Platyhelminthes (Flatworms) Comprehensive Notes
Platyhelminthes (Flatworms)
General Features
Actively seek food, shelter, homesites, and mates.
Require different strategies and body organization compared to radially symmetrical sessile organisms.
Introduction to Platyhelminthes
Phylum of simple, soft-bodied invertebrates.
Characterized by flattened bodies (dorsoventrally flattened).
Lack specialized respiratory or circulatory systems.
Most flatworms are parasitic.
Free-living flatworms (e.g., planarians) exhibit cephalization and have a simple digestive system with one opening.
Significant roles in ecosystems and human/animal health due to parasitic species.
Evolutionary Advances
Cephalization: Concentration of sense organs in the head region.
Bilateral Symmetry: Body can be divided along one plane to yield two mirror images.
Active movement with anterior (head), posterior, dorsal, and ventral sides.
Position and Biological Contributions
Simplest animals with primary bilateral symmetry.
Triploblastic: Three defined layers during embryonic development (mesoderm, ectoderm, endoderm). Mesoderm is well-defined.
Mesozoans
No clearly defined body layers.
Development does not include gastrulation.
Highly specialized parasites.
Acoelomates
Some argue free-living planaria are acoelomates.
Typical acoelomates have only one internal space (digestive cavity).
Region between epidermis and digestive cavity filled with parenchyma.
Some members of acillomorpha are typical or atypical acoelomates.
Developmental Characteristics
Define two metazoan clades: Protostomia and Deuterostomia.
Protostomia
Blastopore becomes the mouth.
Spiral or mosaic cleavage development.
Coelom can be formed by schizocoely.
Deuterostomia
Blastopore becomes the anus.
Radial and regulative cleavage development.
Coelom forms by enterocoely.
Platyhelminthes Classification
Acoelomic protostomes: embryonic blastopore will become mouth.
Divided into four classes: Turbellaria, Trematoda, Monogenea, and Cestoda.
Class Turbellaria
Mostly free-living.
Bottom dwellers in marine or freshwater environments.
Example: Freshwater planarians in streams, pools, and hot springs.
Flatworms limited to moist places.
Monogenea, Trematoda, and Cestoda are entirely parasitic.
Forms and Function
Epidermis and Muscles
Most have cellular, ciliated epidermis on a basement membrane.
Secrete a protective mucus sheet.
Rhabdites for defensive mechanisms (secrete distasteful material).
Dual-gland adhesive organs (viscid and releasing gland cells) for attachment and detachment from substrates.
Dual Gland Adhesive Organs
Viscid gland cells: fasten microvilli of anchor cells to substrate.
Releasing gland cells: provide quick chemical detachment.
Syncytial Epidermis
Some tubellarians and all other members have syncytial epidermis. Nuclei are not separated by cell membranes.
Non-tubellarians have a tegument (integumentary system - protective outer layer).
Parenchymal cells fill spaces in the body.
Nutrition and Digestion
Cestodes lack a digestive system.
Others have a mouth, pharynx, and intestine.
Planarians have a pharynx that extends through the ventral mouth. Intestine has three branches (one anterior, two posterior).
Gastrovascular cavity lined with columnar epithelium.
Mouth of trematodes and monogeans opens near the anterior end.
Pharynx is extensible; intestine ends blindly with varying degrees of branching.
Planarian Anatomy
Osmoregulatory Tube: For expelling excess water.
Ladder-type Nervous System: Visible on the back.
Pharynx extends through the ventral mouth.
Oral sucker, pharyngeal muscles, intestine, ovary, and testis are present.
Can reproduce via self-fertilization.
Feeding Habits
Planaria are carnivorous; detect food using chemoreceptors.
Food trapped in mucous secretions from glands and rhabdites.
Wrap around prey; extend proboscis to suck up bits of food.
Monogeans and trematodes feed on host cells, cellular debris, and body fluids.
Extracellular Digestion: Proteolytic enzymes from intestines.
Intracellular Digestion: Phagocytic cells in gastrodermis.
Undigested food is egested out of the pharynx.
Cestodes rely on the host's digestive tract, absorbing digested nutrients.
Excretion and Osmoregulation
Protonephridia used for osmoregulation.
Marine tubellarians lack protonephridia.
Monogeans have two excretory pores near the anterior end.
Flame cells present in protonephridia of parasitic taxa.
Ducts of trematodes open into an excretory bladder with a terminal pore.
Cestodes have two main excretory canals.
Metabolic wastes are eliminated by diffusion through the body wall.
Nervous System
Subepidermal nerve plexus resembles a nerve net.
Decentralized nervous system.
Detect stimuli, coordinate basic movements.
One to five pairs of longitudinal nerve cords lie under the muscle layer.
Sense Organs
Cephalization occurs.
Ocelli (light-detecting) in tubellarians, monogeneans, and larval trematodes.
Tactile and chemoreceptive cells detect food.
Statocysts for equilibrium.
Rheoreceptors sense water current direction.
Sensory nerve endings in oral suckers and genital pores of parasitic groups.
Reproduction and Regeneration
Fission: Body divides, and each part regenerates missing sections.
Regeneration of tail or head.
Severed heads prevent regeneration in headless worms.
Male Reproductive System
One or more testes connected to vasa efferentia, which connect to a vas deferens.
Vas deferens leads to a seminal vesicle.
Papilla-like penis or extensible cirrus is the copulatory organ.
Tubellarians can self-fertilize.
Eggs and yolk cells enclosed in a cocoon attached to plants; embryos resemble small adults.
Some marine forms have ciliated, free-swimming larvae.
Parasitic Flatworm Life Cycles
Larval trematodes emerge as ciliated larvae.
Cestodes hatch after being consumed by a host.
Many animals can serve as intermediate hosts.
Platyhelminthes Classes
Turbellaria
Trematoda
Monogenea
Cestoda
Class Turbellaria Details
Mostly free-living.
Range from 5 mm to 50 cm long (except for polyclads and dulecitol turbellarians).
Have a single gut or no gut at all.
Very simple pharynx.
Small planaria swim via cilia; others glide over a slime track secreted by adhesive glands.
Rhythmical muscular waves pass backward from the head.
Class Trematoda (Flukes)
All trematodes are parasitic flukes.
Most adults are endoparasites of vertebrates.
Adaptations for parasitism include:
Penetration glands
Glands to produce cyst material
Hooks and suckers for adhesion
Increased reproductive capacity
Some retain ancestral characteristics: alimentary canal, reproductive, excretory, and nervous systems.
Sense organs are poorly developed.
Subclass Aspidogastrea
Least known subclass of trematodes.
Most have only one host (usually mollusks/snails).
If a second host exists, it's usually a fish or turtle.
Subclass Digenea
Indirect life cycle involving mollusks.
First intermediate host: mollusk.
Definitive/final host: vertebrate (sexual reproduction occurs here).
Second and third intermediate hosts may be required.
Parasitize a wide range of hosts (mollusks, fish, turtles).
General Trematode Life Cycle
Egg passes from definitive host in excreta and reaches water.
Egg hatches into ciliated larva (miracidium).
Miracidium penetrates snail tissues and transforms into a sporocyst.
Sporocysts reproduce asexually to form more sporocysts or rediae.
Rediae reproduce asexually to form more rediae or cercariae.
Cercariae emerge from the snail, penetrate a second intermediate host, or encyst on objects.
Develop into metacercariae (juvenile flukes).
Metacercariae develop into adults after being eaten by the definitive host.
Some are serious parasites of humans and domestic animals.
Examples of Trematodes
Sheep Liver Fluke (Fasciola hepatica): Adult fluke lives in bile passages of sheep/ruminants.
Human Liver Fluke (Clonorchis sinensis): Most important human liver fluke; common in China, Japan, and Southeast Asia. Infects cats, dogs, and pigs. Adults are 10-20 mm long with oral and ventral suckers.
Blood Fluke (Schistosoma): Infects over 200 million people, common in Africa, South America, West Indies, Middle, and Far East. Sexes are separate. Include:
S. mansoni
S. haematobium
S. japonicum
Lung Fluke (Paragonimus westermani): Parasitizes humans, pigs, rodents, etc.
Fasciolopsis buski: Lives in human intestines; contracted from eating raw aquatic vegetation.
Leucochloridium: Produces colorful sporocysts in snail's head, attracting birds to eat snails to continue life cycle.
Class Monogenea
Monogenetic flukes originally placed in Trematoda but now considered sister taxa because both have posterior attachment with hooks.
External parasites of fish (especially gills), bladders of frogs and turtles.
Direct life cycle in a single host.
Oncomiracidium attaches to host via posterior hooks.
Posterior hooks may become the posterior attachment organ of the adult (opisthaptor).
Opisthaptor varies widely in terms of hooks, suckers, and clamps to withstand water flow.
Can cause serious economic problems in fish farming.
Class Cestoda (Tapeworms)
Long, flat bodies with a scolex (head-like structure) having suckers and hooks for attachment.
Scolex followed by a linear series of reproductive units called proglottids.
Lack a digestive system.
Muscle, excretory, and nervous systems are similar to other flatworms.
Lack sensory organs except for modified cilia.
Scolex attaches to the intestinal wall using hooks or suckers (depending on species).
Proglottids reproduce via self-fertilization.
Cestoda Tegument
Distal cytoplasm tegument.
Microtriches at the end/margin of the body.
Mitochondria, nerve processes, and muscles (circular and longitudinal).
Tegument lacks cilia.
Entire surface covered with projections similar to microvilli in the vertebrate small intestine.
Microtriches increase surface area for food absorption.
Subclass Eucestoda
Larvae have six hooks on the scolex.
Chain of proglottids called a strobila.
Proglottids originate in the germinative zone behind the scolex.
Practice self-fertilization or cross-fertilization.
Shelled embryos form in the uterus.
Embryos or whole proglottids are expelled.
Proglottid formation is through segmentation.
Nearly all cestodes require two hosts.
Adults are parasitic in the digestive tract of vertebrates.
Over 1,000 known tapeworm species, infecting almost all vertebrates.
Most tapeworms do little harm to the host, but must reside insides
Examples of Eucestoda
Taenia saginata (Beef Tapeworm): Adults live in the alimentary canal of humans; juvenile forms found in intermuscular tissue of cattle. Mature adults can reach over 10 meters in length with over 2000 proglottids. Scolex has four suckers but no hooks. Gravid proglottids contain shelled infective larvae that is released in feces.
Lifecycle:
Cattle swallow shelled larvae that hatch as oncospheres.
Oncospheres use hooks to burrow through the intestinal wall into blood/lymph vessels.
Reach voluntary muscle and encyst to become bladderworms.
Infected meat is eaten; cyst wall dissolves, and scolex evaginates to attach to intestinal mucosa.
New proglottids develop in 2-3 weeks. Infected individuals expel proglottids daily.
Infection avoided by thoroughly cooking beef.
Taenia solium (Pork Tapeworm): Adults live in the small intestines of humans; juveniles live in muscles of pigs. Scolex has suckers and hooks. If eggs/proglottids are ingested, embryos migrate to organs and form cysticerci. Cysticercosis commonly occurs in eyes or brain, leading to blindness, neurological symptoms, or death. Prevented by cooking pork properly.
Diphyllobothrium latum (Fish Tapeworm): Adults found in intestines of humans, dogs, cats, and mammals. Immature stages in crustaceans and fish. Largest cestodes of humans (up to 20 meters in length).
Unilocular Hydatid: Adults parasitize dogs and canines; juveniles infest many mammals. Humans may serve as an intermediate host.
Key Takeaways
Phylum Platyhelminthes includes four classes: Turbellaria, Trematoda, Monogenea, and Cestoda.
Trematoda, Monogenea, and Cestoda are mostly parasitic.
Turbellaria includes free-living planaria.