terrestris. They have few bristles.
Class Hirudinea – An example includes leeches, which are primarily ectoparasites. They lack setae.
Hirudo medicinal can consume five to ten times their body weight in blood. Leeches are used to extract
fluid around an injury.
"Worm" is a non-specific term that may be applied to elongated animals of several different phyla.
Most of the large, familiar worms are members of the phylum Annelida — the segmented worms.
This phylum includes earthworms, leeches, and others.
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Annelids develop a third embryonic tissue layer between the outer and inner layers called the
mesoderm. This middle layer can differentiate into other tissue types such as true muscle and
connective tissue.
Key points
 Triploblastic: three embryonic germ layers.
 More embryonic tissue layers = more complex body.
 True coelom = more contact between different tissue layers = more complex
body.
 Any fluid-filled space in an animal's body can act as a hydrostatic skeleton if
muscles can compress the fluid and make it relatively rigid. They have the
capacity for more varied movements, organs, and body forms. This middle
layer also separates to form a body cavity — the coelom — that allows the
internal organs to move independently of the outer body wall.
Annelids provide the clearest examples of segmented bodies. The body is divided into segments by
internal septa, with the result that each segment has its own coelom. Segmentation in these animals is
obvious when you see it, and it’s also reflected in the way annelids move. Muscles surrounding the
body can squeeze the fluid in the coelom, stretching out the worm’s body. When the worm pulls
back, the segments can shorten. The fluid in each coelomic compartment acts as a hydrostatic
skeleton, becoming rigid when the muscles compress it. Each segment can lengthen or shorten
independently, making annelids much more effective at burrowing than are other worms that aren’t
segmented.
The segmented hydrostatic skeleton of annelids allows them to move in ways that non-segmented
worms (such as nematodes) cannot. Annelids aren't the only examples of animals with hydrostatic
skeletons. You'll see other examples in other kinds of animals -- including humans!
The segments of an annelid are connected by the intestine, circulatory system, and nervous system,
all of which run the length of the animal. Some segments are specialized, for example containing eyes
or the brain, but mostly they all contain the same structures.
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Earthworm Anatomy:
Figure 2. The internal anatomy of an earthworm.

Earthworms also have setae, but you'll have to look closely to see
them. These help the worms squeeze through the soil.

Earthworms are hermaphrodites – they make both eggs and
sperm (they don’t fertilize their own eggs, though). You may be able to
see the tiny openings that release eggs and sperm. Also note the
Figure 3. A cross section of an
earthworm.
clitellum, which secretes a mucus layer involved in reproduction.

Earthworms have a true coelom, lined with mesodermal tissue
both on the outer body wall and also on the intestine. Note the two distinctive tissue layers of
the intestine. Also note the typhlosole hanging down into the intestine to increase the surface
area of the gut. The inner lining of the intestine (an epithelium) is derived from endoderm,
while the outer layer facing the coelom is derived from mesoderm. Annelids have a closed
circulatory system: you can see a large blood vessel on the dorsal side and another on the
ventral side. The circulatory system is derived from mesoderm. There are no lungs for
respiration. Instead oxygen passes through the slimy skin layers.
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Annelid Survey
Earthworm (model).
Annelid preserved specimens, which include polycheates, oligocheates, and leeches.
Dissection of a Preserved Earthworm –
Earthworm dissection video, ~8 minutes https://www.youtube.com/watch?v=u9HHS1uPFSo
External Anatomy
1. Place a preserved 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. Observe all parts of the worm, externally. Locate the conspicuous clitellum, a saddle-like swelling
on the dorsal surface. The clitellum produces a mucus sheath used to surround the worms during
mating and is responsible for making the cocoon within which fertilized eggs are deposited. The
anterior of the animal is more cylindrical than the flattened posterior and is the closest to the
clitellum. The ventral surface of the earthworm is usually a lighter colour than the dorsal
surface. The mouth is located on the ventral surface of the first segment while the anus is found at
the end of the last segment. Find the anterior end by locating the prostomium (lip), 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. Look for the worm’s setae, which are the minute bristle-like spines located on every segment
except the first and last one. Run your fingers over the ventral surface of the earthworm’s body. You
should be able to feel bristle-like setae used for locomotion
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
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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.
Internal Anatomy
Hint: Position your preserved earthworm dorsal side up and pin it down through the first segment and then
again further back behind the clitellum. Cut a slit in the dorsal surface near the posterior pin. Using fine
scissors extend the cut forward to the first segment. Be careful not to cut too deep. Starting at the first
segment, cut the septa (thin membranes) that internally divide the segments, so the skin can be laid flat. Use
additional pins to hold the integument open and expose the organs. Continue to lay the skin back until you
have uncovered a centimeter or so of the intestine.
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.
Phylum Mollusca
Mollusk means soft, and the members of this phylum have soft bodies – but generally inside hard
shells. Mollusks have a true coelom, which seems to open the way to more complex development.
One key effect of the coelom is the circulatory system. Mollusks have circulatory system with a heart,
which develops in the coelom. Octopus and squid, the most active mollusks, have a closed circulatory
system similar to that found in vertebrates. Most other mollusks have an open circulatory system, in
which hemolymph is pumped by a heart but is not confined to blood vessels. In adult mollusks, the
coelom is small, typically just a space that contains the heart.
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The mantle cavity of mollusks provides an excellent example of the way a structure that functions a
particular way can change over evolutionary time to become some- thing quite different. In the
earliest mollusks, the mantle cavity was a space beneath the shell containing
the gills. The mantle cavity serves different functions in various classes of
mollusks, as described below.
Class Polyplacophora – An example is the exclusively marine chitons. Shell
Figure 4. A chiton.
consists of 8 plates. Chitons are grazers; they use a radula (hard, tonguelike
scraper) to scrape algae off rocks. The mantle cavity contains gills, used for gas exchange.
Class Gastropoda – Examples include
nudibranchs and freshwater and marine,
and terrestrial snails and slugs.
Gastropods often have a single shell,
often curled into a tight spiral. Many
Figure 5. A nudibranch and a land snail.
snails are grazers, like chitons, and scrape algae or plants with their radula. They have gills, used for
gas exchange, in the mantle cavity. Terrestrial gastropods don’t have gills; the mantle cavity functions
like a lung. Some gastropods, such as cone snails are predators.
Class Bivalvia – Examples include clams, oysters, scallops, and
mussels. Bivalves have two shells that can be closed together
tightly. Bivalves are filter feeders. They use siphons to pump
water into and out of their mantle cavity. In the mantle cavity, the
Figure 6. A bivalve
water passes over the gills. The gills not only perform gas
exchange, they also capture small particles of food from the
water.
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Class Cephalopoda – Examples include
octopus, squid, cuttlefish, and nautilus.
Cephalopods may have a single shell,
which becomes very small or disappears in
some cephalopods (such as octopus).
Figure 7. An octopus and a cuttlefish
Cephalopods are active predators, with
large focusable eyes, large brains, a closed circulatory system, and the ability to swim rapidly.
Cephalopods can pump water through the mantle cavity for gas exchange and also use this pumping
action for jet propulsion when swimming.
Keypoints:
• Three distinct tissue layers in embryo, leading to multiple tissue types
in adult.
 Symmetry: Bilateral, with cephalization.
 Body cavity: Coelom.
 Proto/deutero: Protostome (Lophotrochozoa).
 Digestive tract: Complete digestive tract (mouth at one end and anus
at the other).
 Other features: Body not clearly segmented.
• Mantle tissue secretes a shell (some mollusks don't have a shell).
• Mantle cavity contains gills.
• Many mollusks have a radula, which is a hard, tonguelike structure
used for feeding.
• Circulatory system may be open (for example, in snails) or closed (in
octopus).
Observe Mollusk specimens
Preserved specimens of various mollusks. Take a look at the various species. How would you
recognize them as mollusks? How would you know what class they belong to?
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Slides
Snail radula. (prepared slide). Snails use this hard,
tongue-like organ for scraping off bits of food to eat.
You may also be able to see the radula on live snails, if
they are feeding – especially if they are scraping algae
from the inside of a glass aquarium.
Figure 8. Micrographs of radulas.
Dissect Squid
Squid dissection video, ~15 minutes https://www.youtube.com/watch?v=OueQ9kU36i0
Model of squid anatomy. Note the mantle cavity, gills, siphons, foot, and gonad. These same basic
features are also found in most other kinds of mollusks, though they
may look very different.
External Anatomy
1. Locate the water jet. The water jet is found on the ventral side of the
Figure 9. A squid.
squid.
2. The tentacles (long) and arms (short) are attached to the head of the squid.
3. Find the two large eyes on the side of the head.
4. Locate the body, which is covered by the mantle, and locate the two fins.
5. Each arm has sucker disks, count the number of sucker discs on one arm: ______
Sketch the external view of the squid; label all the parts that are underlined above.
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Finding the Jaw
Open up the arms and remove any that are in your way. Deep in the middle
of the arms is the mouth and a beak-like jaw. These are located in a bulbous
structure called the buccal bulb. Use forceps to remove the bulb and then the
jaw (beak) draw the beak in the box.
Analysis
1. How many arms does the squid have? _____ How many tentacles? ______
2. What is the function of the arms and tentacles? ______________________
3. What is the function of the water jet? ____________________________
4. Name two features that are adaptations for the squid's predatory life. _________________________
5. Name two traits that the squid shares with other mollusks. ____________________________
6. To what kingdom does a squid belong? __________________ What phylum? ________________
What class? _________________________________
7. Name one other organism in the same CLASS ________________________________________
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Figure 10. The internal anatomy of a male (left side)
and female (right side).
Internal Anatomy
Procedure: Turn the squid ventral side up. Pull the mantle up with the scissors where the water jet is,
it should be loose and easy to pull up. Use scissors to cut from the water jet to the fins. Open the
mantle to expose the structures inside and pin.
Find the ink sac, this is a small dark sac near the water jet. Remove the ink sac and use your
dissecting needle to break the pouch. Write your initials on this paper in squid ink or just smudge the
paper.
2. Find the esophagus, this is best found by looking into the mouth and seeing where it leads. The
muscular mass that surrounded the beak can be pulled up (and out) to show the tube that is the
esophagus.
3. To find the stomach, follow the esophagus toward the posterior.
4. The anus empties into the water jet, use scissors to cut the water jet down the center so you can see
the small opening of the anus.
5. Locate the gills, these are feathery structures that may be hidden under other things, there are two
of them.
6. Follow the gills toward the interior to find an enlarged structure at their base: this is the gill
heart.
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7. All the way toward the fin is a whitish or yellowish structure: this is the gonad. The male gonad is
generally white, the female gonad is usually more yellow to clear. Is your squid male or female?
____________________
8. The hard shell-like structure that lies along the backside of the squid is the pen. See if you can
remove the pen in one piece. The pen serves to stabilize the squid while it swims (like our
backbone).
Phyla Annelida and Mollusca Worksheet
Sources
1. Lab content http://faculty.deanza.edu/heyerbruce/bio6a and
www.biologyjunction.com/earthworm_dissection.htm
2. Figure 1 www.biology.fullerton.edu, www.en.wikipedia.org
3. Figure 2 www.tutorvista.com, www.youtube.com
4. Figure 3 www.gwu.edu
5. Figure 4 www.science.kennesaw.edu
6. Figure 5 www.catalyst.uw.edu, www.uic.edu
7. Figure 6 www.museum.zoo.cam.ac.uk
8. Figure 7 www.faculty.college-prep.org, www.en.wikipedia.org
9. Figure 8 www.shells.tricity.wsu.edu
10. Figure 9 www.en.wikipedia.org
11. Figure 10 www.jb004k12.us, www.youtube.com
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