PHYLUM NEMATODA
Members of the phylum Nematoda
are worms with long, cylindrical bodies and for this reason are commonly
referred to as roundworms. The phylum Nematoda is amoung the largest of the animal phyla. Although only
approximately 15, 000 species have been named, biologists estimate that if all
the species were scientifically described, the number would be closer to
500,000. Most nematodes are found free-living within the soil, but a great many
are important parasites of plants and animals.
Roundworms have two morphological advances
absent in the flatworms. First, in addition to their digestive cavity,
roundworms have a body cavity called a pseudocoelom
consisting of a fluid-filled space between the body wall and digestive
tract. The body cavity of these animals, the pseudocoelom,
is derived from the blastocoel of the embryo rather
than from a secondary cavity within the embryonic mesoderm (which results in a
true body cavity or coelom). The internal organs of pseudocoelomate animals are actually found free within the
fluid-filled body cavity, as there is no layer of mesodermal
epithlium lining the cavity and surrrounding
the organs. Second, nematodes have a complete digestive tract with a mouth and
an anus. Examine slide #
88 which is a cross section through the anterior end of the phlatyhelminth Ascaris
(which inhabits the intestines of both pigs and humans) and refer Figure 7 to
identify the cuticle, epidermis, pseudocoelom,
muscle cells, and the pharynx.
The specific feeding method utilized by a
particular species of nematode is a reflection of its habitat and way of life.
Therefore, nematodes exhibit a wide range of feeding habits: carnivorous,
herbivorous and parasitic. Although the diet may be extremely varied, the
nematode digestive system is relatively uniform throughout the phylum. The mouth
opens into a muscular pharynx, which acts as a pump to bring food
from the mouth into the intestine. From the pharynx, a long tubular
intestine travels the length of the body to the anus. Digestive enzymes
are produced by the single layer of epithelial cells lining the intestine.
Digestion begins extracellularly within the
intestinal lumen but is then completed intracellularly.
Nematodes lack respiratory and circulatory
systems. The majority of free-living nematodes are less than 2.5mm in length
and are often microscopic. These animals are largely the inhabitants of the institial spaces of aquatic sediments and soils. In the
soil, free-living nematodes live in the film of water surrounding each soil
particle, so they are essentially always in a moist environment. Free-living
species obtain their oxygen requirements by diffusion from the environment.
Species of nematodes that live as animal
parasites are facultative anaerobes.
We will examine the digestive and
reproductive systems in the common roundworm Ascaris,
one of the best-known parasitic nematodes. Males and females
live in the intestines of man where they graze on the intestinal contents. Eggs
pass out with the feces and if they contaminate food, are introduced to another
host. The larvae hatch in the intestine of the new host and then burrow through
the walls to be carried by the bloodstream to the lungs. At the lungs they
burrow through the alveoli (air sacs) and crawl up the trachea and down the
esophagus. Occasionally larvae get lost and crawl up the esophagus and exit the
nose. In some areas of the world Ascaris is so
common that a child is not considered to be part of the tribe until a larva is
sneezed out and found in the bed!
Use forceps to obtain one Ascaris for your group and place it in a dissecting
pan with a small amount of water. You will also need several dissecting pins.
Identify the three lips which distinguish the anterior from the posterior end
of the animal. Pin the anterior and posterior ends of the animal to the bottom
of the pan and carefully slit the body wall longitudinally using the fine
scissors in your dissection kit. Pin the sides of the wall open to expose the internal
contents. Be very careful not to remove or disturb any organs you will need to
identify. The intestine will appear as a thin ribbon-like structure that
extends from the mouth to the anus (see Figure 7). All other
internal structures are related to reproduction!
Ascaris
are diecious animals (i.e. have separate sexes).
Larger animals with the straight tails are females. The
thinner animals with hooked tails are males (see
Figure 7). When you are finished with your dissection, be sure to trade with a
group that dissected an animal of the opposite sex.
Female Ascaris:
Locate the genital pore (a slit-like opening about one third of the way
down from the anterior end and connected to a short vagina). The vagina
divides into two uteri. At the end of each uterine segment, a thin oviduct
can be seen, followed by the thread-like ovaries. Eggs are produced by
meiosis within the ovaries and move along the oviductrs
to the uterus. Fertilization of the eggs takes place in the uterus. Click here to see a photo
of the female Ascaris dissection.
**CAUTION: Ascaris eggs are extremely resistant to chemical
treatment. Although it is unlikely, some eggs may survive immerison
inpreservatives for short periods. For this reason
you should keep your hands away from your mouth and nose while performing this
dissection and wash your hands afterward.**
Male Ascaris: The anus serves both an excretory and a reproduction function. The copulatory spicule (a hook-like appendage used to hold the females's genital pore open) should be seen at the edge of the anus. At the posterior end of the intestine, you will find a branch. The area below the branch is the cloaca (which is connected to the anus on the outside). The cloaca serves as a common collecting area for fecal material from the intestine and spermatozoa from the seminal vescile (the thick organ connected to the intestine at the cloaca). The relatively thick tubes associated with the seminal vescile are the vas deferens. The finest of the threads are the testes. Sperm are produced in the testes and they mature as they move along the vas deferens to the seminal vesicle. During copulation, the sperm enter the cloaca before being deposited in the female. Nematode sperm are unusual because they are amoeboid, not flagellated. Click here to see a photo of the male Ascaris sp. dissected
Figure 7: Ascaris sp.
internal anatomy of female
cross section