Phylum Platyhelminthes

Dr. Elizabeth A. Bergey and Dr. Eric G. Bright, University of Oklahoma

Modified, with permission, from Invertebrate Anatomy OnLine
copyright 2003 by Richard Fox (Lander University)

Platyhelminthes Overview

Flatworms, or platyhelminths, are bilaterally symmetrical and have three tissue layers.  Unlike most triploblastic animals, they have no body cavity and no blood system.  The gut, if present, has a single opening to the exterior.  As in other bilateral animals, there is an anterior brain and associated sense organs.  Flatworms have elaborate hermaphroditic reproductive systems.  Fertilization is internal. 

Class Neoophora

Neoophorans includes 4500 described species of free-living flatworms. The body is cylindrical in small species (that may be as small as ciliated protozoa) and dorsoventrally flattened in larger species.  The epidermis is ciliated.  The mouth is located somewhere on the ventral midline and opens into a blind gut, or gastrovascular cavity, which lacks an anus and sometimes lacks a lumen.  Most are aquatic, inhabiting the oceans and fresh water (where most are benthic), but a few are found in moist terrestrial environments.

Osmoregulation and fluid regulation is accomplished with protonephridia.  There is no blood system and transport is by diffusion, which is facilitated by small body size and flattening.  Nutrients are delivered to tissues by diffusion from the gut, whereas oxygen diffuses across the body surface.  The end product of nitrogen metabolism is ammonia, which is lost by diffusion across the body surface.  The nervous system consists of a bilobed brain, or cerebral ganglion, from which longitudinal nerve cords arise and extend posteriorly for the length of the body.  Light-sensitive pigment-cup ocelli occur in various places.  Turbellarians are hermaphroditic with internal fertilization.

Order Tricladida

Dugesia (a planarian)

Dugesia is a triclad, so has a gut with three branches. Dugesia and other freshwater planarians are found under stones or other objects (they are negatively phototactic).

Look at a whole mount of a fixed and stained planarian with the dissecting microscope or low power of the compound microscope.  Note the dorsoventral flattening and anterior-posterior axis, which is the axis of symmetry which divides the worm into right and left sides.  Is the dorsal or ventral side mounted upward?

The anterior end of the body is the head and the remainder is the trunkDugesia tigrina has a pointed triangular head but the head of many planarians is blunt and rounded. The posterior limit of the head is marked by the auricles, a pair of lateral, chemosensory protrusions. Two dark ocelli, or eyespots, occur on the head.

Locate the cylindrical pharynx lying on the midline at the center of the body. The pharynx is a muscular, protrusible tube. During feeding, the pharynx is extended far out of the mouth to reach the food.  The proximal end of the pharynx opens into the intestine which, in triclads, immediately divides into three branches one anterior and two posterior branches.  The name Tricladida means ‘three branches’.  Each branch ends blindly and bears abundant ceca (singular = caecum), so that no area of the body is beyond diffusion distance from food. Protonephridia, the reproductive system, and the nervous system are hard to distinguish on these slides. 

Dorsal view of Dugesia tigrina
Cross-section of Dugesia tigrina

Cross-section slides

Examine a prepared slide of planaria cross sections; the sections on a slide are taken at different levels along the worm: there is usually one through the anterior end of the body, one through the pharynx, and one through the posterior part of the worm.  

Locate and identify the locations of each of the three sections on your slide. The ventral surface is usually flatter than the arched dorsal surface.  The pharyngeal section is usually easy to recognize because it shows the large, unmistakable pharynx as a hollow, red circle surrounded by a narrow white ring in the center of the section.  The anterior and posterior sections have one to many irregular circles distributed through the cytoplasm.  These circles are intestinal branches (rami) and ceca (singular = caecum).

The body is covered by a monolayered, secretory epidermis.  The ventral, but not the dorsal, epidermis is ciliated, a fact that can be verified by careful observation with high power (400X).  The cilia are used for locomotion.  The epidermis is underlain by a distinct basal lamina, which is visible as a thin, dark line just inside the epidermis. 

The dorsal epidermis contains numerous secretory vesicles and rod-shaped rhabdites (rhabd = rod). Rhabdites become a sticky mucus that may help trap small invertebrate prey when expelled at the surface. Clusters of adhesive gland cells occur at the lateral edge of the worms. These are part of a cilia-free adhesive zone that encircles the worm. These cells secrete an adhesive that helps the animal grip the substratum.  The ventral epidermis bears numerous gland cells that secrete mucus. 

A thick layer of body wall muscles, consisting of outer circular and inner longitudinal fibers, lies just inside the epidermis.  Inside the muscle layer, the interior of the worm is filled with a mesenchymal connective tissue, the parenchymaDorsoventral muscles can be seen passing vertically through the parenchyma connecting the muscle layers of the dorsal and ventral body walls.  These muscles maintain a flat shape.

Scattered about in the interior are sections through the intestinal branches and their ceca.  These are irregular circles of various sizes.  The clear space in the interior of each is the gut lumen and is surrounded by a monolayered epithelium of large, vacuolated cells, which may be secretory, absorptive, or phagocytic.  This epithelium is usually referred to as the gastrodermis.

Pharyngeal Cross-section

Look at the pharyngeal cross section.  Review the now familiar features you identified on the other sections and then study the enormous pharynx in the center. 

Cross section through the pharyngeal region of Dugesia

The pharynx occupies almost the entire center of the section and the body wall is very thin above and below it.  Locate and identify the two white, unstained spaces associated with the pharynx.  The one in the center of the pharynx is the pharyngeal lumen (= the space within the pharynx) whereas that surrounding the pharynx is the pharyngeal cavity (= the cavity that surrounds the pharynx when it’s retracted). 

Living Specimens

Place a living worm in a drop of water on a microscope slide without a coverslip.  Living worms can be picked up with a plastic pipet that has had the tip cut off (so that it is wider), but they must be ejected quickly or they will attach to its wall using their adhesive glands.  They adhere tenaciously.

Observe the worm with the dissecting microscope.  Watch as it moves across the slide. Triclads do not swim.  It moves mainly by the cilia of the ventral epidermis, but muscular activity also plays a role, especially in turning.  Manipulate the worm with a blunt needle to encourage it to change directions.  Which muscles would be involved in making a turn?

Observe the animal with transmitted light and look for the intestine and its ceca.  These may be obvious, especially if the worm has been fed recently.

Place one or more worms in the center of a small culture dish of pond water.  Place a small piece of hard-boiled egg yolk in the dish.  Gently put the dish on the stage of the dissecting microscope.  If you are fortunate, you will see the worm protrude its pharynx and feed. Can you see the egg yolk moving up through the pharynx?


Dugesia reproduces asexually by dividing into two fragments: an anterior, nearly normal, worm with head, mouth, pharynx and most of the gut, and an incomplete, headless posterior mass of tissues which must replace its missing parts.   Following division, the anterior end behaves normally but the posterior end remains immobile until regeneration is complete and the missing parts replaced.  Dugesia also reproduces sexually. Females produce protective cocoons from which emerge the young planaria.


Bdelloura, the horseshoe leech, is a small colorless flatworm (not a leech!!) found on horseshoe crabs. Long though to be a commensal, it might actually damage the horseshoe crab’s skeleton and thereby damage it’s host. We’ll look at horseshoe crabs later in the semester.

Overview Parasitic Flatworms

Parasitic flatworms include flukes and tapeworms.  Most parasitic flatworms are endoparasites with complex life cycles with multiple hosts. The definitive host is inhabited by adult, sexually reproductive worms and one or more intermediate hosts are inhabited by juvenile stages of the worm.  Parasitic flatworms have a specialized epidermis, the neodermis. The neodermis is 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.   

Infraclass Trematoda (flukes)

Flukes are important animal endoparasites and several important human and livestock diseases are caused by them.

Flukes have a blind gut with mouth and pharynx, but no anus.  Osmoregulation is by protonephridia.  The integument (neodermis) lacks 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.

The life cycle includes at least two hosts.  The definitive host is always a vertebrate and the intermediate host is usually a gastropod (snail). An additional intermediate host, if present, is an arthropod or fish.  In the definitive host, flukes usually live in either the circulatory system or the gut.

Opisthorchis (= Clonorchis) sinensis  (Chinese liver fluke) and Fasciola hepatica  (sheep liver fluke)

The anatomy of digenetic trematodes is usually studied using wholemount slides of the Chinese liver fluke or the sheep liver fluke. 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 Asia.  The sheep liver fluke inhabits of the liver of sheep, cattle, and humans.

This exercise is based on slides of Opisthorchis. However, we will look at Fasciola, so comments are added that pertain to Fasciola; there are also a couple of added figures).

Note the size and shape of the worm.  The anterior end of the worm narrows gradually to a pointed tip.  The wider posterior end is bluntly rounded.   A conspicuous oral sucker surrounds the mouth and is used to attach to the host.  The circular ventral sucker is located on the ventral surface a short distance posterior to the oral sucker.  The ventral sucker also attaches to the host but is not connected to the worm’s gut.

The body wall consists of an outer syncytial neodermis. There is no cuticle.  Muscles underlie the neodermis and parenchyma fills the interior of the animal.  Organs are embedded in the parenchyma.  Trematodes, like other flatworms, are compact and lack a body cavity. 

The mouth opens into a muscular, bulbous pharynx, which is used to suck food into the gut.  Opisthorchis feeds on blood, epithelium, mucus, and fluids from the wall of the bile duct.  Immediately posterior to the pharynx is a short, inconspicuous esophagus. The esophagus quickly bifurcates to form the gastrovascular cavity, within two long, unbranched intestinal ceca that extend along the sides to the posterior end of the worm.  (Note: in Fasciola, the ceca have branches or diverticula, extending laterally from the two longitudinal ceca; these may be obscured by the vitallaria; see below). Digestion and absorption occur in these ceca.  They end blindly and there is no anus.  Flatworms have no hemal system.  The gut, itself, distributes food to the tissues and its lumen is called the gastrovascular cavity in recognition of its dual roles in digestion and transport.  Flukes also absorb nutrients directly across the cuticle-free neodermis. 

Protonephridia, which are scattered through the mesenchyme, and the branching lateral excretory canals are usually not visible.  The lateral canals drain into a large median excretory duct (bladder) at the posterior end of the worm.  This duct opens to the exterior by a large posterior excretory pore (nephridiopore).  Opisthorchis inhabits a low oxygen environment and is a facultative anaerobe with no special respiratory surface.  The nervous system is usually not visible.  The brain is a bilobed 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 abundant transverse commissures.

Opisthorchis, like other flatworms, is hermaphroditic and has 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 but if the structure is in bold type, it should be visible in most preparations.

The male system begins with two large, irregularly branched testes located in the posterior third of the worm (these are more forward in Fasciola).  The testes 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 arises near the center of each testis. The two vas efferens 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 opens with the uterus in the common genital pore (gonopore or genital atrium).  In ventral view the seminal vesicle is hidden by the uterus and may be difficult to observe.  The common 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.

The female system is more complex, partly because trematodes produce eggs with an oocyte surrounded by yolk cells.  Oocytes are produced by the ovary whereas yolk cells are produced by two independent vitellaria. The single branched ovary (germarium) stains dark pink and lies on or near the midline anterior to the testes.  The two large vitellaria produce yolk cells.  They lie lateral to the intestinal ceca.  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.   The large brown uterus extends anteriorly to its opening in the common gonopore.  It is a wide convoluted tube packed with dark, yellow-brown eggs.  The uterus and seminal vesicle share the common gonopore.

Ventral view of an adult Chinese liver fluke
Ventral view of an adult sheep liver fluke

Sperm from the testes are stored in the seminal vesicle.  During copulation, sperm are forced by contraction of 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. 

Later, eggs leave the ovary and are fertilized by sperm from the seminal receptacle. The fertilized eggs are then enclosed in a layer of yolk cells from the vitellaria.  Shelled eggs are stored in the uterus where meiosis is completed, fertilization accomplished, and development begins.  The eggs in the uterus are actually shelled embryos, sometimes referred to as embryonated eggs to distinguish them from ova.

Development 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. These eggs pass down the bile duct of the definitive host and into the gut. They exit the body of the host with its feces and some end up in freshwater habitats.                           

The Chinese liver fluke has three hosts.  The first intermediate host is an aquatic snail, the second intermediate host is a fish, and the definitive host is a mammal (human, pig, dog, cat, rat, and probably any other mammal that eats raw fish).  The first intermediate host (snail) ingests an egg with its fully developed miracidium.  The miracidium hatches in the snail’s gut and migrates to the snail’s digestive cecum.  In the cecum, the miracidium metamorphoses into a saclike sporocyst.  Cell division and development produce several rediae within each sporocyst. The rediae which escape when the sporocyst ruptures after about two weeks. The rediae remain in the snail.  Within each of the redia, more germinal cells mature into another larval stage, the cercaria.  This clonal reproduction, known as polyembryony, by both the sporocyst and the redia, greatly increases the number of larvae in the first intermediate host. The cercaria looks like a little fluke with a tail.  It has eyes to help it locate a passing fish.

In the Chinese liver fluke, the 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. The definitive host is infected when it eats raw or poorly cooked fish with metacercariae in their muscles.  With the help of the host’s digestive enzymes, young flukes emerge in the intestine. Young flukes simply migrate up the bile duct to the liver.

In the sheep liver fluke, the metacercaria encyst on aquatic palnts- especially water cress, and the metacercaria are transmiited to the mammalian host when these plants are eaten. Beware watercress sandwiches!!!!
The 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 fish before they are eaten.

Life cycle of Fasciola hepatica

Aspidogaster (a bivalve fluke)

Aspidogaster is a bivalve fluke, found in freshwater bivalves. Adults are found in the renal or pericardial cavity. Eggs can develop in place or eggs or larvae are expelled from the bivalve and can be taken up and infect other bivalves, turtles and fish.


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.  Among the 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.  In mating, the female worm lies within a ventral groove of the male, an obvious adaptation to these worms living in a "turbulent" environment (the host's blood stream). 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.

As the eggs of the schistosomes penetrate the walls of the veins and the small intestine or urinary bladder, they cause a significant amount of damage to the tissues.  The tissues hemorrhage, so blood often appears in the urine or feces.  As the infection progresses the tissues become inflamed and fibrotic and unable to function normally.  Many of the eggs produced by the female worms do not escape from the veins, but are swept up in the circulatory system and deposited in the host's liver.  The liver responds to the presence of the eggs by encapsulating them in a fibrous granuloma.  The damage to the small intestine (or urinary bladder) and liver accumulate over time and result in a chronic, disabling disease that can be fatal.

As with most trematode infections, diagnosis most often depends on finding the parasite's eggs.  In the case of S. haematobium, eggs are most often recovered in the urine; eggs of the other two species are most often recovered in the feces.

Infraclass Cestoda (tapeworms)

Tapeworms are highly specialized internal parasites. Adults of most species inhabit vertebrate intestines (i.e., vertebrates are the definitive host).  The life cycle almost always includes an intermediate host, which can either be an invertebrate or a vertebrate.  The intermediate host is usually the prey of the definitive host.

The cestode body is composed of an anterior scolex, with which the worm attaches to its host, and a long posterior strobila which is divided into numerous segment-like units known as proglottids.  Most organ systems are reduced or absent except for the reproductive system, which is well developed.  There is no gut. Small organic compounds are absorbed across the body surface. The syncytial neodermis bears microvilli to facilitate with this absorption. 

Cestodes are compact worms whose interior is filled with parenchyma, as it is in other platyhelminths.  Excretion and osmoregulation are accomplished via protonephridia which empty into two pairs of excretory canals draining to the posterior end of the strobila.  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.

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.  Use the compound microscope to study whole mount slides of the scolex, a mature proglottid, and a gravid proglottid.

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 on low power with 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, using 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. Scolex morphology varies widely among taxa.

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. The pair of nerve rings will not be evident.

The scolex and anterior strobila of Taenia pisiformis
Anatomy of a mature Taenia pisiformis proglottid

 The strobila is long and wormlike, with hundreds or thousands of proglottids that may reach 15 or more meters in length.  (There are records of Taeniarhynchus saginatus, a human parasite, reaching lengths of more than 20 meters.)  The anterior end of the strobila consists of the youngest immature proglottids produced just behind the scolex. New proglottids are short and wide and do not yet contain differentiated cells or organs.  This is the opposite of segmented animals such as annelids and arthropods in which the segment-producing cells, and the youngest segments, are at the posterior end of the body.  Behind the young immature proglottids is 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. The strobila is dorsoventrally flattened and 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.

Examine a mature proglottid at 40X.  Your slide probably contains several proglottids, some of which may be better than others.  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. 

Tapeworms have no mouth, gut, or digestive system.  Organic molecules are absorbed across the specialized neodermis and metabolism is largely anaerobic.  There is no need for a gas exchange mechanism and there is none.  There is no organized fluid transport system.

The body wall consists of a syncytial, microvilliated, absorptive neodermis, a basal lamina, and layers of circular and longitudinal muscles. Inside the body wall is connective tissue consisting of the parenchyma in which the reproductive, excretory, and nervous systems are embedded.    

The excretory system consists of flame bulb protonephridia that are not evident.  Protonephridia drain into nephridial canals (these may be visible on each side of the proglottid) which ultimately open to the exterior at the posterior end of the strobila.  The larger ventral nephridial canal is a pale, wide, longitudinal band lateral to the testes.  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. 

The right and left lateral longitudinal nerve cords arise from nerve rings in the scolex and pass posteriorly in the sides of the proglottids.  They are slender longitudinal lines lateral to ventral nephridial canals but are not visible in most preparations.

Most structures in the proglottid belong to the reproductive system.  The common genital pore 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 (= penal sac).  Inside the sac is the convoluted, eversible, tubular cirrus, which is the intromittent organ, or penis.  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.  Each is drained by a tiny tributary of the sperm duct, but these cannot be seen.  There is no seminal vesicle.        

The smaller and more posterior of the two ducts entering the genital atrium is the female gonoduct, which is also regionally specialized.  The first region is the vagina.  It receives the partner's penis during copulation.  The vagina extends medially and posteriorly to become the small seminal receptacle.  This is a clear, unstained, oval chamber where sperm 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. 

Details of the Taenia pisiformis female reproductive system 

The ovary (= germarium) 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 narrow oviduct arises between the ovaries 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.  Fertilization occurs in the oviduct.  Yolk cells are produced by the single yolk gland (= vitellarium) at the posterior end of the proglottid.

A small duct, usually not discernible, extends from the oviduct to the uterus.  Shelled eggs move through this duct into the uterus.  Within the uterus, development proceeds to the oncosphere 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 the entire proglottid.   Other parts of the reproductive system are degenerate but the nervous and excretory systems are present and functional. The body wall musculature remains and gravid proglottids are mobile and very active.  There is no opening of the uterus to the exterior and eggs are released by rupture of the proglottid. 

Tapeworms are hermaphroditic and commonly self-fertilize, a convenience for an animal that lives in a habitat where it may be the only member of its species. Cross-fertilization may occur if more than one worm is present in the host.

In the intestine of the definitive host (typically a dog in this species), egg-filled gravid proglottids break away from the posterior end of the strobila and are eliminated with the feces.  Sometimes, after breaking away from the strobila, they migrate under their own power out the anus and onto the perianal region of the host.  Pet owners may have noticed small, white, rice-shaped proglottids crawling about the hind ends of their pets.  Proglottids may also be found scattered about on the furniture and floor after they have dried. Outside the body, the proglottids rupture, releasing thousands of eggs, each containing an oncosphere larva.  The oncosphere is a small mass of poorly differentiated cells with a pair of protonephridia, some muscles, and six hooks radiating from its body. 

The eggs are ingested by the intermediate host (rabbit) and the oncosphere escapes into its gut.  The oncosphere bores through the gut wall and penetrates an abdominal organ, usually the liver.  Here it metamorphoses into a cysticercus larva, or bladderworm.  The cysticercus is relatively large, swollen, and fluid-filled.  It has an inverted scolex, with rostellum.  If the rabbit is ingested by a carnivore, the cysticercus finds itself in the intestine of its definitive host.  Here it everts its scolex, attaches to the gut wall, and begins production of its strobila to become an adult worm.

Human tapeworms

Taeniidae includes two species that occur in humans. The beef tapeworm, Taeniarhynchus saginata (= Taenia saginata), is a human parasite for which cattle are the intermediate host.  Humans become infected by eating raw or poorly cooked beef but the worm causes little damage.  Taenia solium, the pork tapeworm, is a more dangerous human parasite. The danger comes, not from the adult worms in the intestine, rather from the cysticercus larva which, although they normally infest the intermediate host (a pig), can also develop in humans. The cysticerci, known also as bladderworms, create and inhabit large fibrous cysts in virtually any tissue of the body, including the brain where they are extremely dangerous.  Adult tapeworms usually do surprisingly little damage and appropriate very little of the host's resources.

Other tapeworms (from

Echinococcus granulosus (a dog tapeworm)

Echinococcus granulosus is a tapeworm of dogs, where adult worms occur in the gut (and cause little problem). Embryonated eggs leave via the feces and are consumed by the intermediate host via food or water. Larvae burrow out the gut and enter the circulatory system. Many end up in the liver; others lodge at sites throughout the body. Larvae develop into hydatid cysts, which are consumed by the dog. Hydatid cysts can be large and impair the health of the host, even causing death. A wide variety of mammals (including humans) can be intermediate hosts, SO KEEP YOUR DOGS WORMED.

Diphyllobothrium latum  (the broadfish tapeworm)

Many "fish eating" vertebrates can serve as the definitive host for Diphyllobothrium latum (the broadfish tapeworm), including humans, dogs, foxes, cats, mink, bears, and seals.  The adult tapeworm lives in the host's small intestine, and in humans the tapeworm can reach a length of 10 meters (>30 feet) and produce over a million eggs a day!

The life cycle of D. latum involves two intermediate hosts. The first intermediate host is a copepod, the second intermediate host is a fish, often pike or salmon, and the definitive host is infected by eating raw or undercooked fish.  In humans the tapeworm is more prevalent in areas where humans eat lots of fish; this includes Scandinavia and areas bordering the Great Lakes in the US.  Dogs and cats are often infected when they are fed the offal remaining after cleaning fish.

Moniezia expansa  (the sheep tapeworm)

The life cycle of Moniezia expansa involves sheep as the definitive host and soil mites as the intermediate host.  The tapeworm's eggs are passed in the sheep's feces, and mites are infected when they eat the eggs; the metacestode stage in the mite is called a cysticercoid.  Sheep are infected when then ingest infected mites.  This species of tapeworm is unusual in that each proglottid contains two sets of female reproductive organs.

© Copyright by Elizabeth Bergey and Eric Bright 2016

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