WORMS, WORMS WORMS
PHYLUM PLATYHELMINTHES: FLATWORMS
Prepared slides – planaria whole mount
-planaria sections
-taenia pisiformis (tapeworm)
-dwarf tapeworm – hymenolepsis nana
Demo slides – giant liver fluke (Fasciola gigantica)
-liver fluke (Clonorchis sinensis)
-tapeworm scolex
-planaria digestive tract
-schistosoma
The name Platyhelminthes means flat worms; there are about 15,000 living species. They live in marine and
fresh water habitats, moist terrestrial environments, and inside other organisms as parasites. All are
triploblastic. In other words, their tissues are derived from the three germ layers: ectoderm (gives rise to their
outer covering – the epidermis), endoderm (gives rise to the lining of the gut), and mesoderm (giving rise to the
tissues in between). As in the Cnidaria, the gut is called a gastrovascular cavity and has the functions of both
digestion and circulation; the mouth serves as both entrance and exit from the gut.
The body is bilaterally symmetrical, and there is a well-defined rostral or head end and, opposite it a caudal or
tail end. At the head end the nervous system has a coordination center which can be called a brain. Many
species of Platyhelminthese have defined structures at the head such as eyespots and auricles. There is no
specialized circulatory, respiratory or excretory systems.
Flatworms are acoelomate. There is no coelomic cavity and so the gut is not free inside the body. The mesoderm
is muscular and is used for movement. Ectodermal cells may be ciliated and provide another means of
locomotion.
The Phylum is divided into
1. Class Turbellaria
2. Class Trematoda
3. Class Cestoda
Class Turbellaria – free living flatworms
Members of the Class Turbellaria are free living (not parasitic). Planaria (Genus Dugesia) lives in fresh water
ponds and is a carnivore. A pharynx can be protruded from the mouth which is in the middle of the ventral side
of the animal. The diet consists of such foods as insect larvae, small crustaceans, and other small living and dead
animals. Planarians reproduce asexually and sexually; individuals have both testes and ovaries.
Exercise 1 - Planaria whole mount: Observe the Planaria prepared slides and draw what you see. In the whole
mount, the animals has been fed particles of carbon, and these blacken the gastrovascular cavity. Behind the
pharynx, the GV cavity divides into two lateral canals called intestines. These intestines are highly branched,
because they serves also for circulation of nutrients to every cell. These smaller branches off of the intestines
are called diverticula. Identify them in your slide and your drawing. Can you find the pharynx? Label it in your
diagram. Identify and label the head of the Planaria, including
the eye spots and auricles
Exercise 2 - Planaria representative cross sections: The most
obvious section in your slide will be the one from the middle
with the great circular, muscular tube of the pharynx; the
space around it is actually outside of the body proper. The
gastrovascular cavityis comprised of a median tube in the body
anterior to the pharynx; posterior to the pharynx it is divided
into two lateral tubes. There are only lateral tubes of the
gastrovascular cavity posterior to the pharynx, but no median
one. The gut is continuous with the cells of the surrounding
mesoderm so there is no coelom. Recall that the gut is lined
with cells of endoderm and the outside of the animal is
covered with ciliated cells of ectoderm. The mesoderm is
muscular, and some bands of muscle cells run from the dorsal
to ventral sides of the worm.
Look at your transverse slide and identify the pharynx, the gastrodermis lining the pharynx, the median and
lateral canals (i.e. intestines) of the GV cavity.
Class Trematoda – the flukes
Trematodes are commonly called flukes, and they are parasitic. The life cycles of parasites are very complex, and
there are intermediate hosts and life stages; read the description of this genus in the text book. Slide:
Exercise 1 – Clonorchis sinensis whole mount: this worm (shown below) is known as a liver fluke. The anterior
end has a mouth at the end and a sucker disc for holding on; a ventral sucker is somewhat farther posterior. The
intestine divides into two simple tubes near the head end. The dark colored organ in the center of the body is
the uterus and it is filled with eggs; the ovary is just posterior to it and appears as a pink mass; it has a seminal
receptacle at the posterior end. The branched pink organ in the posterior part of the body is the testes. Look at
your slide and draw what you see. Identify the structures listed above in your slide and on your drawing.
Class Cestoda – the tapeworms
The Cestoda are parasitic tapeworms. The life cycles are complex (see the text). All tapeworms are extremely
flat; the body is divided into segments, and there is no digestive system. So they do not have a gastrovascular
cavity. They absorb nutrients across their body walls. Preserved tapeworms are available in jars in this lab.
Exercise 2 – Taenia pisiformis: this worm is known as a tapeworm. The tapeworm adheres to its host via a
scolex. The scolex is the term for the head end of a
tapeworm; it has a disc of hooks at the tip, which anchor it
into the lining of the host's intestine, and four large suckers
for holding on. New segments or proglottids are generated
behind the scolex. As you move down the worm away from
the head, these segments get larger. Each is a complete
reproductive machine with testes, ovaries and uterus. The
chance of any one egg hatching and completing the life cycle
to parasitize the final host is so small that high reproductive
capacity is a must.
Observe your slide of Taenia pisiformis and draw the head of the worm, a small immature proglottid and a more
mature one further down the worm. Identify the parts of the scolex listed above. In the mature proglottid,
identify the branched uterus containing hundreds of eggs.
Exercise 3 – Demo slides: Observe demo slides of the Giant Liver Fluke (Fasciola gigantica) and compare its
structures to those of the liver fluke you just looked at. Identify the same parts you found in the liver fluke.
Observe the demo slide of the scolex of a tapeworm and identify the suckers and the hooks. Finally, observe the
slide of the trematode known as Schistosoma.
PHYLUM ANNELIDA – THE SEGMENTED WORMS
Models – earthworm
Prepared slides – earthworm (c.s)
-leech
Preserved samples – leeches
-Nereis clamworm
-sand worm
-scale worm
Demo slides – metanephridium
The Phylum Annelida, annelids, are segmented worms. They live in marine and fresh water and in moist
terrestrial habitats; some are parasitic. They are bilaterally symmetrical, have a true coelom, and a segmented
body. The cellular layer that lines both sides of the coelom is called peritoneum. The digestive system begins at
the mouth at one end and runs to the anus at the other. The body consists of the three basic tissue layers; the
nervous system has a brain; the blood vessels are closed tubes. Many kinds have gills for respiration, but some,
like the earthworm, have no specialized respiratory structures. There are no hard parts, and the segmented
worms are said to have a hydrostatic skeletal support.
The body of the annelid is segmented. The segments (or metamers) are easy to see on the outside of the worm,
but inside these segments are formed by walls called septa.
Class Oligochaeta – the earthworm
The name refers to the few (oligo-) bristles (-chaeta) along the body of these earthworms and fresh water
worms. Examine the earthworm genus Lumbricus. The dorsal or top side is slightly darker than the ventral side.
Earthworms burrow through soil and eat decomposing plant and other organic material. The mouth end is
pointed and conical, and the tail end is dorsoventrally flat. The anus is a vertical slit in the last segment. As you
run your fingers along the sides of the preserved worm, you might feel the rasp of the setae (singular seta) or
bristles. If there are living samples of earthworms, do this carefully!!
The earthworm has two layers of muscles: circular and longitudinal. The circular muscle layer runs around the
body; when they contract the body narrows and the segments lengthen. This action is just the opposite of the
longitudinal layer. Together they produce the kind of rippling motion characteristic of your own intestine (it
likewise has two muscular layers) that is called peristalsis. In order for a worm to move forward through the
ground, it contracts the longitudinal muscle layer in the posterior part and sticks the setae into the soil; this end
of the worm becomes fatter and serves as an anchor to push against. The circular muscle layer in the anterior
part of the worm contracts to thin and extend the front end of the worm; the setae in this portion are
retracted.
Tiny excretory pores from the metanephridia can be found on the lateral or ventral surfaces of all segments
except those at the ends. The earthworm has both ovaries and testes and is hermaphroditic. The openings of
the oviducts can be seen on the sides of segment 14 and those of the sperm ducts, which have swollen edges
on segment 15. The enlarged ring that begins at segment 31 or 32 and ends at 37 is called the clitellum. It is
glandular and secretes a slimy mucus around two copulating individuals.
Exercise 1 – Earthworm cross section: Observe your slide of the earthworm known
as Lumbricus. Draw what you see and try and identify the following structures:
Epidermis, circular muscles, dorsal blood vessel, ventral nerve cord, intestinal
lumen. Look for the typhlosole – a folding of the intestinal wall that increases the
animal’s absorptive surface area. Does your slide have one?
Exercise 2 – Earthworm model: Observe the model of a typical earthworm.
Identify the following structures: prostomium, pharynx, esophagus, crop, gizzard,
intestine with the typhlosole, dorsal blood vessel, hearts, ventral nerve cord, metanephridia, testes,
seminal groove, ovaries, clitellum, setae, circular and longitudinal muscles.
Exercise 3 – Earthworm Dissection: Examine your preserved earthworm and determine the anterior and
posterior ends in addition to its dorsal and ventral sides. The ventral side will be slightly flattened and will have
two openings types of openings. The openings toward the anterior of the worm are the male genital pores or
sperm ducts. There are two pairs. The first pair is anterior to the second pair. The second pair is called the
genital setae. They are tiny and are located just above the clitellum. They may be too small to see but may be
visualized using a dissecting microscope. Sperm are produced in the testes and pass out through these pores.
Just above the first pair of sperm ducts are the female genital pores. Again, these may be too small to see
without a dissecting microscope. Eggs are produced in the ovaries and pass out of the body through these
pores.
Locate the worm's mouth and anus. Near the mouth will be a fleshy protruberance called the prostomium. The
prostomium is located on the dorsal surface of the worm. Note the swelling of the earthworm near its anterior
side - this is the clitellum. Label the clitellum on the drawing to the right. The clitellum is active in the formation
of an egg capsule, or cocoon.
Place a preserved earthworm dorsal side up in a dissecting pan. Anchor the anterior
end with a pin through the tip, stretch the worm slightly, and put a second pin
through the posterior end. Before dissecting, look for the following
Starting at the posterior end near the anus, use a scalpel or fine scissors to make a longitudinal, mid-dorsal
incision through the skin; don't cut deeply. Continue the cut up the worm using fine scissors. This will ensure a
shallow cut. Open the incision with your fingers or forceps. Pin the skin back on both sides so the internal
organs are exposed to view.
Identify the following structures in your dissection: prostomium, pharynx, esophagus, crop, gizzard, intestine
with the typhlosole, dorsal blood vessel, hearts, ventral nerve cord, metanephridia, testes, seminal groove,
ovaries, clitellum, setae, circular and longitudinal muscles, pygidium with anus.
Reproductive System: The first structures you probably see are the seminal vesicles containing the testes. They
are cream colored and located toward the anterior of the worm. These are used for producing sperm. The
seminal receptacles of the female reproductive system may be seen attached to the wall of the worm next to
these as a single cream colored structure. Sperm released by one worm will travel through a seminal groove on
the body of another worm and enter through the female genital pores. The sperm will be stored in these
receptacles and will be used to fertilize the eggs as they are laid and pass out through the female genital pores.
Circulatory system: The dorsal blood vessel appears as a dark brownish-red vessel running along the intestine.
The hearts (or aortic arches) can be found over the esophagus (just posterior to the pharynx) and will look like
enlarged blood vessels. Carefully tease away the tissues to expose the arches of the heart, the run across the
worm. If you are careful, you can expose all 5 of them. The ventral blood vessel is opposite the dorsal blood
vessel, and cannot be seen at this time because the digestive system covers it. It is found on top of the ventral
nerve cord.
Label the diagram (use the bold words from above)
QUESTION: Does the earthworm have a closed or open
circulatory system? ___ ___________________
Digestive System: Use tweezers to remove the seminal vesicles
from over the top of the digestive system that lies underneath
it. The digestive system starts at the mouth. You will trace the
organs all the way to the anus and identify each on the worm.
Find the mouth opening, the first part after the mouth is the pharynx. You may see stringy things attached to
either side of the pharynx (pharyngeal muscles). The esophagus leads from the pharynx but you probably won’t
be able to see it, since it lies underneath the hearts. You will two structures close to the clitellum. First in the
order is the crop, followed by the gizzard. These are prominent structures that were initially seen below the
seminal vesicles.The gizzard leads to the intestine which is as long as the worm and ends at the anus.
*Use your scissors to cut open the crop and the gizzard.
QUESTION: In which organ would you expect the contents to be more ground up? _____________________
Excretory System: In each segment, you may be able to see a pair of minute, white coiled tubes. Use the
dissecting microscope to help you see these tubes. These are the metanephridia and they function in
osmoregulation.
Label the following structures on the drawing.
1. Metamers
2. Mouth/Prostomium
3. Pharynx, esophagus & crop
4. Gizzard & Intestine & Anus
5. Clitellum
6. Brain & Ventral nerve cord.
8. Hearts
Color the Organ Systems
For the picture to the right, color code the
organ systems for the earthworm using the
following key:
Circulatory System - Red | Reproductive
System - Blue | Digestive System - Green |
Nervous System - Yellow
Skin (outer) - brown or beige
Class Polychaeta
Polychaetes (many bristles) live in the marine
environment and are very common in the shallow,
intertidal zone. Many are carnivores and prey on a
variety of invertebrates. Nereis has pinching jaws
with a horny covering at the mouth; short sensory
palps are present immediately behind these, and
tentacles arise on segments immediately behind.
Lateral appendages (on the sides) are called
parapodia. These are used for locomotion--to row
the worm through the water and to excavate a burrow--and for gas exchange--they serve as gills.
Bunches of black setae arise in the body wall and extend beyond the ends of the parapodia.
Class Hirudinea
The name simply means leech. Leeches live in both marine and fresh water and in damp terrestrial habitats. The
body is segmented, but this is not clearly visible on the outside. A sucker at each end is used for locomotion on
a surface; swimming is achieved by contractions of the body wall musculature. Leeches lack setae and
parapodia. Some are predators, others are parasites. Those that feed on blood have sharp jaws and secrete an
anticoagulant that prevents clotting of the blood. Medicinal leeches are a fresh water variety that is still in use.
Exercise 1 – Leeches: Compare what you see in your slide of the common blood leech to what you saw for the
earthworm
PHYLUM NEMATODA – THE ROUNDWORMS
Prepared slides – trichenella spiralis (roundworm)
-ascaris (c.s)
-pin worm (enterobius vernicularis)
-hook worm
Preserved samples – round worms dissected and whole
Dissection – Ascaris male and female
Nematodes, like bacteria, live in a wide variety of environments. Most are harmless, but a few are parasitic to
humans and our livestock. Nematodes are able to survive in some many hostile environments due to their
tough cuticle. Because of this cuticle and the fact that it must be shed in order for the nematode to grow,
nematodes are considered to be ecdysozoans – an animal that surrounds itself with a protective exoskeleton
called a cuticle. The epidermis is located under this cuticle
Nematodes are slender pseudocoelomate worms, typically less than 2.5 mm (0.098 in) long, but greater than
0.1 mm long. They are approximately 5 to 100 µm thick. The smallest nematodes are microscopic, while freeliving species can reach as much as 5 cm (2.0 in), and some parasitic species are larger still, reaching over a
meter in length. The body is often ornamented with ridges, rings, bristles, or other distinctive structures.
The head of a nematode is relatively distinct. Whereas the rest of the body is bilaterally symmetrical, the head
is radially symmetrical, with sensory bristles and, in many cases, solid 'head-shields' radiating outwards around
the mouth. The mouth has either three or six lips, which often bear a series of teeth on their inner edges. The
mouth often includes a sharp stylet, which the animal can thrust into its prey. In some species, the stylet is
hollow, and can be used to suck liquids from plants or animals.
The oral cavity opens into a muscular, sucking pharynx. There is no stomach, with the pharynx connecting
directly to a muscle-less intestine that forms the main length of the gut. The intestine produces enzymes for
extracellular digestion followed by absorption of nutrients. The last portion of the intestine forms a recturm
which ends as an anus, just below and in front of the tip of the tail. Movement of food through the digestive
system is the result of body movements of the worm.
Nitrogenous waste is excreted in the form of ammonia through the body wall, and is not associated with any
specific organs. So these worms do not possess nephridia. However, in many marine nematodes, one or two
unicellular 'renette glands' excrete salt through a pore on the underside of the animal, close to the pharynx.
Exercise 1 – Ascaris lumbricoides preserved slide: Ascaris is an excellent model of a typical nematode. Observe
your slide of Ascaris in cross section and draw what you see. Identify the following: Intestine, pseudocoel, cuticle
and epidermis, longitudinal muscles. Is the worm in the slide male or female?
QUESTIONS:
1. What is the scientific name of the intestinal roundworm found in humans?____________________
2. Is the male or female nematode larger? _________________________
3. What substance is found in the nematode cuticle that is also found in human connective tissue?
___________________________
4. Why is the cavity of the nematode called a pseudocoel? _______________________________
Exercise 2 – Slides of the pin worm (Enterobius vernicularis) and the hookworm (Necator americanus): Observe
these nematodes under the microscope and draw what you see.
Exercise 3 – Ascaris dissection: Using the handout provided, dissect an Ascaris nematode. Identify the following
structures:
Lateral line
Pharynx
Intestine
Uterus (U-shaped) - uniting to form a vagina (ends as the genital pore)
Oviducts
Vas Deferens
Testes
Seminal vesicles
QUESTION: Do you have a male or a female Ascaris? _________________________
QUESTION: If you have a male – can you see the spicules at the posterior end. Use a dissecting scope to help
you. ________________________________