This exercises uses descriptions from Invertebrate Anatomy OnLine,
http://www.lander.edu/rsfox/310taeniaLab.html. It also relies heavily on images obtained from
http://www.biosci.ohio-state.edu/~parasite/images.html and life cycles images from
http://www.dpd.cdc.gov/DPDx/default.htm
The parasitic flatworms.
Introduction
Neodermata is a subgrouping that includes the parasitic flatworms, most of which are flukes (Clade or Class
Trematoda) or tapeworms (Clade or Class Cestoda). Most parasitic flatworms are endoparasites with complex
life cycles requiring multiple hosts including a definitive host inhabited by the adult worm and one or more
intermediate hosts inhabited by juvenile stages of the worm. The defining characteristic is the neodermis, or
tegument. Neodermis is an epidermis specialized for living in a potentially hostile environment from
which it must absorb food but reject toxins. The neodermis is a syncytium with its cell nuclei submerged
below the basal lamina.
Trematoda
Trematoda consists mostly of flukes belonging to Digenea. Digeneans are compact, bilaterally symmetrical,
endoparasitic flatworms known as flukes. A blind gut with mouth and pharynx is present but there is no anus.
Osmoregulation is accomplished via protonephridia. The integument is an elaborate syncytial neodermis
without a cuticle. There are no respiratory or hemal systems and hermaphroditism is the rule. About 11,000
species are known making this the second largest taxon, after nematodes, of parasitic worms. Size ranges from
less than 1 mm to 6 cm. Flukes are important animal parasites and they cause several important human and
livestock diseases.
At least two hosts are required to complete the life cycle and this fact is the basis for the name “digenea”. The
definitive host is always a vertebrate and the intermediate host is usually a gastropod mollusk. An additional
intermediate host, if present, is an arthropod or fish. In the definitive host the parasite typically inhabits either
the hemal system or the gut and its derivatives.
The adult Chinese liver fluke inhabits the bile ducts of the liver of any of several mammals including humans,
cats, and dogs. It is an important human parasite in the orient. It is estimated that this species infects more than
30,000,000 humans in these areas. The parasite also infects a number of other animals, including dogs, cats,
pigs, and rodents, and these animals serve as reservoirs of infection.
The adult worms measure between 10 and 25 mm in length. The do not actually live in the host's liver, but are
found in the bile ducts inside of the liver. Eggs are passed in the host's feces and the first intermediate host is a
snail. Cercariae emerge from the first intermediate host and infect the second intermediate host. The second
intermediate host is a fish, and over 100 species of fish are susceptible to infection. The definitive host is
infected when it eats raw or undercooked fish.
The sheep liver fluke causes fewer problems for its human host. The parasite resides in the bile ducts inside the
liver rather than the liver itself. This species is a common parasite of sheep and cattle and, therefore, relatively
easy to obtain. Infection in sheep and cattle results in animals that show low productivity (low weight gain, low
milk production, etc.). The adult parasites reside in the intrahepatic bile ducts, produces eggs, and the eggs are
passed in the host's feces. After passing through the first intermediate host (a snail), cercariae encyst on
vegetation. The definitive host is infected when it eats the contaminated vegetation. The metacercaria encysts
in the definitive host's small intestine, and the immature worm penetrates the small intestine and migrates
through the abdominal cavity to the host's liver. The juvenile worm penetrates and migrates through the host's
liver and finally ends up in the bile ducts. The migration of the worms through the host's liver and the presence
of the worms in the bile ducts, are responsible for the pathology associated with infection.
The schistosomes are unusual trematodes in that the sexes are separate (they are dioecious), they reside in the
blood vessels of the definitive host, and there are no second intermediate hosts in their life cycles. There are a
number of species of schistosomes that can infect humans, but most human infections are caused by one of the
three following species: Schistosoma mansoni; S. haematobium; S. japonicum. Considering the distributions of
all three species, schistosomiasis is distributed throughout almost all of Africa, parts of Southeast Asia, parts of
northwest South America, and some islands in the Caribbean Sea. It is estimated that approximately
200,000,000 million people are infected with schistosomes, resulting in 1,000,000 deaths each year. The life
cycles of the three primary species of human schistosomes are similar. The male and female worms average
about 10 mm in length and live in the veins of the abdominal cavity. Here they mate and the females produce
eggs. The adult worms can live 20-30 years and, depending on the species, and each female can produce
several hundred eggs each day. The eggs escape from the body by penetrating the walls of the veins and small
intestine or urinary bladder, and they are passed in the feces or urine. The eggs hatch in water, the first
intermediate host (a snail) is infected, and cercariae are liberated from the snails. When humans come in
contact with water containing cercariae, the cercariae penetrate their skin and they become infected. This
occurs when the humans swim, bath, wash clothes, etc., in rivers and streams. After the cercariae penetrate the
skin the immature worms enter the circulatory system and migrate to the veins of the abdominal cavity, and in
about six weeks they reach sexual maturity
1. Anatomy of flukes
Adult flukes. The anatomy of digenetic trematodes is usually studied in introductory laboratories using
commercially prepared whole mount slides. The Chinese liver fluke, Opisthorchis (= Clonorchis) sinensis,
or the sheep liver fluke, Fasciola hepatica, are the most frequently used species. We are fortunate to have
slides of Schistosoma mansoni; a very important human disease causing species and an unusual fluke
because the adults are dioecious or the sexes are separate.
a. Your assignment is to obtain and label as best you can, one photograph of Fasciola or an
adult of some other species of “typical fluke. When you look at your specimen be sure that you
can distinguish the difference between the diverticula of the intestine, the vitelline glands, and the
testes. The appearance of the diverticula is best seen near the mouth where it is the only structure
visible. b. Then obtain and examine one slide of Schistosoma mansoni. c. In your notebook,
compare the structures most evident in these specimens to those found in the free-living
flatworms.
You can find labeled diagrams and descriptions for flukes at the end of this lab. These are not
to be memorized, but used to help you identify stages and structures in the adult.
2. Life cycles:
a. If living cercariae are available in laboratory, examine these. Snails containing rediae or
sporocyts may also be available for dissection. Obtain photographs and movies of all stages that
you can find.
The only stage that can be usually supplied easily living is the cecaria. Usually a species is chosen that
infects beetles or mice as adults. Some care should be taken whenever handling parasites. Wear gloves
when you dissect the snails that are carrying cecariae.
Prior to the lab, Snails will probably be placed in the dark for two hours to stimulate them to release
cercariae and then placed in a bowl of water.
You will be able to examine the water from the bowl under the microscope. If cecariae are present,
snails may be cooled and dissected to see rediae or sporocyts. We will have to wait for specific
instructions, which will come with the snails, as these snails can be river or pond snails and infected
with a number of trematodes
If cecariae from snails are not available:
b. Examine the prepared slides of the various stages of the fluke. Each pair should contribute to
their notebook and a folder that will be available to the class, one photograph of one stage of the
life cycle. Be careful and label soon after taking photographs because some stages in the life cycle
vary more in morphology from species to species (such as rediae and sporocysts) than others (such
as cecaria). Can you determine why this may be? Each photograph should be labeled with the
name of the state and species. After examining the photographs, please take the following self test
and record your answers in your journal.
Self-test: Identify the stage in the following photographs
2a______
2b_____
2c_________
2d_________
2e____________
2f________________
A description of the typical life cycle and diagrams of specific life cycles are included to help you associate
stage with proper host.
Most life cycles include two intermediate hosts (the hosts in which the parasite undergoes asexual reproduction) and a definitive host
(the host in which the parasite undergoes sexual reproduction). Most adult digenetic trematodes are monoecious (a single organism
contains both male and female reproductive organs), and most live in the gastrointestinal tract of the definitive host. The adult
parasites produce eggs, and the eggs are passed in the definitive host's feces. The egg either hatches so the larva inside escapes and
infects the first intermediate host, or the first intermediate host eats the egg. The first intermediate host is always a snail, most often
an aquatic or marine snail. Inside of the snail the parasite goes through several developmental stages, and a larval stage called a
"cercaria" is finally produced. The cercaria leaves the snail and swims around until it comes in contact with a second intermediate
host. The cercaria penetrates the skin of the second intermediate host and encysts. This encysted stage is called a
"metacercaria." The definitive host is infected when it eats an infected second intermediate host. Fish commonly serve as second
intermediate hosts for digenetic trematodes, and the metacercariae encyst in the fish's muscles. In such a case the definitive host
would be infected by eating the fish. Depending on complexity, these various life stages could be recognized.
Egg leaves the adult containing a miricidium. Miricidia through chemoreception finds a snail, penetrates and develops into
sporocysts. The sporocyst is an asexual stage with no mouth or digestive system, that absorbs nutrients through tegument and
produce rediae, daughter sporocysts or cecariae.
Redia are elongate forms with a muscular pharynx, that may form cercariae, or free swimming forms that leave the snail to seek
another host. They encyst eventually as metacercaria to eventually be eaten by a definitive host.
Oriental liver fluke: Clonorchis sinensis
The sheep liver fluke: Fasciola hepatica
Schistosoma
3. CESTODA or tapeworms
Cestodes are compact worms whose interior is filled with unspecialized, mesenchymal connective tissue known
as parenchyma, as it is in other platyhelminths. Excretion and osmoregulation are accomplished via
protonephridia, which empty into two pairs of excretory canals. The nervous system includes anterior nerve
rings, lateral longitudinal nerve cords, and transverse commissures. Cestodes are hermaphroditic and may
utilize either self- or cross-fertilization
Use the compound microscope to study whole mount slides of the scale, a mature proglottid, and a gravid
proglottid. a. Choose one slide to photograph and label. b. Compare the anatomy of tapeworms with
flukes in your notebook.
________________________________________________________________________________________
Information for labeling tapeworms. Use the following description as well as labeled drawing to identify
important characteristics of tapeworms on prepared slides. These descriptions are taken from a online
lab manual that stresses taxonomy and so most of the terms used are not to be memorized, but there as
the author requests his work not be modified to any great extent. Be grateful our course is basically an
appreciation course.
A more colorful diagram
Laboratory Specimens
Commercially prepared slides of Taenia pisiformis (= T. serrata), the dog tapeworm, are frequently used in the laboratory as
examples of tapeworm anatomy. Other species can also be used but differ in some respects. The adults of Taenia pisiformis occur in
dogs, cats, and other carnivores and the intermediate host is a rabbit.
The tapeworm body consists of an anterior, head-like scolex and the body, or strobila, consisting of a linear series of
segments, or proglottids. Look at the whole mount of a scolex with the scanning lens (40X) of the compound microscope. The scolex
is wider than the anterior strobila to which it joined by a narrow neck.
The scolex attaches the worm to the gut wall of the host. For this purpose it has a retractable rostellum armed with two rings
of hooks. Examine the hooks with higher power. Just posterior to the rostellum is a ring of four suckers, two dorsolateral and two
ventrolateral. Scolex morphology varies widely with taxon.
Two lateral nephridial canals may be visible on each side of the scolex. They connect with each other near the rostellum via
a set of convoluted nephridial canals and extend posteriorly through the strobila. A pair of nerve rings are present but will not be
evident.
The strobila is long and wormlike, sometimes reaching 15 or more meters in length. (There are records of Taeniarhynchus
saginatus, a human parasite, reaching lengths of more than 20 meters.) The strobila is strongly flattened dorsoventrally and is a linear
series of hundreds or thousands of proglottids.
The strobila includes a series of young immature proglottids at its anterior end, then a region of sexually mature proglottids in
the middle, and a length of gravid proglottids at the posterior end. The gravid proglottids contain viable embryonated “eggs” ready to
infect a new host should the get the opportunity. Proglottids increase in size posteriorly.
Each proglottid contains its own complete hermaphroditic reproductive system as well as its share of the common excretory
and nervous systems. There is no digestive system. The reproductive systems of the youngest, anteriormost proglottids are not yet
formed but more posterior proglottids have increasingly better-developed reproductive systems. Most tapeworms are protandric with
the male system of each proglottid maturing first.
Examine a mature proglottid using 40X of the compound microscope. Your slide probably contains several proglottids, some
of which may be better than others. You will probably want to make use of them all as you look for the best representation of each
structure.
Find the anterior and posterior ends of the proglottids. In Taenia, the posterior end of each proglottid is wider than the
anterior end of the next proglottid.
Respiratory, and Fluid Transport Systems
Tapeworms have no mouth or gut. Organic molecules, mostly glucose, other sugars, and amino acids, are absorbed across
the specialized, heavily microvillar neodermis and metabolism is largely anaerobic. There is no need for a gas exchange mechanism
and there is none. Food and waste molecules diffuse to and from the body surface and there is no fluid transport system. The
microvilli of the neodermis absorb food molecules from the environment.
Excretory System
The excretory system consists of numerous flame bulb protonephridia scattered throughout the parenchyma of each
proglottid but they are not evident in these preparations. Individual protonephridia drain into an elaborate system of nephridial canals,
which ultimately open to the exterior at the posterior end of the strobila. Dorsal and ventral lateral nephridial canals on each side of
each proglottid extend the length of the worm. In addition to the two nephridial canals, each side possesses a longitudinal nerve cord
which may be confused with the canals. The visibility of these structures depends on the quality of the staining and they are often
indistinct.
The larger ventral nephridial canal is a pale, wide, longitudinal band lateral to the testes (Fig 2). The dorsal canal is much
smaller in diameter and is located medial to the ventral canal, between it and the testis. In each proglottid the right and left ventral
canals are connected by a transverse nephridial canal extending across the posterior edge of the proglottid just anterior to the junction
with the next proglottid.
Reproductive System
Male
Most structures in the proglottid belong to the reproductive system. The male and female systems share a common genital
pore ( = gonopore) and genital atrium but are otherwise independent of each other. The common genital pore (Fig 2) is a large
aperture on either the right or left side of the proglottid. It opens into a shallow, cuplike genital atrium. The male and female systems
both open into the atrium via its own gonoduct.
Find the two ducts joining the medial border of the genital atrium. The anterior duct is the thicker and is the male gonoduct.
The gonoduct is regionally specialized. The wide portion of the gonoduct attached to the atrium is the muscular cirrus sac. Inside the
sac is the convoluted, eversible, tubular cirrus, which is the intromittent organ, or penis. During copulation the cirrus is extended from
the genital pore and inserted into the genital atrium and vagina of another proglottid. On your slide it may be everted in some
proglottids, in which case it looks like a slender worm extending from the genital pore.
The next region of the male gonoduct is the tubular sperm duct, also convoluted, which extends to the testes. Its entire length
is not visible. The testes are numerous small spheres scattered throughout the parenchyma. A tiny tributary of the sperm duct drains
each, but these cannot be seen
Female
The smaller and more posterior of the two ducts entering the genital atrium is them female gonoduct, which is also regionally
specialized. The first region is the vagina. It receives the partner's cirrus during copulation. The vagina extends medially and
posteriorly to become the small seminal receptacle . This is a clear, unstained, oval chamber where allosperm received through the
vagina are stored. It is usually easily visible. A short duct exits the posterior end of the seminal receptacle and joins the oviduct.
The germarium (= ovary) is divided into large right and left lobes lying on either side of the seminal receptacle. It is the site of
oogenesis and produces large numbers of small, yolkless oocytes. The two lobes of the germarium are connected across the midline
by a short, wide, transverse isthmus.
The follicles of the germarium open into small ducts which drain into the isthmus. The narrow oviduct arises from the isthmus
and extends posteriorly for a short distance before receiving the duct from the seminal receptacle . The isthmus is usually easy to see
but the oviduct is often obscured by the seminal receptacle and is harder to find. After receiving the duct from the seminal receptacle
the oviduct continues posteriorly to the ootype. Fertilization occurs in the oviduct.
Yolk cells are produced by the single vitellarium at the posterior end of the proglottid. A small uterine duct, usually not
discernible, extends to the uterus . Shelled eggs move through the uterine duct into the uterus. Within the shell meiosis is completed,
a zygote forms, and development proceeds to the larval stage. The uterus is a blind sac with lateral branches in which embryonated
“eggs” are stored. The size and visibility of the uterus vary with the maturity of the proglottid.
As the proglottid ages the accumulating “eggs” cause the uterus to become larger, darker, and more visible. They will
eventually fill it, distending it so it occupies the entire proglottid. There is no opening of the uterus to the exterior and eggs are
released by rupture of the proglottid.
_____________________________________________________________________________________
Information for labeling flukes:
External Features
Note the size and shape of the worm. Does anterior end of the worm narrows gradually to a pointed tip? Is the wider
posterior end is bluntly rounded? At the extreme anterior end you should note a conspicuous oral sucker surrounds the mouth and is
used to attach to the host and maintain its position. The circular ventral sucker, or acetabulum, may be located on the ventral surface a
short distance posterior to the oral sucker. It also attaches to the host but is not associated with the gut.
Body Wall
The body wall consists of an outer syncytial neodermis whose nuclei are sunken deep within the body. The true epidermis is
lost during larval development. There is no cuticle. Muscles underlie the neodermis and a mesenchymal parenchyma fills the interior
of the animal. Organs are embedded in the parenchyma. Trematodes, like other flatworms, are compact (acoelomate) and have
neither mesodermal body cavity.
Digestive System
The mouth opens into a muscular, bulbous pharynx, which is used to suck food into the gut. The esophagus quickly
bifurcates to form two long, unbranched intestinal ceca that extend along the sides to the posterior end of the worm. Hydrolysis and
absorption occur in these ceca. They end blindly and there is no anus. . Flukes also absorb nutriment directly across the cuticle-free
neodermis.
Excretory/Osmoregulatory System
Protonephridia are scattered through the mesenchyme but they are not visible in these preparations. They drain into a pair of
branching, lateral excretory canals, which likewise are usually not visible. The lateral canals drain into a large median excretory
bladder at the posterior end of the worm. The bladder, which is more easily observed, opens to the exterior by a large nephridiopore at
the posterior end of the worm.
Reproductive System
Most flukes are hermaphroditic and have a complex reproductive system with independent male and female systems. The
reproductive system is large and well-developed, occupying a large proportion of the interior, as is often the case with parasites.
Many of the ducts mentioned in the following descriptions are difficult to find. The exception to this rule is the blood fluke. In this
species females can be found lying within a ventral groove of the male called the gynecophoral canal, an obvious adaptation to these
worms living in a "turbulent" environment (the host's blood stream).
Male System
The male system begins with two large, irregularly branched testes located in the posterior third of the worm . The testes are
large, easily seen, and usually stain dark pink. Each has a central area from which extend wide, blunt, branched lobes reminiscent of
the pseudopodia of Amoeba. A slender, inconspicuous vas efferens, which you probably will not see, arises near the center of each
testis.
The two vasa efferentia unite near the middle of the body to form the short vas deferens which is obscured by the voluminous
uterus in the center of the worm. The vas deferens quickly widens to become the muscular seminal vesicle. This meandering tube
extends anteriorly and eventually joins the uterus and the two open together to the exterior via the common gonopore. In ventral view
the seminal vesicle is hidden by the uterus and may be difficult to observe. The seminal vesicle stores autosperm produced by the
testes.
The common gonopore, or genital pore, is on the ventral surface immediately anterior to the ventral sucker. It is small and
obscure but usually appears as a small pink spot on the anterior edge of the ventral sucker. Fasciola has an eversible copulatory cirrus,
Clonorchis relies instead on the muscular seminal vesicle to transfer sperm.
Female System
The female system is more complex, partly because the trematodes produce eggs consisting of an oocyte surrounded by yolk
cells. Oocytes are produced by the ovary whereas yolk cells are produced by two independent vitellaria.
Begin with the easily located seminal receptacle, a large, ovoid organ where all sperm (from another worm) are stored. The
seminal receptacle is drained by the sperm duct, which joins the oviduct.
The single germanium (ovary) stains dark pink and lies on the midline immediately anterior to the seminal receptacle. The
germarium is smaller (and usually darker) than the seminal receptacle. The germarium connects with the ootype via the oviduct,
neither of which is visible. The ootype is a glandular chamber near the center of the worm but it usually cannot be seen in these
preparations .
The two large vitellaria produce yolk cells. They lie lateral to the intestinal ceca, between the ceca and the edge of the body.
They are large and are usually brownish-yellow in stained preparations. Each is drained by a large, easily seen vitelline duct (=
vitellarium duct). The two ducts join each other to form a short common duct that empties into the ootype.
The large brown uterus exits the ootype and extends anteriorly to its union with the seminal vesicle at the common gonopore.
It is a wide convoluted tube packed with dark, yellow-brown eggs. The uterus and seminal vesicle share the common gonopore.
Some diagrams to help you
Fasciola hepatica
Clonorchis sinensis