Download Sandworm Dissection - Manasquan Public Schools

THE CLAMWORM
To be handed in checklist:
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Description of behavior (paragraph)
Labeled external drawing
Labeled internal drawing
Post-lab questions
External Anatomy of the Clamworm/Sandworm (Nereis sp.)
The clamworm is marine living in sediment. Obtain a preserved clamworm and place it in
your dissecting pan. Add a little bit of water to your dissecting pan. Note the well-developed
head region (Figure 7) with tentacles and other sensory structures. You may also see jaws. If
you do not see the jaws at this point, you will see them later when you dissect your specimen.
Also, note that the body of the clamworm is distinctly segmented and that each segment bears a
pair of fleshy appendages, called parapodia. You will study the specific function of these
appendages later, when you observe the respiratory system. Protruding from the fleshy
parapodia are many setae from which the class derives its name (poly-many, chaeta-setae). You
will also observe the setae later when you examine a demonstration slide of the parapodium.
Figure 7: External features of the clamworm (A) and a close-up view of the head region (B).
Nereis sp.
Record your observations of the living worm and make a labeled drawing.
Internal Anatomy of the Clamworm
Digestive System
Place the specimen dorsal side up in a dissecting pan. Pin the anterior and posterior ends
of the animal and carefully cut through the skin to expose the internal organs as you did for the
earthworm. The marine clamworm, Nereis, is carnivorous therefore its digestive system differs
from that of the herbivorous earthworm, Lumbricus. The mouth and jaws are retractable
(Figure 8, 9). If the jaws are retracted, cut along the midline of the pharynx and pull the jaws out
with a pair of forceps. The jaws are used to capture small animals. The mouth opens into the
muscular pharynx, which leads to the esophagus. Lateral to the esophagus are two esophageal
caeca. Posterior to the esophagus is the stomach-intestine.
Figure 8: Internal Anatomy of the Clamworm.
Figure 8: A Photograph of a dissected anterior region of the clamworm (left) and exposed jaw
(right).
Suggest reasons for the differences in the organization between the digestive tract of Lumbricus
and Nereis.
With respect to diet and behaviour, why does Nereis have a much more elaborate sensory system
than does Lumbricus
Respiratory and Circulatory Systems
As you have observed earlier, each body segment of the clamworm bears a pair of lateral
parapodia, fleshy paddle like appendages that may be used as legs in crawling or as oars for
swimming. The parapodia have a large surface area, are well vascularized with blood vessels
and are the major sites of gas exchange (Figure 10). In addition, the movement of the parapodia
keeps fresh water with a high oxygen concentration constantly moving over the respiratory
surface. Parapodia can be considered primitive gills.
Examine a demonstration slide of an intact parapodium and note the bristle-like setae.
Figure 10: Cross-section of Nereis sp. showing the arrangement of blood vessels to the internal
organs (left) and showing parapodium (right).
In what ways are the parapodia of Nereis sp. morphologically suited to the function of gas
exchange?
Reproductive System
The reproductive organs in Nereis are difficult to see and will not be examined in this
dissection.
POST LAB QUESTIONS
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4)
What adaptations does the sand worm have for completing respiration (breathing)?
Describe the structures that enable locomotion. Explain the role of body systems at work.
How is the sand worm adapted to carrying out ingestion and digestion?
State and explain at least five ways in which sand worm structure leads to function that
makes them well adapted to their environment.
5) What hypothetical adaptation can you think of that could make them more successful?
Explain.
6) IMPORTANT - Make a Venn diagram that looks at the similarities and differences of a
Lumbricus (earthworm) and the polychaete you saw today.