Name _________________________________________ Date ______________ Period ______
Partner’s Name ____________________________________________________
Title: The Pseudocoelomate Body Plan: Aschelminthes
Learning Objectives:
1. Describe the evolutionary significance of the pseudocoelomate phyla.
2. Describe the organ-system level of organization.
3. Describe the nature of body cavities and the advantages they give organisms that possess
them.
4. Recognize organisms in the phylum Rotifera.
5. Recognize important structures of rotifers and describe the functions of those structures.
6. Recognize organisms in the phylum Nematoda.
7. Recognize important structures of Nematodes and describe the functions of those structures.
Pre-laboratory Quiz
1. A body cavity that is entirely lined by mesoderm is a _______________.
A. coelom
B. pseudocoelom
C. gut
D. cloaca
2. All of the following are true of body cavities EXCEPT __________.
A. they are usually filled with parenchyma
B. they provide room for organ development
C. they provide area for storage
D. they act as hydrostatic skeletons
E. they provide more surface area for the diffusion of gases, nutrients, and wastes.
3. All of the following are aschelmenthes EXCEPT ___________.
A. nematodes
B. rotifers
C. flukes
D. nematomorphs
4. The development of an individual from an unfertilized egg is called _____________. It is
encountered in this week’s laboratory investigation of the phylum _______________.
A. vitellogenesis/Nematoda
B. parthenogenesis/Nematoda
C. vitellogenesis/Rotifera
D. parthenogenesis/Rotifera
E. parthenogenesis/Nematomorpha
5. True/False The aschelminthes include a diverse group of closely related phyla.
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6. True/False All nematodes are parasites.
7. True/False Mesenteries are mesodermal sheets that suspend the gut tract of pseudocoelmate
animals within their body cavities.
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PHYLUM ROTIFERA
Rotifers are mostly freshwater pseudocoelomates that derive their name from a ciliated corona.
When cilia are beating, the impression is of rotating wheels. The body is elongate and consists
of a head, a trunk, and a foot. Rotifers are dioecious. Frequently, reproduction occurs by the
development of unfertilized diploid eggs. This is called parthenogenesis. In some species, males
are unknown. In other species, parthenogenesis occurs in the late spring and summer. In
autumn, parthenogenic males are produced. Fertilization results in a zygote that overwinters and
hatches into a female in the spring. This is an adaptation to withstand the harsh winter
conditions of temperate lakes. Other rotifers exhibit normal sexual reproduction.
Obtain a rotifer on slide and add a cover slip. Examine the specimen first under scanning and
low power. Many details of the anatomy will only be visible under high power.
1. Draw a picture of the rotifer. Label the anterior and posterior ends as well as the cilia,
corona, the head, the trunk, and the foot.
2. Observe the movements of the rotifer. Describe what you see. ________________________
_____________________________________________________________________________
3. What is the purpose of the cilia on the anterior end? Name two different purposes.
_______
______________________________________________________________________________
______________________________________________________________________________
4. What is the function of the foot? ________________________________________________
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5. Using your textbook, label the internal structures of the rotifer. Then COLOR CODE the
structures of the various systems as follows: digestive-red, nervous-blue, and reproductivegreen, excretory-orange. (Hint: Two structures will not be colored.)
6. What is the function of the mastax? ______________________________________________
7. What is the function of the pedal glands? _________________________________________
8. What is the function of the cloacal bladder? ________________________________________
______________________________________________________________________________
9. Organisms in the phylum Rotifera have several differences in digestion when compared with
Gastrotrichs. Name two of these differences. ____________________________________
______________________________________________________________________________
10. What is the habitat of most rotifers? ____________________________________________
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PHYLUM NEMATODA
Characteristics: Round in cross section, covered by a flexible, non-living cuticle; lack motile
cilia and flagella; longitudinal muscles only. Examples: Ascaris, Trichinella, Turbatrix
Ascaris—External Structure
Examine a preserved specimen of Ascaris. Nematodes are dioecious. Males are smaller, and the
posterior tip is hooked. Females are larger and lack the hook.
11. Is the preserved specimen male or female? ________________________
Note the cylindrical appearance of the worm. Distinguish anterior form posterior. The anterior
tip of both sexes is less pointed than the posterior.
12. Draw a picture of Ascaris. Label the anterior and posterior ends. Color your picture to
match the coloration of the specimen.
Free-Living Nematodes
Although Ascaris and other nematodes are parasitic, most nematodes are free-living. They occur
in virtually every aquatic and terrestrial habitat. One commonly used in the laboratory is the
vinegar eel, Tubatrix aceti. It is relatively transparent, thus showing internal structure in living
specimens. Unlike the Platyhelminthes, nematodes only possess longitudinal muscles just inside
the epidermis. When the muscles contract, they act against the hydrostatic skeleton. Because
there are no circular muscles, a thrashing movement occurs.
Make a wet mount of the vinegar eels. Examine it under low and high power.
13. Describe the movements of the vinegar eels. Describe what you see. __________________
______________________________________________________________________________
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14. Draw a picture of the vinegar eel. Label the anterior and posterior ends.
One member of each lab group should take the pH of the liquid in which these animals live.
15. What is the pH of the liquid? _____________ Is it acidic, basic, or neutral? ___________
16. What is the habitat of vinegar eels? _____________________________________________
Analysis and Conclusions
17. What is a hydrostatic skeleton? ________________________________________________
______________________________________________________________________________
18. What is parthenogenesis? _____________________________________________________
______________________________________________________________________________
19. Compare the movements of Planaria to vinegar eels. How are they different? ____________
______________________________________________________________________________
20. What is the anatomical reason for this difference in movements between the two creatures?
______________________________________________________________________________
21. List four advantages of having a body cavity. ____________________________________
______________________________________________________________________________
______________________________________________________________________________
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