flukes

Lab Topic 19:
Animal Phyogeny:
Investigating the Evolution of Body Plan

Porifera

Cnidaria

Platyhelminthes

Phylum Nematoda

Phylum Synopsis: The 90,000 species of roundworms are remarkably similar; all have cylidrical, non-segmented bodies that are bilaterally symmetrical and covered by a tough proteinaceous exoskeleton called a cuticle. Body size is from less than 1 mm to more than 1 meter in length.The internal organs are free in a body cavity called a pseudocoel. They have a complete digestive tract running as a tube from the mouth to the anus. Body wall muscles are well developed but run only longitudinally along the body, not circularly around the body. This allows them only to thrash from side to side but not to extend forward as other worms do. They have a nervous system that coordinates movement but shows little cepahalization. Sense organs are not prominent. An excretory system is present. Nematodes lack a distinct circulatory and respiratory system. Circulation occurs when fluids in the body cavity slosh back and forth in the pseudocoel. Respiration occurs by diffusion of gases across the cuticle. Sexes are usually separate and internal fertilization results from mating. Development is direct to minature forms of the adults. The cuticle is shed to allow growth. Nematodes are found in aquatic habitats, moist soils, and as internal parasites of plants and animals. They play important ecological roles as decomposers of detritus and as parasites.

Modern molecular evidence (DNA sequence analysis) has lead to a hypothesis that nematodes may be regressive protostomes related to arthropods and together the two phyla are grouped in clade called Ecdysozoa, a term that reflects a common characterisitc of shedding the cuticle during growth periods.

Ascaris, representative genus

Ascaris are intestinal parasites normally found in horses and swine.

External Anatomy

A female Ascaris is shown of left and a male on the right.

What type of symmetry does this organism exhibit?

Do these animals show sexual dimorphism?

Note curved (hooked) posterior end of male.

What is the sex of this worm?

A. Male

B. Female

C. BothD. This organism does not reproduce sexually

The outside of the worm is covered by a cuticle with a mouth at the anterior end and an anus on the opposite end. The two are connected by a tubular digestive tract.

Photos of the anterior and posterior ends of a male Ascaris

How does the digestive system of Ascaris differ from that found in cnidarians and flatworms? Is it complete or incomplete? What does that mean?
What is the ccuticle made of?
When a nematode grows, does it shed its cuticle?

Digestive System

Cutting through the cuticle at the anterior end reveals the muscular, pumping pharynx leading into gut.

As the cut is extended, the tubular digestive system is see and then the smaller tubules of the reproductive system,

Do these animals have a coelom? What does that mean?
The body plan of a nematode is

(A) One-hole sac plan;
(B) channel network plan;
(C) tube-within-a-tube plan

To what phylum does this organism belong?
Describe the sketelal component of this organism
What type of musculature does this organism have:

A. No specialized muscles
B. Circular muscles only
C. Longitudinal muscles only
D. Circular and longitudinal muscles

What type of digestive system does this organism have?
Does this animal have a circultory system? How are the circulatory functions carried out?
Where is the heart located in this animal?

Reproductive System

Female system. As you cut through the cuticle along the length of a female, the tubular reproductive system is revealed. It consists of a Y-shaped uterus. Each of its arms connects distally to smaller oviducts that eventually join small tubular ovaries.

Male system. If your animal is a male, the tubular male reproductive system is revealed. How, from its external anatomy, can you tell if your specimen is male or female? What is sexual dimorphism?

Microscopic cross-sectional anatomy

Looking at a microscope slide of a stained cross section of an Ascaris shows that the tubes of the internal organs (reproductive and digestive) are free in a body cavity.

Stained cross section of a female nematode.

Point to the cuticle and longitudinal muscle layers in the body wall.

Identify the body cavity, digestive system, and reproductive system.

This organism is classified as having which of the following body cavity types:

A. acoelom

B. pseudocoel

C. eucoelom

D. none of the above

General and Comparative Questions

1. Describe the changes in the digestive sytem as you move through the series Porifera, Cnidaria, Platyhelminthes to Nematoda.
2. What are the advantages of a tube-within-a-tube body plan compared to a body plan with a sac-like gut. Give an example of animals having them for both types.
3. Contrast radial and bilateral symmetry. How is nervous system development correlated with bilateral symmetry?
4. What is a body cavity and why is one beneficial to an animal?
5. None of the animals studied in this lab (Porifera, Cnidaria, Platyhelminthes, or Nematoda) had organs specialized for gas exchange or circulation. How do these animals perform these important physiological functions?
6. Of the four phyla studied, which do you consider complex and which simple? Why?
7. Given the following hypothesis: There is no evidence that the four phyla studied in this lab can be arraged from simple to complex. What observations have you made in your investigation of these four phyla that would proove this hypothesis false?
8. How do nematodes feed?
9. Describe what the coeloms (body cavities) of Cnidaria, Platyhelminthes, and Nematoda look like and how they illustrate increasing complexity.
10. How do nematodes carryout the important functions of respiratory gas exchange and circulation?
11. What evidence from this lab do you have that nematodes have muscles and nervous systems?
12. Describe how the cuticle acts as a skeletal system.

For further information about nematodes click on :

National database of images at Ohio State University showing various species of parasitic round worms
USDA Plant/Soil Nematode Lab Homepage

Click here to return to Porifera
Click here to return to Cnidaria
Click here to return to Platyhelminthes

Click here to return to index page for Biology 201

Credits

Photos by Maria Oehler ,Allison Hall, and Linda Westgate
Text and layout by W. D. Dolphin