Ch. 8 Flashcards
(22 cards)
key features of platyhelminths (flatworms)
Has no defining characteristics
1st phyla to have combination of bilaterally symmetric, triploblastic, and acoelomate.
- Diversity and Classification:
• Approximately 34,000 described species, with more than 80% being parasitic.
• Includes free-living turbellarians and parasitic classes (monogeneans, trematodes, and cestodes), believed to have evolved from free-living turbellarian ancestors. - Body Plan and Symmetry:
• Acoelomate (lack of a body cavity), triploblastic (three germ layers), and bilaterally symmetrical.
• Free-living forms may be basal bilateral animals, potentially ancestral to all coelomate animals.
• There’s a debate about whether flatworms evolved from coelomate ancestors or if their acoelomate condition is a secondary simplification. - Development and Protostome Features:
• Development is protostome-like, with spiral and determinate cleavage, and the mouth forming before the anus from the blastopore.
• Most species have a conspicuous anterior brain and longitudinal nerve cords (usually one pair in advanced species).
• The mesoderm develops into parenchyma tissue, occupying the space between the body wall and gut. - Digestive System:
• No anus: both food entry and waste exit occur through a single opening (like cnidarians) - Flat Body and Respiration:
• Flat body increases surface area relative to volume, allowing for efficient gas exchange via simple diffusion across the body surface.
• No specialized respiratory or circulatory systems; a few species may have hemoglobin for oxygen transport.
• Metabolic wastes are also likely removed by diffusion. - Excretion and Osmoregulation:
• Protonephridia (primitive excretory organs) consisting of ciliated or flagellated cells (flame cells or solenocytes) that help in ultrafiltration and osmoregulation.
• These organs regulate ionic balance, water content, and metabolic waste removal.
• Fluid is drawn through the cups by the beating of cilia or flagella, which creates negative pressure, resulting in ultrafiltration.
• The fluid composition changes as it passes through the tubule, allowing selective absorption or secretion of ions and adjustment of water content. - Evolutionary Debate:
• There’s ongoing debate about the evolutionary relationships of flatworms, including whether they belong in a separate phylum (Acoelomorpha) or if some flatworms may be secondarily acoelomate.
• The possibility that parasitic flatworms (like cestodes) share a common ancestor within a clade called Neodermata is suggested.
• The Neodermata clade includes parasitic groups (like Cestoda, Monogenea, Digenea, and Aspidogastrea), which are united by a syncytial tegument replacing the larval epidermis during metamorphosis, indicating a possible common ancestry for parasitism. - Reproduction:
• Simultaneous hermaphrodites: Most flatworm species have both male and female reproductive organs, allowing them to function as both sexes simultaneously.
• Sperm exchange and fertilization occur between individuals of the same species, though self-fertilization is typically not possible.
key features of Class Turbellaria
- Habitat and Distribution:
• 15% of all flatworm species are turbellarians.
• Most species are free-living, with about 150 species being commensal or parasitic to other invertebrates.
• They primarily live in aquatic
environments (marine, freshwater), but some species are terrestrial in humid
areas.
• Most species are small (<1 cm), but some terrestrial and marine species can be much larger. - Nervous System:
• Primitive spoules have a diffuse nerve
• More advanced turbellarians possess a cerebral ganglion (primitive brain) and one to three pairs of longitudinal nerve cords.
• Sensory capabilities include pairs of eyes (typically anterior), chemoreceptors for sensing food, and cells sensitive to pressure ano mechanical stimuli.
• Statocysts in some species help with body orientation. - Locomotion and Surface Features:
• The outer surface is ciliated, with more cilia on the ventral surface.
• Mucus secretion from the ventral surface aids in movement by allowing
cilia to beat within it, creating slippery traction.
• Multiciliated epidermal cells (with several to many cilia) help with this movement.
• The locomotion is assisted by pedal waves of muscular contraction along the ventral surface, which move the body forward.
• Circular muscles push the body forward, while longitudinal muscles pull it in the direction of movement.
• Some turbellarians can swim, either by ciliary action or by using waves of body wall contractions.
• Swimming species may do so routinely (especially shallow-water species) or only when conditions worsen (for benthic species).
• Acoel flatworms (a small group of turbellarians) may be permanently
planktonic, suggesting they live as part of the plankton community in warm, oceanic surface waters. - Movement and Musculature:
• Muscle contractions for movement are subtle but effective, with multiple pedal waves moving simultaneously down the body, aiding in slow but controlled movement.
• Body wall musculature: Turbellarians have a complex arrangement of muscle fibers (longitudinal, circumferential, dorsoventral, and diagonal) that assist in locomotion. This allows some species to engage in looping (a type of movement where the body alternates attachment and detachment to the substrate while
moving forward). - Looping Movement:
• Looping involves attaching at the anterior, contracting longitudinal muscles to pull the posterior forward,
then attaching at the posterior and contracting circular muscles to propel the body forward.
• For effective looping, turbellarians must be able to adhere temporarily to the substrate using specialized cells, while also being able to detach when needed for movement. - Adhesion Mechanism:
• Turbellarians have paired secretory cells (duo-glands) on the ventral surface. One cell secretes a viscous glue to attach to the substrate, while the other pruduces a chemical to break this attachment, enabling movement. - Rhabdites and Mucus Secretion:
• Many species have rhabdites (cylindrical structures) on the body surface, which release thick mucus.
• The function of rhabdites is unclear, but they likely play a role in defense (e.g., in response to predation) or in preventing desiccation. - Digestive System:
• Simple digestive system: Most turbellarians have a simple gut, but with significant variation across species.
• Some turbellarians (like Acoela flatworms) lack a true gut cavity and have a mouth on the ventral surface through which food is pushed into a mass of dig ive cells.
Acoel Flatworms and Their Classification:
• Acoels may not belong in the phylum Platyhelminthes. Recent molecular studies, particularly those involving microRNAs (miRNAs), show significant differences between acoels and other
flatworms.
• miRNA differences: Acoels express far fewer unique miRNAs than other flatworms, supporting the idea that they are not closely related to typical platyhelminths.
• These findings suggest acoels might represent a primitive group of bilateral animals, potentially forming a new phylum called Acoelomorpha.
• The exact evolutionary relationships of acoels are still unclear, but they may be more closely related to deuterostomes (like sea urchins or fish) rather than other flatworms.
- Turbellarian Evolution:
• Turbellarians have evolved primarily through increasing complexity in two areas:
• Digestive system: More advanced turbellarians have more specialized guts, which may be straight, branched or multi-branched. Some species have a protrusible pharynx for feeding.
• Reproductive system: Turbellarians exhibit a high degree of reproductive specialization, with both male and female reproductive organs typically present in a single individual (hermaphroditism). The male system is particularly complex. - Feeding and Digestion:
• Most turbellarians are active carnivores, although some consume detritus or algae, and some have algal symbionts.
• Digestion occurs in two stages:
- Extracellular digestion: Enzymes are secreted to break down food outside of cells.
- Intracellular digestion: The digested particles are then engulfed and further processed inside cells. - Reproductive Systems and Mating:
• Hermaphroditic reproduction:
Turbellarians typically possess both male and female organs in the same individual.
• Complex male reproductive system:
The male organs are often highly specialized.
• Mating behaviors:
- Triclad flatworms engage in recipretal copulation, where each individual inserts its penis into the other’s female opening.
- Other turbellarians may use hypodermic impregnation, where the male pierces the partner’s body with a stylet to transfer sperm.
Development and Reproduction:
Regenerative Abilities:
Research on Regeneration:
- Development and Reproduction:
• After fertilization, turbellarians typically develop directly into miniature flatworms within a protective capsule, bypassing a free-living larval stage in most species.
• Some marine species exhibit a free-swimming larval stage after
fertilization, typically a Müller’s larva (a short-lived, microscopic stage), before becoming a miniature flatworm. - Regenerative Abilities:
• Remarkable regeneration: Turbellarians are known for their ability to regenerate entire body parts, not just repair wounds. This ability goes far beyond typical regeneration seen in other organisms.
• Neoblasts: These undifferentiated cells, unique to turbellarians, play a crucial role in regeneration. Neoblasts have versatile developmental plasticity, meaning they can differentiate into various cell types as needed.
• Some turbellarian species reproduce by asexual fission, a process tied closely to their regenerative capabilities. This means they can essentially “clone” themselves by splitting into two or more individuals. - Research on Regeneration:
Developmental biology studies:
Turbellarians, particularly freshwater triclads, have been important models for studying cellular differentiation and regenerative processes.
• It remains unclear why some turbellarian species can regenerate while others cannot. This difference may be influenced by specific biological or environmental factors.
• Notably, the fixed cell fates that characterize protostomes (where the fate of cells is determined early in development) do not necessarily limit regenerative abilities in turbellarians.
Class Cestoda (Tapeworms)
Defining Characteristics:
1. Scolex: Small anterior attachment organ, often equipped with hooks and/or suckers for anchoring in the host.
2. Proglottids: The body is divided into segments, called proglottids, which arise from behind the scolex.
3. Absence of Digestive Tract: Tapeworms do not have a mouth or digestive system;
instead, they absorb nutrients directly through their tegument.
• Tegument: Unlike turbellarians, which have a ciliated epidermis, cestodes have a nonciliated tegument. This tegument is syncytial, meaning it contains many nuclei but lacks separate cell membranes. Its surface is folded into projections to increase surface area for nutrient absorption directly from the host’s gut.
Body Structure:
• Scolex: The anterior end of the cestode is known as the scolex, which is equipped with hooks and/or suckers to attach securely to the host. Some cestodes, particularly those in the subclass Cestodaria, lack a scolex.
• Proglottids: The body of the tapeworm is made up of proglottids, which bud from the neck just behind the scolex. Each proglottid is involved in sexual reproduction and contains both male and female reproductive organs, making the tapeworm a simultaneous hermaphrodite. Proglottids continually form behind the scolex, and can number in the thousands per tapeworm.
Reproduction:
• Proglottids: Each proglottid is essentially a reproductive unit. It contains both testes and ovaries, and in some species, hundreds to thousands of eggs are produced per proglottid. Fertilization often occurs between sperm from different proglottids (or from the same proglottid), and each proglottid can produce around 50,000 eggs, leading to a large egg produ on rate. Tapeworms can produce hundreds of thousands of fertilized eggs daily.
Lifecycle:
• Endoparasitic Existence: Tapeworms rely entirely on nutrient absorption from the host, using their specialized tegument. They lack digestive systems, as they absorb nutrients directly through the body surface.
• The main focus of a tapeworm’s life cycle is the continuous production of eggs within proglottids, which then pass out of the host via feces to continue the lifecycle in a new host.
Lifecycle of Cestodes (Tapeworms)
Proglottids and Egg Release
• Proglottid Maturation: The posteriormost proglottids (the segments farthest from the scolex) often break off periodically, or they may burst open, releasing fertilized eggs into the host’s digestive tract.
• Egg Passage: The eggs then pass out of the host’s body with the feces.
• Egg Development: Once outside the host, the fertilized eggs are typically not immediately infectious to the definitive host. They must first he consumed by an intermediate host, which can be either a vertebrate or an invertebrate. The type of intermediate host depends on the species of cestode.
Oncosphere Stage in Intermediate Host
• When an intermediate host (such as a mammal, fish, or arthropod) ingests a fertilized egg, the egg hatches into a larval form known as the oncosphere.
• The oncosphere has 3 pairs of hooks and muscles to help it attach to the host’s intestinal wall. It also possesses flame cells for excretion.
• The oncosphere penetrates the intestinal wall and migrates through the host’s body, where it becomes encysted (i.e., forms vesting stage) in specific tissues or in the coelomic space.
Cysticercus (Bladder Worm) Stage
• In Taenia species (a genus of tapeworms), the oncosphere develops into a cysticercus, commonly called a bladder worm.
• The cysticercus is a resting stage that often forms in organs like the muscle tissue or liver of the intermediate host. It can remain in this stage until consumed by the definitive host.
Hydatid Cyst Formation
• In some species, particularly in the Taeniidae family, the oncosphere undergoes asexual replication and forms a much larger
structure known as a hydatid cyst. These cysts can contain multiple cysts (daughter cysts) within, leading to huge cystic formations.
• Hydatid cysts can be potentially dangerous, as their growth may cause severe tissue damage, especially in the liver or lungs of the intermediate host.
Final Host and Completion of Lifecycle
• The lifecycle is completed only when the intermediate host (containing the
cysticercus or hydatid cyst) is eaten by the definitive host-usually a vertebrate like a fish, cow, pig, or dog, which serves as the final host.
• In the definitive host’s gut, the cysticercus (or hydatid cyst) releases the larval tapeworm (the scolex) that attaches to the intestinal wall.
• The tapeworm then matures into an adult, and the cycle begins again as it starts producing proglottids with fertilized eggs.
Hosts
• Intermediate Hosts: Various animals, including fish, cows, pigs, and birds, can serve as intermediate hosts.
• Definitive Hosts: These hosts are typically vertebrates, such as dogs, carnivorous mammals, and occasionally humans, where the adult tapeworm resides in the gut.
• Humans can be either final or intermediate hosts, depending on the species of tapeworm. For example, Taenia solium (pork tapeworm) can
cause cysticercosis if the eggs are ingested, Ieading to larvae encysting in human tissues, a dangerous condition.
Precautions:
• Undercooked Meat: Humans can become infected by eating undercooked meat (e.g., beef, pork, or fish) that contains tapeworm larvae, or by accidentally ingesting tapeworm eggs through contact with contaminated food or water.
• Dog Licks: Dogs, as definitive hosts, can shed tapeworm eggs, which may be transmitted to humans if they come into contact with the dogs feces.
Class Monogenea (Monogenetic
Flatworms)
Monogeneans are ectoparasitic flatworms primarily found on the skin or gills of fishes. These parasites have evolved highly specialized features to attach to their hosts, and their life cycle is typically direct, meaning they do not require intermediate hosts.
Defining Characteristics of Monogeneans:
- Posterior Attachment Organ (Haptor): The most distinguishing feature of monogeneans is their haptor (also called opisthaptor). This organ, located at the posterior end of the body, consists of suckers, hooks, and sclerites (hard structures), which enable the monogenean to anchor itself securely to the host’s skin or gills.
- Anterior Adhesive Organ: In addition to the haptor, monogeneans often have an anterior adhesive organ (or prohaptor), which helps in the initial attachment to the host. This organ may include additional suckers or adhesive glands.
- Larval Stage (Oncomiracidium): The larva of monogeneans is called the oncomiracidium. This larval stage has 3 bands of cilia and often 1 or 2 pairs of eyes, aiding its movement and host detection. The oncomiracidium is the free-swimming stage that searches for a suitable host to attach to.
- No Intermediate Hosts: The life cycle of
monogeneans is direct and involves no intermediate hosts. The stages typically include:
• Sexual maturity: The adult reaches maturity and starts producing eggs.
• Egg production: Eggs are laid either in or on the host fish.
• Larval stage: The egg hatches into an oncomiracidium.
• Attachment to host: The larva swims to find and attach to a new host.
Host Specificity and Ecological Niche:
• Monogeneans show a high degree of host specificity. A single species may be restricted to a specific location on the host, such as the base of the gill filaments or the tips of the gill filaments. Some species are also specific to the skin of the fish.
• With around 8,000 described species and estimates of up 25,000 species in total, monogeneans are among the most speciose groups of flatworms.
Taxonomy and Evolutionary Relationships:
• The taxonomic status of monogeneans has been debated. Initially considered a type of trematode (flukes), monogeneans show significant differences, especially in their life cycle, making this classification uncertain.
• Some studies propose that monogeneans are more closely related to cestodes (tapeworms) than to trematodes, primarily based on the similarity between the oncosphere (larval stage) of cestodes and the oncomiracidium of monogeneans.
• Molecular date suggest that monogeneans may form a separate class within the flatworms, distinct from both trematodes and cestodes. If true, the similar morphology of monogeneans and
trematodes is likely the result of convergent evolution rather than common ancestry.
Summary of Life Cycle:
- Sexual Maturity: Monogeneans mature and produce eggs in or on their host.
- Egg Production: Eggs are laid and remain on the host or in its environment.
- Oncomiracidium: The eggs hatch into larvae with cilia and eyes, which swim in search of a new host.
- Attachment: The oncomiracidium attaches to the host, where it matures into an adult, completing the lifecycle.
Class Trematoda (Flukes)
Problems Faced by Trematodes as
Parasites:
• Reproduction within the host:
Trematodes must be able to reproduce inside the definitive host (where they reach maturity).
• Egg and embryo release: After fertilization, they need to release eggs or embryos from the host to continue the lifecycle.
• Finding and recognizing a new host:
Trematodes must locate an appropriate host in their environment.
• Host entry: They must penetrate the host’s defenses and enter it.
• Locating a suitable environment: Once inside, the trematode must find the right habitat within the host.
• Maintaining position within the host:
Trematodes must stay attached to their host without being dislodged.
• Surviving anaerobic environments:
Many trematodes live in areas with little oxygen.
• Avoiding immune attack: They must avoid detection and destruction by the host’s immune system.
• Preventing host death before reproduction: Trematodes must avoid killing the host too early in the lifecycle.
Class Trematoda (Flukes)
External Body Structure:
• Syncytial Tegument: Like cestodes and monogeneans, adult trematodes have a non-ciliated, syncytial tegument (a multi-nuclear layer without separate cells), which helps them evade the host’s immune response and absorb nutrients from the host.
• Mouth and Digestive System:
Trematodes have a mouth opening and a blind-ended digestive tract (bilobed in most species). Unlike cestodes, they do have a digestive system, and they feed by ingesting host tissues and blood.
• No Segmentation: Trematodes are not segmented, distinguishing them from cestodes and monogeneans.
Class Trematoda (Flukes)
Host Interaction:
• Schistosomiasis: Trematodes include the blood flukes responsible for schistosomiasis, a disease that affects over 200 million people globally, particularly in parts of Africa and Asia.
Schistosomiasis can lead to serious health issues, including cancer and liver damage, and is one of the most significant parasitic diseases worldwide.
• Infection in Livestock: Trematodes also cause economic damage in livestock, contributing to financial losses in agriculture.
Class Trematoda (Flukes)
Types of Trematodes:
- Digenetic Trematodes:
• The major group of trematodes, comprising over 6,000 species. These trematodes typically have a complex lifecycle involving at least two hosts: an intermedia host (often a mollusk) and a definitive host (usually a vertebrate).
• The adult trematode resides in the definitive host, and larvae or other stages infect the intermediate host.
Lifecycle of Digenetic Trematodes:
The lifecycle of digenetic trematodes involves several stages, typically including:
• Eggs released from the definitive host.
• Miracidium larvae hatch from the egg and infect an intermediate host (often a mollusk).
• The larvae develop into cercariae
• The cercariae infect the definitive host (e.g., a vertebrate), where they mature into adult flukes and reproduce.
Class Trematoda (Flukes)
Types of Trematodes:
2. Aspidogastreans (or Aspidobothreans):
• A smaller group with fewer than 100 species. These flukes are less common and have a simpler lifecycle compared to the digenetic trematodes.
The Digeneans (Digenetic Flukes)
Extra info
• Digenetic trematodes require at least two hosts to complete their life cycle
• generally greater specificity for the first intermediate host (the
mollusc).
• High Reproductive Output: To overcome the difficulty of ensuring that at least some offspring reach their final host, digenetic trematodes produce large numbers of larvae at each stage.
• Behavioral Adaptations for Host
Transmission: Trematodes exhibit fascinating behavioral adaptations to enhance their chances of reaching the next host.
- For example, some species of schistosomes release their cercariae at specific times of day, aligning with when their target vertebrate hosts are most likely to be near the water.
The Digeneans (Digenetic Flukes)
Miracidium Larva and its host
require at least one intermediate host to complete their life cycle, in contrast to some other flatworms like cestodes
that may not need intermediate hosts.
• Miracidium Larva: The first free-living larval stage is the miracidium, a ciliated and gutless larva that hatches from the fertilized egg. It is actively ciliated and capable of swimming, seeking out and penetrating a molluscan host (usually a snail) to begin its development.
• The miracidium has specialized glands that help it, penetrate the mollusc’s body, using secretions to bore through the tissues
The Digeneans (Digenetic Flukes)
Development in the Intermediate Host:
• After entering the mollusc, the miracidium transforms into a mother
sporocyst, which lacks a mouth and digestive system. Instead, it absorbs nutrients from the host’s blood.
• Inside the sporocyst, germ balls develop into daughter sporocysts or rediae.
• Rediae are active feeders with a mouth and gut, and they continue the process of asexual reproduction by producing cercariae.
• Daughter sporocysts don’t feed but form cercariae through a similar process of asexual division. Some rediae serve as “soldiers,” attacking
and consuming other larvae from competing trematode species or even their own clones.
The Digeneans (Digenetic Flukes)
Cercariae:
• Cercariae are non-ciliated, actively swimming larvae with a muscular tail.
They are capable of moving through the water, seeking to encyst in another host or on vegetation.
• Penetration and Tail Loss: When the cercaria encounters a new host (whether an intermediate host or definitive host), it penetrates the host’s tissues, detaching its tail (sometimes before and sometimes after entering the host).
The Digeneans (Digenetic Flukes)
Second Intermediate Host:
• Some digeneans require a second intermediate host. These hosts could be a vertebrate or an invertebrate species, and many digeneans exhibit low specificity for their second intermediate hosts, meaning they can infect a wide range of species.
• For example, Echinostoma species may use freshwater snails or amphibian larvae as second intermediate hosts, while their miracidia only infect specific species of snails.
The Digeneans (Digenetic Flukes)
Metacercariae:
• Once in the second intermediate host, the cercaria transforms into a
metacercaria, which is an encysted form. The metacercaria is dormant and retains infectivity until the second intermediate host is eaten by the
definitive host.
• Encystment helps the trematode survive in the environment or within the host for extended periods (even months), ensuring that it can reach its definitive host.
The Digeneans (Digenetic Flukes)
Definitive Host:
• Upon ingestion by the definitive host (often a vertebrate, such as a fish, bird, or mammal), the metacercaria excysts (breaks free from its cyst) and migrates through the digestive tract to the appropriate internal location (e.g., the liver, blood vessels, or lungs, depending on the species).
• The juvenile trematode matures into an adult, where it develops a mouth and digestive system (usually a blind-ended gut), along with suckers to anchor itself to the host tissues. The adult produces eggs that are released into the host’s tissues, often in large quantities.
The Digeneans (Digenetic Flukes)
Reproduction:
• Most digeneans are hermaphroditic, meaning each individual contains both male and female reproductive organs, though some species are gonochoristic (have separate sexes).
• In some cases, the adult digenean can release thousands of eggs daily, contributing to the high reproductive capacity of these parasites.
The Digeneans (Digenetic Flukes)
Summary of the Digenean Life Cycle:
- Eggs are released from the adult trematode inside the definitive host.
- Miracidium larvae hatch from the eggs and penetrate a suitable molluscan host.
- The miracidium transforms into a sporocyst (mother sporocyst), which generates rediae or daughter sporocysts.
- Rediae produce cercariae, which exit the host and search for a new host.
- Cercariae penette a second intermediate host or encyst on vegetation, forming metacercariae.
- The definitive host consumes the infected intermediate host or vegetation, and the metacercariae excyst and mature into adults in the host’s body.
- The adult trematodes reproduce, and the cycle continues.
life cycle of digenetic trematodes is remarkably complex and highly adaptive process,
Aspidogastreans (also called
Aspidobothreans)
Defining Characteristics of Aspidogastreans:
- Ventral Sucker:
• Aspidogastreans possess a large ventral sucker that is divided by septa,
often forming a row of suckers along the body. This is a distinctive feature that sets them apart from both
mannaendans and digenetic
trematodes, who typically use different attachment organs. - Simple Life Cycle:
• Most species have a relatively simple life cycle, often involving a single host, which is always a mollusc-either a freshwater mussel or a gastropod.
• This life cycle structure resembles that of monogeneans, which also typically have a single host, though monogeneans use a specialized posterior attachment organ (the haptor). - Host Specificity:
• Aspidogastreans tend to have low host specificity, unlike digenetic trematodes, which often have more specific requirement or intermediate hosts. While the mollusc serves as an intermediate host, aspidogastreans do not require a variety of intermediate
hosts like digenetic trematodes do. - Life Cycle and Development:
• Some species of aspidogastreans may require a second host, which is typically a fish or a turtle, for the completion of their life cycle. In these species, the mollusc serves as the intermediate
host, and the adult fluke develops in the final host. This is similar to the life cycle of digenetic trematodes, but there are key differences:
• Unlike digenetic trematodes, aspidogastreans do not exhibit asexual replication within the interm vite mollusc host. This is a key distinction, as most digenetic trematodes undergo asexual replication stages, which are critical for their high reproductive output.
- Attachment and Body Structure:
• Instead of the posterior attachment organ (the haptor) found in
monogeneans, aspidogastreans have a modified ventral surface, which forms a powerful attachment sucker that aids in their ability to adhere to the host.
