Invertebrate Anatomy OnLine
Copyright 2001 by
is one of many exercises available from Invertebrate
Anatomy OnLine , an
Internet laboratory manual for courses in Invertebrate Zoology. Additional
exercises can be accessed by clicking on the links on the left. A
glossary and chapters on supplies and laboratory techniques are also available. Terminology
and phylogeny used in these exercises correspond to usage in the Invertebrate
Zoology textbook by Ruppert, Fox, and Barnes (2004). Hyphenated
figure callouts refer to figures in the textbook. Callouts
that are not hyphenated refer to figures embedded in the exercise. The glossary
includes terms from this textbook as well as the laboratory exercises.
Neodermata, Trematoda C,
Digenea sC, Opisthorchiida O,
Opisthorchidae F, (Fig
or platyhelminths, are bilaterally symmetrical metazoans with three tissue
most triploblastic animals, they are compact and have no coelom (body cavity)
surrounding the viscera and no hemal system. The
gut, if present, has a single opening to the exterior. An
anterior brain with associated concentration of sense organs is present, as
expected of bilaterians. Flatworms
are complex animals with elaborate hermaphroditic reproductive systems. Fertilization
is internal with copulation. They
may be free-living or parasitic.
includes the parasitic flatworms, most of which are flukes or tapeworms. 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 neoderm 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 (Fig 10-33*, 10-36).
consists mostly of flukes belonging to Digenea. Flukes
are important animal parasites and several important human and livestock
diseases are caused by them. The small taxon Aspidogastrea also belongs to
Digenea s C
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.
least two hosts are required to complete the life cycle and this is the basis
for the name “ digenea”. The
definitive host is always a vertebrate and the intermediate host is usually a
gastropod mollusc (Fig 10-34B). 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.
anatomy of digenetic trematodes is usually studied in introductory laboratories
using commercially prepared wholemount slides. The
Chinese liver fluke, Opisthorchis (= Clonorchis)
sinensis, or the sheep liver fluke, Fasciola
hepatica, are the most frequently used species. Due
to its convenient small size and transparency Opisthorchis is
an especially good subject for an introductory study. This
exercise is based on commercially prepared wholemount slides of Opisthorchis. The
adult 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.
quality of staining varies in commercially prepared slides and greatly affects
the visibility of structures. Few
slides are perfectly stained and there will undoubtedly be structures you cannot
find on your slide. It
will help to use more than one slide.
the compound microscope to examine a stained wholemount of Opisthorchis (Fig
1, 10-34A*, 10-37).
the size and shape of the worm. This
species reaches 25-30 mm in length and is strongly flattened dorsoventrally. The
anterior end of the worm narrows gradually to a pointed tip. The
wider posterior end is bluntly rounded. At
the extreme anterior end a conspicuous oral
sucker (Fig 1, 10-34A)
surrounds the mouth and
is used to attach to the host and maintain its position. The
circular ventral sucker,
or acetabulum, is 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 consists of an outer syncytial neodermis whose nuclei are sunken below
the basal lamina (Fig 10-36). 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
are embedded in the parenchyma. Trematodes,
like other flatworms, are compact (acoelomate) and have neither mesodermal body
cavity nor mesothelium.
The mouth opens
into a muscular, sucking pharynx (Fig
1, 10-16C), which is used to pump food into the gut. Opisthorchis feeds
on blood, epithelium, mucus, and fluids from the wall of the host’s bile duct. Immediately
posterior to the pharynx is a short, inconspicuous esophagus.
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.
are scattered through the mesenchyme but they are not visible in these
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 easily observed (Fig 1, 10-34A, 10-37), opens
to the exterior by a large nephridiopore at
the posterior end of the worm.
an anoxic environment and is a facultative anaerobe with no special respiratory
Fluid Transport System
have no hemal system. The
gut itself distributes food to the tissues and is often called the
gastrovascular cavity in recognition of its dual roles in digestion and
transport. Flukes also absorb nutriment directly across the
cuticle-free syncytial neodermis.
nervous system is usually not visible in preparations of this type. The
brain is a bilobed cerebral ganglion dorsal to the esophagus. Three
pairs of longitudinal nerve cords (dorsal, lateral, and ventral) exit the brain
and are connected with each other by numerous transverse commissures.
like other platyhelminths, is hermaphroditic and has a complex reproductive
system with (nearly) independent male and female systems sharing only the common
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 but
if it is in bold type, it should be visible in most preparations.
Figure 1. Ventral
view of an adult Chinese liver fluke, Opisthorchis
male system begins with two large, irregularly branched testes located
in the posterior third of the worm (Fig 1, 10-34A, 10-17). The
testes are large, easily seen, and usually stain dark pink. One
is the anterior testis and
the other the posterior
testis, situated as their
names suggest. Each has a central area from which extend wide, blunt, branched
lobes reminiscent of the pseudopodia of Amoeba. A
slender, inconspicuous duct, the vas efferens, which you probably will not see,
arises near the center of each testis.
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
usually stains pink in contrast with the brown uterus. 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
seminal vesicle stores autosperm produced by the testes.
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 pink spot on the anterior edge of
the ventral sucker. Opisthorchis,
unlike most flukes, has no eversible copulatory cirrus (Fig 10-37) relying
instead on the muscular seminal vesicle to transfer sperm.
female system is more complex, partly because trematodes produce ectolecithal
eggs consisting of an oocyte surrounded by yolk cells. Oocytes
are produced by the ovary whereas yolk cells are produced by two independent
and internal fertilization also contribute to the complexity of the system.
with the easily located seminal
receptacle, a large, ovoid organ where allosperm (from another worm)
are stored (Fig 1, 10-34A, 10-37). It
is usually stained pink. The seminal receptacle is drained by the sperm duct
which joins the oviduct.
single germarium (ovary)
stains dark pink and lies on the midline immediately anterior to the seminal
germarium is smaller (and usually darker) than the seminal receptacle. The
ootype is a glandular chamber near the center of the worm but it usually cannot
be distinguished in these preparations (Fig 10-37). The germarium connects with
the ootype via the oviduct, neither of which is visible.
two large vitellaria produce
vitellocytes (= 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 (usually) vitelline
duct (= vitellarium duct). The
two ducts join each other to form a short common duct that delivers vitellocytes
to the ootype.
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 embryonated eggs. The
uterus and seminal vesicle share the common gonopore.
Reproduction and Life Cycle
Opisthorchidae parasitize the branches of the bile duct of fish-eating mammals,
including humans. The
life cycle requires two intermediate (snail and fish) and one definitive
(mammal) host for completion.
from the testes pass via the vasa efferentia and vas deferens to the seminal
vesicle for storage. During
copulation sperm are forced by the muscular seminal vesicle from each worm into
the uterus of the partner. The
sperm move up the uterus to the seminal receptacle where they are stored.
oocytes leave the germarium and are fertilized by sperm from the seminal
receptacle as they enter the ootype. The
fertilized oocytes, still in the ootype, are then enclosed in a layer of
vitellocytes from the vitellaria. Vacuoles
in the yolk cells release liquid protein which hardens to form a tough eggshell. Additional
components of the eggshell may be secreted by the epithelium lining the ootype. Shelled
eggs are stored in the uterus where meiosis is completed, union of pronuclei
accomplished, and development begins. The
so-called “eggs” in the uterus are actually shelled embryos, sometimes referred
to as embryonated eggs to distinguish them from unicellular haploid ova.
in the uterus is rapid and by the time they are released from the common
gonopore of the adult worm each "egg" contains a miracidium larva (Fig 10-34B,
embryonated “eggs” pass down the bile duct of the definitive host and into its
gut. They exit the body of the host with its feces and some end up in freshwater
The Chinese liver fluke has three hosts, the last of which inhabits the bile
ducts of humans and other mammals. The
first intermediate host is an aquatic snail, the second intermediate host is a
fish, usually a cyprinid (e.g. carp), and the definitive host is a mammal
(human, pig, dog, cat, rat, camel, and probably any other mammal that eats raw
If slides of fluke larval stages are available in the laboratory, examine them
with 400X of the compound microscope. All
the larval stages have protonephridia but they are not usually visible. Rediae,
cercariae, and metacercariae have guts.
embryonated egg consists
of a zygote, embryo, or larva enclosed in a capsule, or eggshell.
A door, or operculum,
is located at one end of the eggshell.
first intermediate host (snail) ingests an “egg” with its fully developed miracidium (Fig
miracidium, now in the snail’s gut, emerges from the eggshell and migrates to
the snail’s digestive cecum, which is a diverticulum of the gut. In
miracidium slides you should be able to see cilia on
the surface and abundant germ
the snail’s digestive cecum, the miracidium metamorphoses into a saclike sporocyst soon
after ingestion (Fig 10-35B). It
lacks mouth or gut. Within the
sporocyst numerous asexually produced germinal cells each develop into a
multicellular but undifferentiated germ
balls which then develop
into larvae, known as rediae, which escape from the sporocyst when the sporocyst
ruptures after about two weeks.
The rediae (Fig
10-35C) remain in the snail. Each
has a gut consisting of mouth, muscular pharynx, and a short, unbranched
on the quality of your slide, you may be able to see the oral
sucker surrounding the mouth and
the developing gut. Undifferentiated gem balls and immature
cercariae can be seen with
no difficulty. The
oldest cercariae have tails and should be recognizable.
each of the hundreds of redia, more germ cells mature into another larval stage,
the cercaria (Fig
clonal reproduction, known as polyembryony, by sporocyst and redia greatly
increases the number of larvae in the first intermediate host even though only
one miracidium may have entered the host. The cercaria looks like a little fluke
with a tail, oral sucker, ventral sucker, gut, and protonephridia but no
reproductive system yet (Fig 10-35D,E). The tail and suckers are
easy to see. Sometimes
a bilobed gut with pharynx and ceca can be made out. It
may have eyes to help it locate a passing fish. Most
species have simple tails but in some it is forked.
cercariae are released into the water where they attach to the skin of a passing
fish (the second intermediate host) and bore through the epidermis to encyst
under a scale or in muscle as a metacercaria (Fig
intermediate host is usually a member of the fish family Cyprinidae, which
includes carp, an important food fish in the orient. On
a good slide you may be able to see the two suckers, pharynx, and bilobed
definitive host is infected when it eats raw or poorly cooked fish with
metacercariae in the muscles. With
the help of the host’s digestive enzymes young flukes excyst in the duodenum of
the definitive host. Although
the route is uncertain, it is thought that young flukes simply migrate up the
bile duct to the liver from the duodenum.
worms mature in the liver, mate and produce the first eggs after about 3 months. The
life cycle is best broken by preventing the contamination of water with human
feces and by thoroughly cooking freshwater fishes before they are eaten.
call-outs, such as this one, refer to figures in Ruppert, Fox, and Barnes
without hyphenation refer to figures embedded in this exercise.
(ed) 1950. Selected
Invertebrate Types. Wiley,
New York. 597p.
TWM . 1944. The
morphology, taxonomy, and life history of Metorchis
conjunctus (Cobb, 1860). Can.
J. Res., Sect. D., 22:6-16.
GO, Roberts LS . 1989. Foundations
of Parasitology, 4th ed. Times
Mirror/Mosby, New York. 750p.
Ruppert EE, Fox RS,
Barnes RB. 2004.
Invertebrate Zoology, A functional evolutionary approach, 7 th ed.
Brooks Cole Thomson, Belmont CA. 963 pp.
Prepared wholemount of Opisthorchis
Opisthorchis sinensis wholemounts
Triarch, Carolina, Ward’s
Trematode egg, miracidium, redia, cercaria, and
Rediae and cercariae