CHAPTER 12 STUDY GUIDE
12.1
12.1
Introduction
A. A World of Nematodes and Position in the Animal Kingdom
1. Nine distinct phyla of animals belong to the pseudocoelomates.
2. Phylogenetic analysis suggests there are at least two large superphyla within protostomes.
3. Molecular evidence divides Protostomia into two groups that diverged in the Precambrian:
Lophotrochozoa and Ecdysozoa.
4. Ecdysozoa go through a series of molts; Lophotrochozoa possess a lophophore and trochophore
larva.
5. Rotifera, Acanthocephala, and Entoprocta belong to Lophotrochozoa.
6. Ecdysozoan pseudocoelomates include Nematoda, Nematomorpha, Kinorhyncha, and Priapulida.
7. Rotifera and Acanthocephala are closely related to Gnathostomulida.
8. The original blastocoel of the embryo persists as a space or cavity between enteron and body wall.
9. Since it lacks the peritoneal lining of a true coelom, it is a pseudocoel.
10. This defines the pseudocoelomates, a Protostomia division of the bilateral animals.
11. More complex animals have a true coelom formed from mesoderm and lined with a peritoneum
(Figure 12.1).
12. All have a body wall that is often syncytial, a dermis and muscles surrounding the pseudocoel.
13. Except in parasitic Acanthocephala, the digestive tract is complete.
14. Digestive tract, gonads and excretory organs are within the pseudocoel bathed in perivisceral fluid.
15. Eutely is a constant number of cells or nuclei per species, a property in several groups.
B. Biological Contributions
1. The pseudocoelom may be filled with fluid or gelatinous material with mesenchyme cells.
2. It generally provides greater freedom of movement.
3. There is space for more development of digestive, excretory, and reproductive systems.
4. It is simpler to distribute materials throughout the body.
5. This provides a storage area for waste products to be later discharged.
6. It serves as a hydrostatic organ.
7. A complete digestive tract is found in most phyla.
Phylum Nematoda: Roundworms
A. Characteristics
1. About 12,000 species are described; perhaps a half million exist.
2. They live in virtually all habitats in all biomes; topsoil may contain billions per acre.
3. Nematode parasites exist in nearly all animal and many plant species; they are economically and
medically important.
B. Form and Function
1. Distinguishing Characteristics
a. They have a cylindrical shape (Figure 12.2) and most are under 5 cm long..
b. They have nonliving cuticle.
c. They lack motile cilia or flagella.
d. Muscles in the body wall only run longitudinally.
2. They lack protonephridia; one or more large glands or similar structures serve for excretion.
3. The pharynx is muscular with a triradiate lumen.
4. The pseudocoel serves as a hydrostatic skeleton against which longitudinal muscles work.
5. An outer, thick, noncellular cuticle is secreted by the underlying hypodermis.
6. Compression and stretching of the cuticle returns the body to resting position when muscles relax.
a. As muscles contract on one side of the body, they compress the cuticle on that side.
b. The force of contraction goes through the pseudocoel stretching the cuticle on the other side.
c. The body returns to normal when the muscles relax producing a thrashing motion.
7. Digestion
a. The alimentary canal consists of mouth, pharynx, a non-muscular intestine, a short rectum and
the anus.
b. The pharynx sucks food in.
c. The intestine is one cell thick; food moves back as new food enters and the body moves.
d. Defecation occurs from opening the anus with pseudocoelomic pressure.
8. Some parasitic adults have an anaerobic metabolism without the Krebs cycle and cytochrome
system.
9. Free-living nematodes and free-living stages of parasitic nematodes or obligate aerobes utilize
both the Krebs cycle and cytochrome systems.
10. A ring of nerve tissue and ganglia around the pharynx lead to dorsal and ventral nerve cords.
C.
D.
E.
F.
G.
11. Most nematodes are dioecious with males smaller than females; the male has copulatory spicules
(Figure 12.3).
12. Fertilization is internal and eggs are stored in the uterus until deposited.
13. The cuticle is shed between each of four juvenile stages.
Some Nematode Parasites
1. Some nematodes are important pathogens of humans (Table 12-1); most are tropical.
2. Ascaris lumbricoides: The Large Roundworm of Humans
a. Ascaris lumbricoides occurs in up to 25% of people in some areas of the southeastern U.S.
(Figure 12.4A).
b. More than 1.2 billion are affected worldwide.
c. A. suum is found in pig intestines (Figure 12.4B).
d. A female Ascaris lays 200,000 eggs a day, passing out in the host’s feces.
f. Eggs survive long periods in soil.
g. When a host swallows embryonated eggs, juveniles hatch and burrow through intestinal wall.
h. Carried through the heart to the lungs, they break into the alveoli and are carried up to the
tracheae.
i. Coughed up and swallowed, they mature and feed in the intestine.
j. Infestations may block or perforate the intestines.
Hookworms (Figure 12.5)
a. The anterior end of these small (9-11 mm) worms has a hook-like curve.
b. Necator americanus is the most common hookworm.
c. The males and females are separate.
d. Large plates in their mouths cut into intestinal mucosa; then they suck the host’s blood.
e. They pump through more blood than they digest; heavy infections cause anemia.
f. Eggs pass in feces and juveniles hatch in soil where they live on bacteria.
g. If human skin comes in contact with soil, infective juveniles burrow through skin to blood.
h. Similar to Ascaris, they travel in blood to lungs, are coughed up to be swallowed, and
mature in the intestine.
Trichina Worm (Figure 12.6)
a. Trichinella spiralis is tiny but causes a potentially lethal trichinosis.
b. Adult worms burrow into the intestinal mucosa and females directly produce juvenile worms.
c. Juveniles penetrate blood vessels and circulate throughout the body to all tissues and spaces.
d. They penetrate skeletal muscle cells, redirecting gene expression of the musculature so it
loses its striations and becomes a nurse cell to the parasite.
e. When meat containing live juveniles is eaten, worms are liberated and mature in the intestine.
f. They infect humans, hogs, rats, cats and dogs; hogs can become infected eating uncooked
scraps of infected meat or rats.
g. There are four other sibling species with variable distribution, freezing resistance, etc.
h. Heavy infections cause death.
Pinworms (Figure 12.7)
a. It is the most common helminthic parasite in the U.S. but causes little disease.
b. Adults live in the large intestine and cecum.
c. Females, about 12 mm long, migrate to the anal region at night and lay eggs, causing itching.
d. Scratching the anal region contaminates hands and bedclothes.
e. Eggs develop rapidly and become infective within six hours at body temperature.
f. When swallowed, they hatch in the duodenum and mature in the large intestine.
g. Members of this order have haploid males from unfertilized eggs; females are diploid and
come from fertilized eggs.
h. Diagnosis is best done by the “scotch tape” method, applying the tape to the anal area to pick
up the shelled embryos, and observing them under the microscope.
i. Pinworms exhibit haplodiploidy.
j. Males are haploid produced parthenogenetically; females are diploid from fertilized eggs.
Filarial Worms (Figure 12.8)
a. Eight species of filarial nematodes infect humans; some cause major and serious diseases.
b. Wucheria bancrofti and Brugia malayi live in the lymphatic system.
c. The worms cause inflammation and blockage of the lymphatics.
d. Females release live young, tiny microfilariae, into blood and lymph.
e. Mosquitoes ingest the microfilariae when they feed; the worms develop to the infective stage
and move into the mosquito bite wound when it feeds.
f.
12.2
Elephantiasis is caused by repeated exposure; swelling and growth of connective tissue causes
enormous swelling of body parts.
g. River blindness or onchocerciasis is carried by black flies and infects 30 million people in
tropics.
h. Dog heartworm, Dirofilaria immitis, is carried by mosquitoes and is most common U.S.
filarial worm (Figure 12.9).
Phylogeny and Adaptive Radiation
A. Phylogeny
1. Analysis of rRNA gene sequencing suggests that after the ancestral deuterostome diverged,
protostomes split into two large groups: Ecdysozoa that molt and Lophotrochozoa that exhibit
lophophore feeding and trochophore-like larvae.
2. Lophotrochozoa consists of several clades with pseudocoelomate animals.
a. Acanthocephala and Rotifera form the clade Syndermata with eutelic syncytial epidermis.
b. The clade Gnathifera consists of Syndermata, Micrognathozoa, and Gnathostomulida with
cuticular jaws.
c. Entoprocta with protostome cleavage and trochophore-like larva belongs within
Lophotrochozoa
3. Ecdysozoa pseudocoelomates include Nematoda, Nematomorpha, Kinorhyncha and Priapulida.
4. Loricifera placement is undetermined at present.
B. Adaptive Radiation
1. The basic pseudocoelomate plan is very adaptable with a cuticle, hydrostatic skeleton, and
longitudinal muscles.
2. Nematodes are by far the most adaptive of this set of organisms, mostly due to their ability to
survive suboptimal conditions.
3. A wide range of unusual strategies including cryptobiosis, hermaphroditism, alternation of
generations, etc., has helped keep some groups extant and adapted.