NEMATODA
The phylum Nematoda is composed of a mixture of free-living and parasitic members.
Nearly 20,000 species have been described in this vast phylum, but biologists estimate
that there may be 4-50 times that number in existence. In fact, nematodes are so
numerous that if you were to remove everything else from the environment, the
remaining nematodes would form a ghostly skeleton outlining the entire terrestrial
biosphere! Nematodes possess two major morphological advances over the flatworms
you studied in the previous chapter. They have a pseudocoelom, a body cavity that lies
between a layer of mesoderm and a layer of gastrodermis, and a complete digestive tract
containing both a mouth and an anus. The evolutionary advantages of each of these
characteristics are discussed later. Whereas flatworms show tremendous diversity in
body form, nematodes tend to display more similarities than differences among the
numerous species. They all possess unsegmented, tapered, tubular bodies devoid of
appendages and covered by a thin cuticle secreted by the epidermis. The cuticle is
permeable only to water, gases and some ions and thus serves as a protective coating,
especially in parasitic forms. Another difference between nematodes and flatworms is
that in nematodes the sexes are usually separate (dioecious) and females are generally
larger than males.
A unique characteristic of this phylum is that each animal has a finite number of cells in
its body and this number is species specific (meaning that every member of the same
species has the same number of cells, but different species may have different cell
numbers). Cell division stops fairly early during embryonic development in nematodes
and future growth during later developmental stages is achieved through cell growth
(rather than through cell division). One common free-living nematode, Caenorhabditis
elegans, is transparent throughout development, making it possible for biologists to trace
the lineage of every cell from the zygote to the adult worm. Biologists have constructed
a fate map of the lineage of all 959 cells in this roundworm and have recently sequenced
its entire genome, making it the first multicellular organism to have its entire genetic
sequence decoded.
Although nematodes are usually only a few millimeters thick, they may range in length
from a few millimeters to several meters. One parasitic species that inhabits the placenta
of female sperm whales may attain a length of 9 meters! The infectious potential of
parasitic nematodes is staggering. For instance, in Africa alone, 40 million people are
presently infected with the larva of Onchocerca spp., a nematode which migrates to the
victims’ eyes causing blindness. Onchocerca infection is currently one of the major
causes of blindness worldwide. The disease commonly referred to as elephantiasis
results from blockage of the lymphatic system brought about by one of several nematode
species. This affliction causes substantial buildup of fluid and subsequent dense growth
of connective tissue, grossly deforming various body regions. This condition is one of
the world’s fastest spreading diseases and presently afflicts nearly 100 million people,
particularly children, in temperate regions of the globe.
Not only are their sheer numbers bewildering, but their ability to reproduce in large
numbers is equally impressive. A single female Ascaris releases some 200,000 fertilized
eggs per day (that’s 73 million per year!) Females of the guinea worm, Dracunculus
medinensis, live just beneath the skin in their human hosts. When the skin comes into
contact with water (as when the host bathes), the nematode protrudes is caudal end
through the sore on the host’s skin and can eject up to 1.5 million offspring into the water
during the course of a day. These offspring cannot directly reinfect humans, but instead
carry out the lifecycle in a species of microscopic aquatic crustacean. Humans are
infected when they drink the water containing these tiny arthropods. The cure for this
infection is simple but tedious. An incision is first made in the skin of the host near the
sore and the one-meter worm is slowly rolled out on a match stick a few centimeters daily
until the entire worm has been removed.
Nematode Anatomy
1.
Obtain a preserved specimen of the roundworm Ascaris lumbricoides. This is a
large, parasitic roundworm that infects humans and pigs. Be very careful when
handling preserved nematodes. Nematode eggs are extremely resilient and may
remain viable even after the females have been preserved. Keep your hands away
from your eyes, nose and mouth to avoid possible contamination.
2.
First determine the sex of your nematode. Males are typically smaller, have a
curved caudal end and have two small, spiny projections called spicules on the
ventral surface near the anus. These spicules are used during copulation.
3.
After you have identified the sex of your specimen, differentiate between the cranial
and caudal ends. This is more apparent on the males due to their curved tails. The
cranial end of both sexes is generally more pointed than the more blunt caudal end.
The anus is located on the ventral surface rather than at the terminal portion of the
tail—another useful feature for distinguishing which end is which.
4.
Use a dissecting scope to examine the cranial end of your specimen. Notice the
triradiate mouthparts (lips) surrounding the mouth. This is a distinguishing
feature of the phylum.
5.
Use a dissecting needle to gently scrape away a piece of the thin cuticle from the
body. Notice that it is composed of a transparent, proteinaceous substance. In
parasitic species such as Ascaris, this cuticle protects the animals from the digestive
enzymes of the host.
6.
Submerge your specimen in water in your dissecting pan and use a sharp dissecting
needle to make an incision through the body wall of the nematode near the mouth.
Keeping the roundworm immersed in water, draw the needle along the length of the
roundworm, extending the incision the entire length of the body.
7.
Use several pins to carefully open your specimen and secure it to the pan. HINT:
Pin your specimen near one edge of the dissecting pan so that you can easily view it
under the dissecting scope.
8.
Use the following figures and Table 10.3 to identify the internal anatomy of
Ascaris.
9.
View slides ZE 2-121 & ZE 3-12.
Table 10.3 • Anatomy of a Roundworm (Ascaris)
Structure
Function
Mouth
Ingestion of food
Pharynx
Muscularized region of digestive tract that “pumps” food through the mouth and
into the intestine
Intestine
Ribbon-like digestive tract where absorption of nutrients occurs
Anus
Elimination of indigestible wastes (egestion)
Lateral lines
Longitudinal canals that function as the excretory system of the roundworm,
releasing nitrogenous wastes in the form of ammonia and urea
Pseudocoelom
Body cavity lined on the inside by a layer of gastrodermis and on the outside by a
layer of mesoderm
Testis (male)
Produces sperm
Ductus deferens (male)
Stores mature sperm and transports them to seminal vesicle
Seminal vesicle (male)
Enlarged tube representing terminal portion of male reproductive tract which
transports mature sperm out of the nematode
Genital pore (female)
Point of entry for sperm and opening through which fertilized eggs are released
from the body
Vagina (female)
Terminal portion of female reproductive tract which receives sperm from males
and directs eggs through genital pore
Branched uterus (female)
Site where developing eggs mature before being released
Oviduct (female)
Repository for eggs produced in ovary until fertilization
Ovary (female)
Produces eggs