Name
Marine Biology--Mr. Nelson
PHYLUM ANNELIDA LAB--Looking at Sea Worms
Background:
The organisms in the Phylum Annelida (Latin annellus, "ring") are referred
to as segmented worms to distinguish them from the non-segmented flatworms
and roundworms. This phylum includes the common earthworm, leeches, and a
large number of freshwater and marine species that are unfamiliar to most
people. The distinguishing characteristic of this phylum is metamerism, the
division of the body into similar segments that are arranged linearly along the
anterior-posterior axis.
The annelids are divided into three major classes:
1. Class Hirudinea (leeches)--Leeches represent a group of annelids
specialized for blood sucking. They may be found attached to marine
fishes and invertebrates. The mouth and the posterior end are
modified as suckers. They have no parapodia.
2. Class Oligochaeta (freshwater species and earthworms)-Conspicuous segmentation; lack a well-developed head; few setae
per segment; hermaphroditic. Usually found in fresh water or moist
soils.
3. Class Polychaeta (sea worms)--Practically all representatives of
the Phylum Annelida in the marine world are polychaetes. Each of
their body segments is provided with a pair of extensions known as
parapodia. Typical parapodia have stiff bristles, or setae. Parapodia
have many functions depending on the lifestyle of the polychaete.
They are flattened and are used as a further aid in locomotion in
crawling polychaetes but may be reduced in burrowing species. All
polychaetes feature a well-developed head with several pair of eyes
and sensory structures. Deposit and suspension-feeding species are
provided with various types of tentacles and feeding structures used
in the capture of food. Carnivorous species often sport a retractable
pharynx armed with jaws.
The polychaete, Nereis, will be used to represent the third class. It
resembles others in this phylum with its distinct segmentation and similar
structures found in each of these segments. There are many species of this
marine form, as well as a variety of habitats and life styles. The Nereis is very
active and a voracious predator. Other polychaetes are less so or burrowers,
each with adaptations to best fit its niche.
Objectives:
1. To identify the external and internal structures of annelids.
2. To recognize some of the major systems, functions, and associated structures.
Procedure:
External Anatomy
1. Get a preserved Nereis (sea worm) and a preserved earthworm to place on the dissection
tray. Mr. Nelson will come around to place a living Nereis on your tray.
2. Compare the Nereis to the earthworm. What external characteristic results in them both
being placed into the Phylum Annelida and not one of the other two phyla of worms (Phylum
Nematoda--the roundworms, Phylum Platyhelminthes--the flatworms)?
3. Notice that the sea worm's body is divided into segments. Measure the length of your
seaworm. Estimate the number of segments on a sea worm and record the number below.
Compare your number with two other groups.
Length:
# of segments:
Does it appear that as annelids grow in length, they add more segments or does each segment
get thicker?
4. Lay out both your sea worms on the dissecting tray with the ventral surface (mouth) down.
Figure 20-2 shows the dorsal and ventral view of the anterior (or head region) and may be
helpful in orientation. With your fingers or the forceps, gently squeeze the anterior region of
your living sea worm. Two sharp hooks (jaws) should stick out from the mouth, showing that
this is the worm's anterior.
5. Now that you have used the location of the mouth to determine dorsal vs. ventral. Describe
two other characteristics on the Nereis, which help to distinguish the dorsal vs. ventral
surfaces.
1.
2.
6. Besides the mouth, describe how one could recognize the anterior vs. posterior regions of a
Nereis?
7. Use the information below and your textbook, to correctly label the following parts which
have been left off Figure 20-2.
Label: Prostomium, mouth, cirri
1.
Notice that the proboscis is extended in the dorsal view. The Nereis is an active species and
has a well-developed head as well as various sensory organs: eyes and cirri (also called
tentacles), at the anterior end. Your specimen will most likely have its proboscis retracted with
only the mouth showing. The head is composed of two segments: the prostomium covers the
dorsal area over the mouth, and the peristomium that contains the mouth.
8. Use the information below, to correctly label the following parts which have been left off
figure 20-3.
Label: parapodium, setae, epidermis
All of the segments will look very much the same, except toward the posterior or tail end.
All segments, with the exception of the last, have parapodia on them. Parapodia are paddlelike appendages (one pair per segment) that also contain hair-like setae. The setae provide
traction and grip when the worm is moving over a surface. The skin of the worm is called
its epidermis.
9. The parapodia aid the worm in locomotion. Observe your living Nereis by placing it in tray
of salt water. Use the theme of “Structure is Related to Function” to describe how do the
parapodia seem to aid in locomotion in the water?
10. Use the internet, textbook and/or this lab to suggest a function for the setae (It is not
protection). Earthworms have them too.
Respiratory System:
The parapodia also act as gills and absorb oxygen from the water and transfer it into the
internal structures. The parapodia are much larger in some species as well as reduced in
others, with the earthworm not having any parapodia, only setae.
Internal Anatomy
12. Now we will look at some internal structures of the Annelids. Due the buoyancy of water,
the Neries does not support have to have the same level of support as the terrestrial
earthworm. Therefore, we will dissect the earthworm because its internal organs/structures
are more defined.
13. Take your preserved EARTHWORM specimen and stretch it out on your dissecting tray,
dorsal side up. Put a dissecting pin in the posterior end to anchor it. Insert another pin in the
tippity-tip-tip of the prostomium. This will stabilize your specimen while you expose its
internal organs. Don't worry it won't be too gruesome.
14. First we will practice. Take your scapula and start at the posterior end, cutting lightly
anteriorly. Remember the epidermis is very thin. Cut a section about 1.5 inches long and pin
back the epidermis with dissection pins. You will know if you go too deep, because wastes
from the cut intestine will be visible. Practice not cutting to deep as to damage internal
structures.
15. Now for the real part. Practice is over. Beginning at the tippity-tip-tip of the anterior end,
cut through the skin with the scalpel. Make the cut just slightly to the right of the midline
(center of worm, lengthwise). Carefully cut the skin without cutting the underlying tissues.
As you cut, pin the skin on both sides to the tray. Cut and pin back only the first quarter of the
worm.
Digestive System:
Its proboscis and jaws are located just within the mouth. Food enters the worm's mouth,
then it is digested in a food tube, the digestive tract. The mouth is connected to a wider part
of the digestive tract called the pharynx. The pharynx connects with a narrow esophagus,
located in segments 6 through 14. Food moves from the esophagus into the crop. The crop,
in segments 15 and 16, temporarily stores the food. Then the food passes into the gizzard,
where it is ground up with sand that is ingested with it. Next, the food passes into the
intestine, where it is further digested and then important nutrients are absorbed into the
blood. Finally, solid wastes are eliminated through the anus, located in the last two
segments. Note that the digestive tract is separated from the skin by the fluid-filled space
called the coelom. Figure 8-9 will help to orient you to the digestive system.
16. When you have located the crop and gizzard of the earthworm, invite Mr. Nelson over to
your group it tell you the function of the crop and gizzard. With a dull probe, gently touch the
crop and gizzard to compare their hardness. Based on the functions of the crop and gizzard,
which is harder and why?
17. Write the following Neries structures in order, starting at the anterior and moving
posteriorly, when the proboscis in not extended:
jaws, intestine, mouth, gizzard, anus, crop
18. As you studied in the Phylum Coelenterata, hydra, sea anemones, and jellyfish have a
two-way digestive systems, some animals (like you and I) have a one-way digestive system.
Which type is found in the Phylum Annelida?
19. Use your text or the internet. What does a Neries eat?
19. Review the function of the digestive system. What important digested substances get
absorbed from the intestine and carried to the rest of the worm's body by the blood?
Circulatory System:
Annelids feature a closed circulatory system, one where the blood is always enclosed in
blood vessels (open circulatory systems have no vessels; blood is just pumped through the
body cavity). The main structures of this system are the dorsal and ventral blood vessels
connected by lateral vessels along the intestine. In the anterior region, these lateral vessels
are enlarged into 5 hearts (see figure 8-9). These blood vessels carry important nutrients
(oxygen and food), to the various organs, and carry away liquid wastes.
20. Use the previous information, to correctly label the following parts, which have been left
off figure 20-4.
Label: blood vessel, hearts.
21. Using the diagram above, is the blood vessel you labeled the dorsal or ventral blood
vessel?
22. In your dissected worm, your best chance will be to see the dorsal blood vessel. Given the
way you cut the earthworm, why is it difficult to see the ventral blood vessel?
22. Worms in the Phylum Annelida have a closed circulatory system. What does this mean?
23. Do you have an open or closed circulatory system?
Excretory System:
Along with the circulatory system, the nephridia take care of filtering liquid wastes from
the body fluid and eliminating them. Pair of nephridia are found in each segment. Each
looks like a curled up tube. You will need the dissecting scope if you want to locate these.
Each nephridium has an external opening called the nephridiopore, located on the ventral
surface of the worm.
24. What structure in your body serves the same functions as the nephridia?
Nervous system:
The nervous system of this group is important for its increased complexity from lower
phyla as well as its similarity throughout the phylum. The paired ventral nerve cord (easily
noticed when lifting the intestine) gathers at the anterior end to become the cerebral
ganglion. This system can control the intricate movements of the muscles for movement and
the hydrostatic pressure (water movement) to maintain shape and structure within the
worm's flexible body.
25. Keeping in mind terms like anterior, posterior, dorsal, and ventral, how does the location
of the nerve cord in the sea worm compare to the location of the nerve cord in your body?
Muscular System:
Muscles are found within the walls of the worm and are used in intricate ways to move
around in its surroundings. The proboscis is also controlled by a set of muscles to reach out
and grasp its prey. When relaxed the jaws and proboscis are retracted into the mouth.
25. How does the living sea worm use its muscles to move when out of the water? Do you see
an "accordion like" motion or a "whip like" motion?
26. The Phylum Platyhelminthes contains "flatworms". One example is a tapeworm.
Tapeworms are parasitic and live in the intestines of many different species of vertebrates
(including marine vertebrates). The head region of a tapeworm has evolved to attach to the
inner wall of the intestine by hooks and suckers. Unlike the Annelid worms you looked at in
lab, tapeworms do not have a mouth or gut. Describe how a tapeworm obtains food and by
what process?