Lab 7: Annelids and Smaller Ecdysozoans

1. Taxonomy for Lab 7

Phylum Annelida – the segmented worms

  • Class Polychaeta – marine bristle worms (Nereis virens)
  • Class Oligochaeta – earthworms & freshwater forms (Lumbricus, Tubifex, etc.)
  • Class Hirudinea – leeches (Hirudo medicinalis,)

Phylum Nematoda: – Ascaris, Trichinella, Ancylostoma, Enterobius, Dirofilaria
Phylum Onychophora – "velvet worms" or "walking worms" (Peripatus)

2. Phylum Annelida - Class Polychaeta

The Class Polychaeta (“many bristles”) is the largest group of annelids, containing over 10,000 species, most of which are Marine. Polychaete characteristics include a well-developed head and lateral appendages called parapodia, which are found on each segment. Parapodia are used for both creeping and swimming. They can also take part in gas exchange, which is possible because parapodia contain a large number of respiratory capillaries. In some forms (fanworms), the parapodia have been modified into large, feathery gills. In terms of nutrition, many polychaetes are carnivorous, seizing prey in chitinous jaws or teeth that are everted rapidly from a muscular pharynx. Some sedentary forms are particle feeders that use ciliary currents and mucus to trap organic food molecules, plankton, etc.

Examples of polychaete worms include: Nereis virens (the clamworm), Amphitrite (the spaghetti worm), Glycera (the beak thrower), Chaetopterus (the parchment worm), Aphrodita (the sea mouse) and fan worms (also called feather duster worms or Christmas tree worms)

Nereis virens

Lab-7 01  

Nereis virens is a common polychaete inhabitant of shallow waters along the Atlantic coast of North America. Hiding by day in burrows in sand or mud bottoms, the worms come out at night to search for food, which is grabbed with powerful jaws on a muscular eversible pharynx. 


Lab-7 02 Lab-7 03

Amphitrite is a genus marine polychaete worm that lives in the mucus tubes, which become thickly covered in fine sand and broken shell material. They are sedentary and feed by waving the tentacles out of the tube into the seawater. These tentacles (which are the most visible parts of the animal) are reminiscent of spaghetti, giving them their common name spaghetti worms. The remainder of the animal remains in a semi-permanent burrow or tube constructed in soft substrates. The tentacles are used to capture fine organic debris (detritus) and small organisms, which are then transported to the mouth by fine cilia. 


Lab-7 04

This marine polychaete is a voracious predator that remains hidden in an elaborate gallery system constructed in a muddy bottom. When a suitable prey approaches the entrance to the gallery, the beak thrower explosively everts its pharynx to nearly one-fifth its body length and seizes it with four jaws that open at the tip. The prey is then subdued with poison injected through ducts in its jaws.


Lab-7 05

This unique polychaete lives in mud flats in a leathery, open-ended U-shaped tube. Its common name comes from the parchment-like appearance of the tubes the worm lives in. Modified parapodia near the middle of the body form fans that move water through the tube from end to end. Food particles carried by these currents are entangled in a sheet of mucus and then carried up a ciliated groove to the mouth.


Lab-7 06

This strange looking polychaete has numerous felt like setae on the dorsal surface and parapodia (on the ventral surface) that allow it to crawl across the substrate. These organisms may grow up to 20 cm and are active carnivores that feed chiefly on other polychaetes, including Nereis, which may be up to three times the length of the sea mouse. The setae on the dorsal surface are iridescent and have a red sheen that may warn off predators.

Dorsal surface

Lab-7 09

This image of a sea mouse show the brilliantly colored setae that emerge from its scaled back. These setae are made of millions of submicroscopic crystals that reflect and filter the faint light of the ocean depths.

Ventral surface

Lab-7 11

This image shows the ventral surface of a sea mouse with its numerous parapodia that permit it to crawl over the substrate in search of prey.

Fan worms

Lab-7 07

These colorful polychaetes (sometimes called feather duster worms or Christmas tree worms) live in tubes constructed of sand grains embedded in mucus. Note the feather-like radioles (tentacles) that bear numerous branches lined with cilia. When threatened, a worm can rapidly withdraw these radioles into the tube, which is then sealed with an protective plug called an operculum.

3. Phylum Annelida - Class Oligochaeta

   Lab-7 08

The Class Oligochaeta (“few bristles”) contains about 3,000 species of earthworms and freshwater forms. Earthworms lack parapodia, are poorly cephalized and in general, are less diverse than polychaetes. Whether crawling on the surface or burrowing through soil, earthworms move by means of peristaltic contractions. Circular and longitudinal muscles are used alternately to extend and thicken each segment, while four pairs of setae on each segment are used to anchor parts of the body to prevent slipping. In terms of reproduction, oligochaetes are monoecious but practice cross fertilization, and development is direct.

Freshwater forms

Lab-7 10

Although most oligochaetes are terrestrial, some live in freshwater. Usually smaller than earthworms (many are microscopic), freshwater forms are normally found on the bottom or burrowed into mud where they feed on algae and detritus. Note the chitinous setae projecting from the sides of the worm in the image above.

An important group of freshwater oligochaetes are tubificid worms that live within tubes they secrete. The blood of such species contains very high concentrations of hemoglobin, which allows them to exist in oxygen-poor waters such as sewage outflows and settling tanks. As such, tubificids serve as important indicators of water quality. 


Lab-7 12

During the reproduction, two earthworms come together from opposite directions in a way so that their clitella are in contact. The clitellum, a saddle-like structure where fertilization actually takes place, is located about one-third way down the worm's body. It produces a mucus that holds the two earthworms together, so that during the exchange of sperm, the connection is not broken. Once the sperm is received from each partner, it is stored in the seminal receptacles until it is used to fertilize the eggs, which are then encased in cocoons produced by the clitellum. 

4. Phylum Annelida - Class Hirudinea

   Lab-7 13

The Class Hirudinea contains about 500 species of leeches that have bodies that are flattened dorsoventrally and lack setae. Although they have a fixed number of metameres (34), leeches appear to have more because of the presence of many superficial rings called annuli. Internal septa are lacking, and the coelom functions as one large chamber that is occupied mainly by spongy tissue and dorsoventral muscles. These muscles along with the others allow leeches to swim gracefully by means of dorsoventral undulations of the body. In terms of nutrition, although popularly considered as parasitic bloodsuckers, most are actually predators that go after anything they can catch or scavengers that feed off dead animals. Some, however, are ectoparasites on various invertebrates and vertebrates, including humans. The best known of these is the medicinal leech.

Medical leech

Lab-7 14

The medicinal leech (Hirudo medicinalis) is best known as the organism used for blood letting (people used to believe many health problems caused by getting rid of "bad" blood). Surprisingly, they are being used once again to remove blood from hematomas (areas of blood leakage) that result from surgical procedures such as re-attaching severed limbs. 

5. Phylum Nematoda

Lab-7 15

Phylum Nematoda contains about 12,000 species of roundworms. Although some species are free-living, many are parasitic on just about every form of plant and animal, causing agricultural damage and as well as disease, discomfort and death to humans and domestic animals. The outer body covering of nematodes consists of a thick, noncellular cuticle that protects parasitic forms from digestive enzymes of the host and free-living forms from abrasion and other hazards.

6. Phylum Onychophora

Lab-7 16

The Phylum Onychophora contains about 70 species of small, nocturnal, caterpillar-like animals called "velvet worms" or "walking worms" that live in rain forests and other tropical and semitropical leafy habitats of the Southern Hemisphere. The best-known genus is Peripatus, which is a “living-fossil” that has not changed much in the past 500 million years! Onychophorans are of interest to zoologists because they share many characteristics of both annelids and arthropods.

7. Nereis c.s.

 Lab-7 17

  1. Dorsal blood vessel
  2. Longitudinal muscles
  3. Coelom
  4. Lumen of intestine
  5. Ventral nerve cord
  6. Ventral blood vessel
  7. Parapodia

This slide shows a cross section through the body of a clamworm (Nereis virens). These predatory polychaete annelids are common in shallow waters along the Atlantic coast of North America. Hiding by day in burrows in sand or mud bottoms, the worms come out at night to search for food, which is grabbed with powerful chitinous jaws on a muscular eversible pharynx. Biramous lateral appendages called parapodia are easily seen on this cross section. These highly vascularized appendages serve both for locomotion (swimming or creeping along the bottom) and respiration, and they may also be supplied with various sensory structures.

Dorsal region (close-up)

 Lab-7 18

  1.  Dorsal blood vessel
  2. Longitudinal muscles
  3. Cuticle
  4. Hypodermis
  5. Circular muscle layer

Ventral region (close-up)

Lab-7 19
  1. Ventral blood vessel
  2. Ventral Nerve Cord
  3. Coelom
8. Earthworm c.s.

Lab-7 20

  1. Dorsal blood vessel
  2. Body wall layers
  3. Nephridium
  4. Seta
  5. Ventral blood vessel
  6. Ventral nerve cord
  7. Typhlosole
  8. Intestinal lumen
  9. Coelom

This slide shows a stained cross section through the body of a common earthworm (Lumbricus terrestris). On the outside of the worm is a thin, non-living cuticle that is secreted by the underlying hypodermis. Beneath the hypodermis is a thin layer of circular muscles and a much thicker layer of longitudinal muscles. Observe the large body cavity (coelom) lined by a thin layer of flattened cells that make up the peritoneum. Much of the coelom is taken up by the intestine, which contains a conspicuous fold of tissue called the typhlosole. It's thought that this structure serves to increase the surface area of the intestine for absorption. Covering the outside of the intestine and most of the inside of the typhlosole is a specialized tissue made up of chloragogue cells. These cells are involved in a variety of metabolic functions including the synthesis of urea, glycogen, fats and hemoglobin.

Earthworm body wall layers (close-up)

Lab-7 21

  1. Cuticle
  2. Hypodermis
  3. Circular muscles
  4. Longitudinal muscles 

Earthworm setae (close-up)

Lab-7 22

  1. Setae
  2. Coelom
  3. Cuticle
  4. Hypodermis
  5. Circular muscle layer
  6. Longitudinal muscles

Earthworm ventral region (close-up)

Lab-7 23

  1. Ventral blood vessel
  2. Ventral nerve cord
  3. Subneural blood vessel
  4. Intestinal epithelium
  5. Longitudinal muscles

Earthworm typhlosole (close-up)

Lab-7 24

  1. Typhlosole (entire structure)
  2. Chloragogue cells
  3. Intestinal lumen
  4. Coelom
  5. Intestinal epithelium
  6. Dorsal blood vessel
9. Leech w.m. 

 Lab-7 25

This slide shows a stained whole mount of a common freshwater leech (Helobdella stagnalis) with its proboscis extended. Although they are known for their blood sucking habits, many leeches are actually predators that feed on soft-bodied invertebrates. During feeding, the proboscis is extended out of the mouth where it is used to penetrate the surface of the prey and suck out its body fluids. In the region of the neck a dark, circular structure can be seen. The function of this structure (which is unique to this species) is unknown!

Anterior Region

Lab-7 26

  1. Proboscis
  2. Eye spot
  3. Proboscis cavity

Middle Region

Lab-7 27

  1. Male atrium
  2. Vas deferens
  3. Salivary glands
  4. Crop
  5. Testis (one of six pairs)

Posterior Region

Lab-7 28

  1. Crop
  2. Gastric ceca
  3. Intestinal ceca 

Posterior Sucker

Lab-7 29

  1. Intestine
  2. Posterior sucker


10. Nereis parapodium

Lab-7 30

  1. Dorsal cirrus
  2. Notopodium
  3. Setae
  4. Neuropodium
  5. Ventral cirrus
  6. Acicula

This slide shows a parapodium from a polychaete worm. This biramous appendage consists of a ventral division called the neuropodium and a dorsal division called the notopodium, each of which is supported by a stiff chitinous rod called an aciculum. A dorsal cirrus and ventral cirrus (richly supplied with sensory receptors) project from the notopodium and the neuropodium respectively. Numerous dark staining setae that extend beyond the parapodium can also be seen on this slide.

11. Earthworm setae

Lab-7 31

This slide shows a another cross section through the body of a common earthworm (Lumbricus terrestris). Note the eight chitinous setae (pointed to by blue arrows) that are found in each segment of the earthworm except the first and last. These structures are used to help anchor the worm when burrowing and moving through the soil. Although each seta in the living worm does project through the body wall, this section does not reveal this fact.

Earthworm setae (close-up)

Lab-7 32

  1. Setae
  2. Cuticle
  3. Hypodermis
  4. Circular muscle layer
  5. Longitudinal muscles
12. Earthworm nephridium

Lab-7 33

  1. Nephridia
  2. Nephridiopore
  3. Coelom
  4. Intestinal lumen

This slide shows a cross section of the common earthworm (Lumbricus terrestris). Note the coiled tubular portions of the nephridia found on each side of the coelom. These excretory organs open through nephridiopores on the ventral surface of the body wall. This specimen has been sectioned in such a way that a portion of one nephridiopore can be seen on the lower right hand side of the slide.

Earthworm nephridiopore (close-up)

Lab-7 34

  1. Nephridiopore
  2. Hypodermis
  3. Circular muscle layer
  4. Longitudinal muscles
13. Earthworm dissection mount 1 (anterior region)

Lab-7 35

  1. Pharynx
  2. Esophagus
  3. Aortic arch
  4. Seminal receptacle
  5. Seminal vesicle
  6. Crop
  7. Gizzard
  8. Intestine

This image shows the anterior region of a preserved earthworm. Note the five pairs of aortic arches (which have been injected with red latex) that serve as "hearts" to pump blood from the dorsal blood vessel to the ventral blood vessel. Digestive structures visible on the dissection mount include the pharynx, esophagus, thin-walled crop, muscular gizzard (which serves to grind up ingested food) and the beginning of the intestine. Also seen on the image are the seminal receptacles (which receive sperm from the mating partner) and seminal vesicles (which store sperm for release to the mating partner).

14. Earthworm dissection mount 2 (anterior region)

Lab-7 36

  1. Intestine
  2. Ventral nerve cord
  3. Dorsal blood vessel

This image shows the middle region of a preserved earthworm. Note the dorsal blood vessel (which has been injected with red latex) and ventral nerve cord that carries information from a pair of cerebral ganglia that serve as a "brain" to all segments of the worm. The large, muscular intestine is also visible on the display.

15. Earthworm model

Lab-7 37   

This image (and the three close-up views linked above) is taken from a plastic model of an earthworm. Earthworms burrow through soil, feeding on decaying organic matter. As pointed out by Charles Darwin, they perform a beneficial role by aerating the soil and enriching it by bringing up nutrients from below.

In terms of nutrition, food is brought into mouth by a muscular pharynx that passes through the esophagus, which is surrounded by calciferous glands that remove excess calcium ions (acquired from eating the soil) from the blood and secrete them into the gut. Food then enters a thin-walled crop and is passed on to a muscular gizzard, which serves as a grinding organ. From there it passes into the intestine to exit out the terminal anus. The dorsal wall of the intestine is folded inward to form a typhlosole that serves to increase the surface area for digestion and absorption. Chloragogue cells surrounding the intestine and filling much of the typhlosole are involved in the synthesis of glycogen and fats.

Although much of the circulation in annelids is handled by the coelom, earthworms also have a well-developed, closed circulatory system consisting of a dorsal vessel that runs above the alimentary canal from the anus to the pharynx. The dorsal vessel receives blood from the body wall and pumps it anteriorly into five pairs of aortic arches that help maintain a steady pressure into the ventral vessel, which delivers blood to the rest of the body.

In terms of excretion, some wastes simply diffuse out through the moist skin, which also serves as the principal gas exchange organ. Other wastes are handled by paired structures called nephridia. Each nephridium (also called a metanephridium) has a ciliated funnel-like nephrostome that collects wastes from the coelomic fluid and then passes it through the transverse septum into the next metamere. The nephridia empty to the outside via a openings called nephridiopores.

In terms of reproduction, although earthworms are monoecious, they practice cross fertilization. Copulation occurs between partners that are joined by mucus secretions from a saddle-like structure called the clitellum and by special ventral setae that penetrate each partner’s body. Sperm are released from the seminal vesicles of one partner and received by seminal receptacles of the other after passing along a seminal groove. After copulation, the clitellum of each worm secretes a cocoon that receives the sperm and eggs, which are then fertilized in the cocoon. The cocoon is then deposited in the ground, where direct development takes place, terminating when a young earthworm that resembles the adult hatches from the cocoon.

Segments 1-8

Lab-7 38

1. Prostomium; 2. Mouth; 3. Buccal cavity; 4. Esophagus; 5. Aortic arch; 6.Nephridia 7. Cerebral ganglion; 8. Circumpharyngeal connective; 9. Ventral nerve cord.

Segments 5-16

Lab-7 39

1. Pharynx; 2. Esophagus; 3. Crop; 4. Aortic arches; 5. Dorsal blood vessel;
6. Ventral blood vessel; 7. Nephridia (a pair in each segment); 8. Seminal vesicles

Segments 13-24 

Lab-7 40

1. Esophagus 2. Crop; 3. Gizzard; 4. Chloragogue cells; 5. Intestine; 6. Dorsal blood vessel; 7. Ventral blood vessel; 8. Setae; 9. Ventral nerve cord; 10. Clitellum

16. Earthworm dissection

Lab-7 41

This dissection of a preserved earthworm shows some of the more conspicuous features of its internal anatomy. Digestive structures that can be seen include the buccal cavity (1), pharynx (2), crop (3) and gizzard (4). Also seen on the image are the seminal vesicles (6), seminal receptacles (7), the dorsal blood vessel (5) that runs along the top of the intestine, two of the many septa (8) that divide the coelom into separate segments called metameres and one of the five pairs of aortic arches (9) that help pump blood from the dorsal blood vessel to the ventral blood vessel.

17. Ascaris lumbricoides male c.s.

Lab-7 42

  1. Cuticle and hypodermis
  2. Longitudinal muscle layer
  3. Vas deferens
  4. Testis
  5. Lateral line with excretory canal
  6. Intestine
  7. Pseudocoelom

This slide shows a cross section of a stained, male, large intestinal roundworm (Ascaris lumbricoides). This nematode is a common parasite of humans, with over a billion people infected worldwide. The section shows the outer protective cuticle secreted by the underlying hypodermis. Longitudinal muscle bands, dorsal and ventral nerve cords, lateral lines (containing the excretory canals), the intestine and pseudocoelom (body cavity) are also visible. The slide also contain various sections through the long, coiled testis and vas deferens, the latter of which contains the ameboid spermatocytes (sperm cells). Note: These structures may differ in shape, depending on the way they were sectioned.


Ascaris male (close-up)

Lab-7 43

  1. Cuticle
  2. Hypodermis
  3. Excretory canal
  4. Testis
  5. Vas deferens
  6. Longitudinal muscles
  7. Pseudocoelom
18. Ascaris lumbricoides female c.s.
Lab-7 44
  1. Cuticle and hypodermis
  2. Longitudinal muscle layer
  3. Ovary
  4. Oviduct
  5. Uterus
  6. Intestine

This slide shows a cross section of a female, large intestinal roundworm (Ascaris lumbricoides). The section shows the outer protective cuticle, which is secreted by the underlying hypodermis. Longitudinal muscle bands, the intestine and body cavity (pseudocoelom) are clearly visible. The slide also contain various sections through the long, coiled tubules that make up the reproductive system. The system begins with the ovaries that lead to thicker oviducts that lead to the two large uteri containing the eggs. These uteri eventually join to form the vagina (not shown on the slide), which exits at the genital pore. Note: The ovaries and oviducts may differ in shape, depending on the way they were sectioned.

Ascaris female (close-up)

Lab-7 45

  1. Cuticle
  2. Hypodermis
  3. Longitudinal muscles
  4. Ovary
  5. Oviduct
  6. Pseudocoelom
19. Trichinella spiralis infected muscle section

Lab-7 46

  1. Encysted larvae
  2. Skeletal muscle fibers

This slide shows a section of skeletal muscle the containing encysted larvae of the nematode parasite Trichinella spiralis. Humans can contract the parasite by eating raw or inadequately cooked pork or bear meat. Once ingested, enzymes in the digestive system trigger the release of the encysted worms, which mature, mate and produce offspring. These juvenile worms then enter the host's lymphatic system and are distributed through the body via the circulatory system to active skeletal muscle into which they burrow and become enclosed in cysts.

20. Ancylostoma caninum male and female w.m. (hookworm)

Lab-7 47

  1. Mouth

This slide shows male and female dog hookworms (Ancylostoma caninum). The male is easily recognized by its conspicuous copulatory bursa, an expanded posterior portion of the worm used for grasping the female during mating. Several species of hookworms infect over a billion people in warmer regions of the world, including the southeastern United States. Juvenile parasites gain entrance their hosts (often barefoot children) by burrowing through the skin. Note the mouth with hook-like cutting teeth that are used to attach to the wall of the intestine where they feed on blood. Heavy infestations can cause anemia, malnutrition and retarded development.

Ancylostoma caninum (close-up)

Lab-7 48

  1. cutting tooth
19. Enterobius vermicularis w.m. (pinworm)

Lab-7 49

  1. Mouth

This slide shows a stained adult pinworm (Enterobius vermicularis). Pinworms are among the most common nematode parasites found in humans, infecting over 30% of preschool children and 10% of the adults in the United States. Although they can cause anal itching, they seldom constitute a serious health hazard because they feed on wastes and bacteria in the cecum and appendix of the large intestine.

Close-up view of the pharyngeal region

Lab-7 50

  1. Mouth
  2. Pharynx
  3. Pharyngeal bulb

This slide shows a close-up of the pharyngeal region of a stained adult pinworm (Enterobius vermicularis). Note how the muscular pharynx on this specimen ends in an expanded pharyngeal bulb, which is a characteristic of nematodes in this group.

21. Freshwater nematode

Lab-7 51

This slide shows a live freshwater nematode. Although some nematodes (including those shown in the previous pages) are parasitic, many species are free-living. Free-living forms may feed on small animals (including other nematodes), plant material, algae and fungi. Many freshwater and marine nematodes are deposit-feeders that ingest substrate particles that contain dead or decaying organic matter.

22. Vinegar eels

Lab-7 52

This slide shows a number of living nematodes that are commonly referred to as "vinegar eels" (Turbatrix aceti). These free-living roundworms exist by feeding on bacteria and fungi found in the sediments of nonpasteurized vinegar. When viewed alive, vinegar eels are seen to be in constant motion, using vigorous dorsolateral undulations to propel themselves through the medium.

Phase Contrast Image of a Clump of Vinegar Eels

Lab-7 53

23. Dog heartworm (Dirofilaria immitis) microfilariae

Lab-7 54

This slide shows a blood smear containing two microfilariae of the dog heartworm (Dirofilaria immitis). Infection by this parasite occurs when the larval heartworms are transferred to the definitive host through the bite of a mosquito, the nematode's intermediate host. Heavy infestations of adult worms (which live in the heart and large arteries of the lungs) can be fatal. The regular use of heartworm medication (obtained from a licensed veterinarian) can prevent the disease.

24. Dog heartworm (Dirofilaria immitis) adults (in situ)

Lab-7 55

This image shows an actual dog's heart infected with adult heartworms (Dirofilaria immitis). These nematode parasites (which are transmitted by the bite of a mosquito carrying the microfilariae stage of the parasite) are a major menace to the health of dogs in North America. The use of heartworm pills can prevent this serious disease!

25. Female Ascaris dissection mount (anterior section)

Lab-7 56

The above image shows a magnified view of the anterior end of the large intestinal roundworm (Ascaris lumbricoides), one of the largest of the parasitic roundworms, with females reaching a length of up to 30 cm in humans!

The digestive system of nematodes consists of a mouth that leads to a muscular pharynx, from there to a long, non-muscular intestine (1), which is injected with yellow latex on the dissection mount, a short rectum and terminal anus. Since the intestine is only one cell layer thick and lacks any musculature, food must be moved posteriorly by body movements and by additional food being passed into the intestine from the muscular pharynx.

26. Female Ascaris dissection mount (middle section)

Lab-7 57

The above image shows a magnified view of the middle portion of the large intestinal roundworm. Except for the thin, non-muscular intestine (1), most of the internal organs are given over to reproduction. In females, a short vagina (2) leads from the genital pore to a point where it splits into two large uteri (3). Each uterus continues to highly coiled oviducts (4) that eventually terminates in thread-like ovaries (shown on the next page).

27. Female Ascaris dissection mount (posterior section)

Lab-7 58

The above image shows a magnified view of the terminal portion of the large intestinal roundworm. Note the highly coiled, thin ovaries (2) in which the eggs are produced. As the eggs mature, they move into the oviduct and from there they are carried to the two uteri (one of which is shown on the image as structure number 1) where they are fertilized by ameboid sperm from a male worm. A single female can produces thousands of eggs each day. These eggs (which are deposited in the soil with the feces of their hosts) can remain viable for months or even years. Not surprisingly, over a billion people on earth are infected with the large intestinal roundworm! Also seen on the image is the end of the non-muscular intestine (3).