Platyhelminthes

Definition Dorsoventrally flattened animals with

primary bilateral symmetry Without coelom, anus, circulatory,

respiratory, or skeletal systems With flame-bulb protonephridia With a cellular mesodermal connective

tissue filling all spaces between organ- systems.

The phylum includes the free living flatworms and the parasitic flukes and tapeworms.

Bilateral symmetry All remaining animal phyla are

bilaterally symmetrical. Correlated with motility. Anterior end

meets environment first and bears most of the sense organs.

Cephalization results. Right and left sides ea. contact environment

in same way so virtually identical, but upper and lower halves differ as do front and back.

Of all bilaterally symmetrical animals these are probably the most primitive.

Germ layers Three distinct germ layers; ectoderm

(outer), (endo)mesoderm(midddle) and endoderm (inner)

Triploblastic (p. 141). Ectoderm surface layer (skin, hair,

etc.) and nervous system. Endoderm Gut lining and internal

organs. Mesoderm Everything else (muscles,

blood, bones, connective tissue, etc.)

Appearance Range in size from microscopic worms no

larger than protozoans to the extreme elongation seen in tapeworms which can be 10 to 15 m, but most are small to moderate size.

Most are white or colorless or owe their color to the food they ingest; free-living (f.l.) worms are often gray, brown or black (when not white), but some are brightly colored.

Appearance May have clearly recognizable head at

anterior end separated from body by neck-like constriction, but anterior is organized as a head even when the constriction is absent.

Can tell it is head because the main nerve center is there, the presence of numerous sensory organs, and because it is directed forward during locomotion.

In parasitic flatworms head usually bears hooks and suckers to attach to host (but these may be elsewhere on body also).

Appearance Ventral surface on which

locomotion occurs, and which also bears mouth and genital pores, differs from dorsal surface in turbellarians.

Difference is less marked in trematodes and virtually absent in cestodes.

Body Wall Ciliated epidermis one layer thick in f.l.

for locomotion. Tegument in parasites (p. 142, 148).

Cells of tegument lie under muscle layers and extend up as a syncytial layer of cytoplasm over surface.

Beneath surface layer generally find circular, longitudinal and often also diagonal muscle fibers of mesodermal origin in f.l.

Acoelomate No internal body space other than

digestive tube. Spaces between organs filled with

a loose connective tissue with more cells than mesoglea = parenchyma.

Solid construction is a diagnostic character of the phylum.

Nervous System N.s. of the most primitive flatworms

derivable directly from cnidarians. = epidermal nerve net and a slight massing of nervous tissue representing a brain.

In most, the n.s. has sunk below epidermis and muscle layer and the main nerve net shows concentration into several longitudinal ganglionated cords - 2 ventral ones are either most conspicuous ones or only ones.

Nervous System System primarily radial becomes

bilateral by elimination of some of the radial cords (around bell of medusa).

Brain = main nervous center. Ladder type of n.s. when only two n.

cords present because joined periodically by transverse connections.

Sense Organs Turbellarians richly supplied with

sensory organs - greatly reduced in parasitic classes chemoreceptors - widespread tangoreceptors (for sensing touch)

widespread statocysts in some ocelli - widespread. Also in larval

flukes and some adult groups.

Digestive System Similar to anthozoans. Has mouth,

pharynx and blind intestine, no anus. Mouth is located at about the center of

the ventral surface. Primitive Pharynx varies in phylum. Usually is a

strong muscular tube which can sometimes be extruded.

Intestine varies from a simple sac to complicated branchings; missing completely in Acoela (primitive turbellarians) and in cestodes (tapeworms).

Excretory System Single or paired protonephridial tubules

ea. with flame bulbs. (p. 143) Flame bulb surrounds small space into wh

tuft of cilia project. Cilia beat like flame and force fluids through system into collecting ducts opening to outside by pores.

b. Used mainly for osmoregulation, not for elimination of wastes. Reduced or absent in marine flatworms.

Metabolic wastes mainly removed by diffusion thru body.

Reproduction Reproductive system is most

complicated system (in tapeworms there is little else).

Almost all are monoecious. Reciprocal fertilization is

accomplished by a complicated copulatory apparatus.

Reproduction Fertilization is internal.

Fertilized, shelled eggs, often already containing embryos are shed to exterior or fastened to objects by adhesive secretions.

Asexual fission in some turbellarians. Reduced pop. density triggers.

Asexual reproduction in many parasitic stages. Important mechanism to increase reproductive potential.

Ecology Either free-living or ecto-or

endocommensals or parasites. Class Turbellaria are mainly free living. Classes Trematoda and Cestoda are

wholly parasitic, chiefly endoparasitic. Adult stage parasitizes vertebrates

(exceptions rare). Larval stages often occur in invertebrates,

usually snails. Hosts may be fresh-water, marine or

terrestrial

Turbellaria Free-living flatworms. Live in fresh

water, in springs, streams, ponds and lakes; or in the ocean, chiefly along shores; or in moist terrestrial habitats, mostly in tropical or subtropical regions. A few Turbellarians are commensal or

parasitic. See text for further information

Trematoda The digenetic (having more than

one host in their life cycle) parasitic flukes.

Many are serious parasites in humans.

Have a complicated life cycle with at least two different hosts.

Trematode Life Cycle (Generalized scheme) Egg is enclosed within an oval shell

with a lid, deposited in the gut of the host and passed to the outside with the host's feces.

Miracidium. = free swimming ciliated larva which hatches from eggs. Enters molluscan host (eaten by it or

penetrates host's epidermis). Usually host is a snail.

Miracidia

The Miracidium is inside the egg. www.path.cam.ac.uk/~schisto/Pictures/ Eggs/S.mansoni.egg.gif

www.volny.cz/schistosomes/ miracidia.htm

Sporocyst Miracidium looses its cilia

after penetration of host and becomes a sporocyst.

Inside the hollow sporocyst, germinal cells give rise to a no. of embryonic masses.

Each mass develops into a daughter sporocyst or another develop mental stage, called a redia.

www.path.cam.ac.uk/.../Flukes_Gen/ Fluke_Life3.html

Redia Redia is also a chambered form.

Germinal cells within the redia again develop into daughter rediae or more usually into a no. of larvae called cercariae.

If there are daughter sporocysts the redia stage is skipped and cercariae develop inside of daughter sporocysts.

Cercaria The cercaria possesses a digestive tract,

suckers and a tail. It leaves the host and is free-swimming. If it comes in contact with a second

intermediate host, an invertebrate (commonly an arthropod) or a vertebrate, it penetrates the host and encysts.

The encysted stage is called a metacercaria

Some cercariae

www.path.cam.ac.uk/.../Flukes_Gen/ Fluke_Life3.html

Metacercaria This is the usual resting stage

in the second intermediate host. Sometimes there is an

intermediate stage called a mesocercaria capable of inhabiting a wide range of hosts and becoming a metaceria when the host is eaten.

Metacercaria Can encyst on vegetation or in a

host. Matures into final adult stage when

host is eaten. Vegetarians not immune since can

eat plant on which metacercariae have encysted.

Adult If the host of the metacercaria is

eaten by the final vertebrate host, the metacercaria actively escapes from its cyst, migrates and develops into the adult form within a species specific location in the host.

Can stay in the intestine or must penetrate gut wall and migrate to final site.

Ceatoda Segmented entoparasites in

digestive tract and associated ducts of vertebrates.

Have no epidermis, mouth or digestive tract. 2 subclasses. Cestodaria Eucestoda

Cestodaria Unsegmented Free swimming and ciliated Ten hooks (not 3 as do larval

eucestodes). No digestive system and hooked larva

make them cestodes. Resemble miracidium of trematodes so

interesting phylogenetically. http://www.path.cam.ac.uk/~schisto/Tapes/Tapes_Gen/Tape5.html

Eucestoda Morphology. 3 body regions Scolex (holdfast organ) located

at the anterior end and provided with suckers and/or hooks for adhering to the host. Can be quite complicated. Small when compared to a mature proglottid.

Eucestoda Morphology. Neck region. Situated immediately posterior to

scolex. Unsegmented, poorly differentiated

area generally narrower than scolex and strobila proper.

Continuously differentiating zone which gives rise to new proglottids (body segments)

Eucestoda Morphology.

Strobila. Main bulk of body. Linearly arranged

segments called proglottids form a chain.

Newest segments at neck and oldest at tail end.

Web site

Eucestoda Morphology (segmentation)

Metamerism (segmentation) is a serial repetition of body parts along anterior-posterior axis.

Differs from "typical" segmentation (metamerism) of annelids, arthropods and chordates. Text says not "true" metamerism but doesn't define metamerism.

If above defn. true, this is segmentation. Big difference is in direction. In cestodes go from scolex --> young

segments. --> old segments. In others get new segments from segment before end.

Cestode body wall. Several layers thick. Outermost is tegument (nonciliated

cytoplasmic syncytium overlying muscle layers).

The syncytium is formed from extensions of cells located in the parenchyma.

Differs from tegument of trematodes in having surface microthrix (= microvilli; surface folds) which increase the surface area through which food can be absorbed. Pinocytosis and exocytosis (excretion) can occur here.

Other Cestode features

Respiration is largely anaerobic, but have enzymes for aerobic resp. also.

Ns. Is typical, but no specialized sense organs.

Reproduction. mono and dioecious.

Other Cestode features Monoecious. Self fertilization within single

proglottid and between proglottids, and copulation between 2 worms if they bump into ea. other in the gut of the host all occur in monoecious.

Selfing between different proglottids of the same worm is probably deleterious. When one worm per mouse (Hymenolopus diminuta) found an increase in freq. of abnormal larvae. Also a decrease in egg viability. No selfed strain can be maintained beyond 5th generation so it is probable that cross fert. to provide hybrid vigor is essential at least occasionally. Usually male organs mature before female so fertilization of same segment is not possible.

Other Cestode features Dioecious. Cross fertilization is necessary.

Only one dioecious order. Complete reproductive system in each

proglottid. Basic structure similar to digenetic trematodes.

Fertilized eggs usually stored in blind uterus. Terminal proglottids packed with eggs break off and eggs are freed and pass out through intestine in feces or proglottids

Life cycles Usually at least 1 or 2 intermediate

hosts (arthropods and vertebrates).

"Typical" life cycle. Egg --> oncosphere larva --> cysticercus or plerocercoid stage --> adult.

Life cycles

Egg --> 1st larv. stage = oncosphere (hooked, free-living larva).

Oncosphere is eaten by intermed. host, us. an arthropod. Membranes covering larv. are digested and oncosphere bores thru intestinal wall of host into body spaces or various organs where develops further.

Life cycles Oncosphere --> develops into

procercoid (resembles cercaria) and no further development until host eaten by appropriate 2nd intermed. host, = a fish or other vert.

Life cycles Larva penetrates intestinal wall and

reaches coelom, liver, spleen, muscles or other parts while gradually developing into plerocercoid. Grows, elongates, anterior end develops into scolex with adhesive organs typical of the species and have young tapeworm, but no strobilation. Encysts in host tissue until host eaten by final host.

In intestine of final host plerocercoid attaches to the wall and develops into mature tapeworm.

Produces many proglottids each day.

Life cycles Cysticercus. Some use a vertebrate host

instead of arthropod to eat oncosphere and grow cysticercus = bladder worm.

Humans can eat and get adult tapeworms. Us. get from raw or incompletely cooked pork.

Can get from beef tapeworm as well. Adult tapeworms not too bad. Symptoms absent or mild.

Much more serious to eat embryonated eggs of pork or beef tapeworm. Bladder worms develop in various organs, including the brain. Symptoms similar to tumor, and can get serious reactions to liberated toxins.

Parasite reports Epidemiology Life cycle, Morphology, Pathology Treatments.