Download 9. phylum platyhelminthes

PHYLUM PLATYHELMINTHES
The 18,500 species making up this phylum are known as flatworms. 80% of all
flatworms are parasitic while the remainder are free-living forms. Ranging in size from 1
millimeter to 12 meters in length, flatworms possess bilaterally symmetrical bodies that
are flattened dorsoventrally. Unlike Cnidarians, flatworms have three tissue layers
(triploblastic); the jelly-like mesoglea is replaced by a cellular mesoderm. Flatworms
lack segmentation and are acoelomate, i.e. possessing only one internal space, the
digestive cavity. The region between endoderm and ectoderm is filled with mesoderm
(mainly consisting of muscle tissue). The presence of excretory, digestive and central
nervous system demonstrates that this phylum has reached the organ system level of
organization.
Class Turbellaria
Turbellarians are usually free-living with soft bodies and a ciliated epidermis used for
locomotion. The mouth is located ventrally, some distance from the head and the
digestive system is incomplete (no anus is present). Genera commonly used in zoology
classes are Planaria (European) and Dugesia (Western Hemisphere). These turbellarians
are common in freshwater ponds and slow moving water hiding under rocks and
vegetation. The two above genera are nocturnal and feed on living forms (carnivores) or
on dead creatures (scavengers).
Place a live flatworm in a watch dish with enough freshwater to fully cover the animal.
Using you dissection microscope observe the triangular head with two lateral lobes or
auricles. These are olfactory organs. On the dorsal surface of the head are two eyespots
for the detection of light but not images. Answer the study questions below.
Figure 1. Draw Dugesia 15 cm long from a slide. Label the external anatomy
and the digestive system.
Study Questions
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What structural advances does this phylum show over the Cnidarians?
Is your specimen uniformly pigmented or is there a pattern?
Can they see?
Touch the head of your specimen with a dissecting probe. What happens?
Can the animal move backwards?
Does it always swim dorsal side up? Turn your worm over and see what
happens.
Gently touch your specimen in various places to see where the most
sensitive areas are located.
Place the point of your dissection needle directly in the path of a
swimming worm. How does the result relate to cephalization?
Stir the water to create a current. What does the worm do?
Observe the internal anatomy of a turbellarian using a prepared slide. Locate the
mouth and the muscular pharynx on the midventral surface. The intestine is 3-lobed
(trilobed) and each of these lobes is diverticulated to increase the surface area for
digestion and absorption of nutrients. Look for the lateral nerve cords and portions of the
reproductive system.
Study Questions
10. How do flatworms respire, excrete wastes, locate food, digest
food, and reproduce?
Class Trematoda
This class includes the flukes, leaf-like or cylindrical flatworms equipped with a pair of
suckers. Flukes are primarily endoparasitic with a few species ectoparasitic on the gills
of fish. They lack cilia on their body covering (tegument). Trematodes have a
fascinating and complex life cycle. Two or more hosts are involved. The final or
primary host is a vertebrate and intermediate host is a mollusc (often a snail). A typical
life cycle involves numerous stages: adult, egg, miracidium, sporocyst, redia, cercaria and
metacercaria. The egg exits the host via the feces and hatches in water as a ciliated
miracidium. This stage enters the tissue of the snail where it becomes a sporocyst, a baglike structure containing embryonic germ cells. The sporocyst reproduces asexually
producing more sporocysts, which then form rediae. The redia contain digestive, nervous
and excretory systems plus many germ cells. They reproduce asexually into additional
redia, and later these produce cercariae. The cercariae is a free-swimming larva that
emerges from the snail, and either enters another host or encysts on vegetation. Later,
they emerge as a juvenile flukes and eventually enter the primary host where they mature
into an adult. The sheep live fluke, Fasciola hepatica, causes liver rot and was the first
fluke life cycle to be fully understood. Below is the life cycle of Fasciola hepatica.
Another fluke, Schistosoma (formerly Bilharzia), a blood fluke causing Schistosomiasis
or in Africa, Bilharziasis.
We will examine a prepared slide of the human liver fluke, Clonorchis sinensis, a 1020 mm. parasitic flatworm common in Asia. Domestic animals also fall victim to this
parasite. The human liver fluke (also called the Chinese or Asian liver fluke) is spread by
improper unsanitary conditions when eggs are deposited into water where the snail,
Paraossarula spp. ingest the eggs which hatch inside these snails. The miracidium
burrows into the tissue of the snail and becomes a sporocyst, later redia are produced.
The redia divide into cercariae which escape into the water. Here they encounter a fish of
the family Cyprinidae and bore into muscle or under the scales and form cysts. If a
mammal eats raw fish (“sushi”), the cyst dissolves releasing the metacercariae which
travel to liver and bile duct to mature. Flukes can live up to 30 years in a host and up to
4000 flukes can infect an individual. Heavy infestations may lead to cirrhosis of the liver
and death. To avoid infection one should eat only thoroughly cooked fish and support the
campaign to eliminate intermediate hosts. Any strategy that disrupts the flukes life cycle
will eliminate the disease.
Examine a slide and observe the esophageal and the ventral suckers. The digestive
system consists of a pharynx, a muscular esophagus and two intestinal pouches or caeca.
The excretory system consists of two protonephridial tubules branching into flame
cells (these are not visible on your slides). It is possible to locate the excretory bladder
and the posterior excretory pore. A nervous system similar to that of turbellarians
consists of paired ganglia and paired lateral nerve cords with transverse connections
(commissures). A complex reproductive system found in these hermaphroditic animals
consists of two multi-branched testes located posteriorly that send sperm down the vas
deferens to exit via the genital pore near the ventral sucker. The ovary is found midbody. Along the lateral sides of the fluke are the yolk gland and yolk duct which join
the uterus near the ovary. The yolk gland or vitellaria produces a proteinaceous shell
material around the egg but does not produce yolk. A seminal receptacle can be found
posterior to the ovary and stores sperm received from another fluke. Eggs are fertilized
before they advance through the uterus. Anteriorly the egg filled uterus opens at the
genital pore.
Figure 2. Draw the human liver fluke, Clonorchis sinensis 15 cm long. Label
all internal anatomy.
Study Questions
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What are the anatomical differences between a turbellarian and a
human liver fluke? Why are there differences?
What are the three hosts in the human liver fluke life cycle?
What is the main purpose of intestinal caeca in Clonorchis?
What steps would you take to eliminate the human liver fluke from an
area? Remember you are an epidemiologist and someone is paying you!
Class Cestoda
The cestodes or tapeworms, are entirely endoparasites. They lack a digestive system
and the nervous system is greatly reduced. The body is covered by a thick, non-ciliated
living cuticle or tegument. The body plan is unique among flatworms. The anterior end
or scolex is a specialized organ equipped with suckers and / or hooks that attaches to
inner wall of the digestive tract of the host. The remainder of the tapeworm is a long
chain (up 12 meters) of connected units termed proglottids. Immediately behind the
scolex is the germinative zone from which new proglottids bud off. As this process
continues, the older proglottids move posteriorly. Tapeworms are not segmented, each
proglottid is actually an individual while the whole worm is a colony.
Tapeworms are usually monoecious (hermaphroditic). The anterior-most proglottids
contain ripe testes and the posterior proglottids have degenerated testes but ripening
ovaries. The posterior-most proglottids are packed with eggs (gravid) and break off the
tapeworm and exit with the feces.
Typically, the life cycle requires two hosts. The adult worm is a parasite in the
intestinal tract of a vertebrate. The intermediate host can be an invertebrate (i.e.fleas in
the dog tapeworm, Dipylidium canium), or a vertebrate (i.e. beef tape worms,
Taeniarhynchus saginatus , the pork tapeworm, Taenia solium, and the fish tapeworm,
Diphyllobothrium latum). Human infection from these species is usually the result of
eating raw or improperly cooked meat. The species used in lab is Taenia pisiformis
found as larvae in the mesentary and liver of rabbits and as an adult, in cats and dogs.
Obtain a whole mount of Taenia pisiformes and observe under 40X. CAUTION: this
slide is thick and can be crushed using a compound microscope.
Figure
Figure 3. Draw a scolex and the first three immature proglottids 6 cm long.
Label all structures.
Figure 4. Draw a mature proglottid 5 cm in size and label all structures.
Figure 5. Draw a ripe (gravid) proglottid 5 cm in size and label all structures.
Study Questions
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What are the functions of the scolex?
How are proglottids produced?
Which proglottids are the youngest and the oldest?
What is the sex of your specimen?
Do tapeworms suck blood? Do they have a mouth.
How do tapeworms obtain nourishment?
What structure prevents a tapeworm from digestion by the host?
Why do tapeworms lay so many eggs?
List six adaptations tapeworms have evolved for parasitic life.
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