Integrated Principles of Zoology
Eighteenth Edition
Chapter 14
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Xenacoelomorpha, Platyzoa and Mesozoa
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Getting Ahead
Radially symmetrical cnidarians and ctenophores can snare prey from any direction but cannot chase prey efficiently.
Animals that actively seek food, shelter, and reproductive mates require directed movements most effectively achieved by elongated bodies that have a head and tail or bilateral symmetry.
Cephalization is the concentration of sense organs in the head region.
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Body Cavities
Most metazoans have triploblastic bodies—have ectoderm, endoderm, and mesoderm layers that produce all body structures.
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Body Plans
Figure 14.1 Acoelomate, pseudocoelomate, and coelomate body plans.
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Phylum Xenacoelomorpha
This is a new phylum.
Two sister clades are found in this phylum:
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Xenoturbellids
Wormlike, ciliated animals.
Two furrows.
Xenoturbella is the lone genus.
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Xenoturbella profunda
©2015 MBARI
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Features of Xenoturbella
Thick epidermis.
Subepidermal nerve net.
Frontal pore.
Ventral glandular network of unknown function.
Circular and longitudinal muscles.
Ventral mouth.
Blind gut.
Direct development.
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Acoelomorpha
Small flat worms less than 5 mm in length.
Typically live in marine sediments; few are pelagic and some live in brackish water.
Mostly free-living but some are symbiotic and a few are parasitic; almost 350 species.
Members were formerly in Class Turbellaria within phylum Platyhelminthes.
Have cellular ciliated epidermis with parenchyma layer that has small amounts of ECM and circular, longitudinal, and diagonal muscles.
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Acoelomorph Worms
Figure 14.3 Acoelomorph worms, Waminoa sp., on a bubble coral, Plerogyra sinuosa.
©L. Newman & A. Flowers/Science Source
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Digestion in Acoelomorphs
Some have digestive system with mouth leading to tube-like pharynx followed by a sac-like gut, but no anus.
Many acoelomorphs have no gut and the pharynx is absent.
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Structure of an Acoelomorph
Figure 14.4 (A) Generalized acoelomorph flatworm. (B) Midsagittal section showing gut cavity filled with endodermal cells.
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Reproduction in Acoelomorphs
Acoelomorphs are monoecious.
Female reproductive organs produces yolk-filled eggs called endolecithal eggs.
Following fertilization some or all cleavage events produce a duet-spiral pattern of new cells which may be one of the defining morphological feature of acoelomorphs.
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Features of Acoelomorphs
Have other defining features proposed for acoelomorphs.
Have distinct anteroposterior axis, but lack a “true” brain.
Have a radial arrangement of nerves in body, not the ladder-like pattern seen in Platyhelminthes.
Statocysts have different structures than Platyhelminthes.
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Phylogeny of Acoelomorphs
Phylogenetic studies using molecular characters such as mitochondrial genome and myosin genes describe acoelomorphs as early-diverging bilaterally symmetrical triploblasts.
Have only four or five Hox genes unlike free-living Platyhelminthes which have seven or eight Hox genes.
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Clades Within Protostomia
Most triploblastic metazoans are divided into two superphlya: Protostomia and Deuterostomia.
Protostomes now divided into two large clades: Ecdysozoa and Lophotrochozoa.
Modern molecular phylogenies have grouped acoelomate and coelomate taxa together within the protostomes.
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Trochophore Larvae
Minute, translucent, and roughly top-shaped.
Have a prominent circlet of cilia and sometimes one or two accessory circlets.
Occur in the early development of other marine members of Annelida and Mollusca and are assumed to be ancestral for these groups.
Trochophore-like larvae also occur in some Platyhelminthes, Nemertean, Echiura, and Sipunculida groups.
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Clade Platyzoa
Clade Platyzoa is a unique group of lophotrochozoan protostomes that contain Platyhelminthes, Gastrotricha, and Gnathifera.
Gnathifera contains four phyla.
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Platyzoan Relationships
Figure 14.5 Hypothetical relationships among members of Platyzoa.
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Phylum Platyhelminthes
Commonly called flatworms; vary in size from a millimeter to many meters in length like tapeworms.
Normally slender, leaf-like form but can also be long and ribbon-like.
Some free-living; others parasitic.
Not a valid monophyletic phylum according to some due to lack of single unique characteristic.
However, parasitic species have an external body covering called a syncytial tegument (neodermis), not the cellular ciliated epidermis of free-living species.
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Example of a Platyhelminth
Figure 14.6 (A) Stained planarian. (B) Bipalium, a terrestrial flatworm.
©Michael Abbey/Science Source
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Platyhelminth Diversity
Platyhelminthes is divided into four classes: Turbellaria, Trematoda, Monogenea, and Cestoda.
Class Turbellaria.
Monogenea, Trematoda (flukes), and Cestoda (tapeworms) are all parasitic.
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Platyhelminthes Relationships
Figure 14.7 Hypothetical relationships among parasitic Platyhelminthes.
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Turbellarian Form and Function
Most have cellular, ciliated epidermis on a basement membrane.
Contains rod-shaped rhabdites that swell and form a protective mucous sheath when discharged with water.
Most turbellarians have dual-gland adhesive organs in the epidermis.
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Anatomy of a Planarian
Figure 14.8 (A) Whole planarian. (B) Cross section of planarian through pharyngeal region, showing relationships of body structures.
©Eric Grave/Science Source
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Dual-Gland Adhesive Organ
Figure 14.9 Reconstruction of dual-gland adhesive organ of the turbellarian Haplopharynx sp.
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Parasitic Platyhelminth Form and Function
Three parasitic classes have a non-ciliated body covering called syncytial tegument that has many nuclei enclosed within a cell membrane.
Many larval forms have temporary ciliated covering that is shed when a host is contacted to prevent host immune response.
Neodermis formed after several surface layers of epidermis are shed, allowing cytoplasmic extensions from cells below basement membrane to reach the surface.
The three parasitic groups are in clade Neodermata.
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Tegument of Endoparasites
Resistant to host immune system and to digestive juices within the host gut which allow tapeworms and other worms to dwell in it.
Syncytial nature of tegument allows more resistance due to lack of penetrable junctions between cells.
Tegument can be absorptive and secretory where secreted enzymes can reduce host digestive system and absorb nutrients from host gut cavity.
Most tapeworms have no mouth and lack complete digestive tract.
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Structure of the Tegument
Figure 14.10 Diagrammatic drawing of the structure of the tegument of a trematode Fasciola hepatica.
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Turbellarian Nutrition and Digestion
Platyhelminthes generally have mouth, pharynx, and intestine.
In turbellarians the pharynx may extend through the ventral mouth.
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Structure of a Planarian
Figure 14.11 Structure of a planarian. (A) Reproductive and osmoregulatory systems. (B) Digestive tract and ladder-type nervous system. (C) Pharynx extended through ventral mouth.
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Parasitic Platyhelminth Digestion
Monogeneans and trematodes graze on host cells, feeding on cellular debris and body fluids.
Cestodes have no digestive system.
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Structure of a Trematode
Figure 14.12 Structure of human liver fluke Clonorchis sinenesis.
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Platyhelminth Excretion and Osmoregulation
Flatworms have protonephridia used for osmoregulation and excretion.
Majority of metabolic wastes removed by diffusion across the wall.
Have flame cells which are cup-shaped structures that have flagella extending from the surface.
Beating flagella drive fluids down collecting ducts and through delicate interlaced projections.
Wall of the duct bears folds or microvilli to resorb ions and molecules.
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Variations in Protonephridia
In Planarians, the collecting ducts join and empty at nephridiopores to regulate water.
Monogeneans have two excretory pores that open laterally near anterior end.
Trematodes have ducts that empty into excretory bladder that leads to the outside via a terminal pore.
Cestodes have two main excretory canals on each side that are continuous along the length of the worm and join on the last segment and opens to the terminal pore.
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Platyhelminth Nervous System
Most primitive type of flatworm nervous system, found in some turbellarians, called subepidermal nerve plexus.
Other flatworms also have one to five pairs of longitudinal nerve cords under the muscle layer.
Freshwater planarians have one ventral pair of nerve cords forming a ladder-type pattern and the brain is a bilobed ganglion anterior to the ventral nerve cords.
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Platyhelminth Sense Organs
Active locomotion favored cephalization and evolution of sense organs.
Ocelli (light-sensitive eyespots) present in turbellarians, monogeneans, and larval trematodes.
Tactile and chemoreceptive cells are abundant over the body especially in the ear-shaped auricles on the sides of the head of planarians.
Some have statocysts for equilibrium and rheoreceptors to sense the direction of water currents.
Sensory nerve endings found in oral suckers and genital pores of parasitic groups.
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Flatworm Reproduction and Regeneration
Many turbellarians reproduce asexually (fission) and sexually.
Trematodes have asexual reproduction in their intermediate hosts, snails.
Some juvenile cestodes have extensive asexual reproduction.
Nearly all flatworms are monoecious (hermaphroditic) but can cross-fertilize.
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Fission in Turbellarians
Figure 14.13 Some small freshwater turbellarians.
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Flatworm Reproductive Development
Endolecithal eggs with spiral determinate cleavage are typical of some turbellarians, and likely ancestral for flatworms.
Parasitic flatworms generally have female gametes with little yolk; yolk is released by separate organs called vitellaria.
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Turbellarian Reproductive Structures
Male structures include one or more testes, connected to vasa efferentia that connect to one vas deferens.
Turbellarians develop male and female organs opening at a common genital pore.
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Parasitic Flatworm Reproductive Structures
Monogeneans hatch free-swimming larvae that attach to hosts and develop into juveniles.
Larval trematodes emerge as ciliated larvae that penetrate or are eaten by the intermediate host like snails.
Cestodes hatch only after being consumed by a variety of intermediate host.
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Class Turbellaria
Mostly free-living; range from 5 mm to 50 cm long.
Live under objects in marine, freshwater, and terrestrial habitats.
Turbellarians are distinguished by the presence or absence of gut, pattern of branching of the gut, and type of pharynx.
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Turbellarian Gut Pattern
Figure 14.14 Intestinal pattern of two orders of turbellarians. (A) Tricladida. (B) Polycladida.
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Turbellarian Movement
Turbellarians combine creeping with ciliary movements.
Others move by gliding over a slime track secreted by marginal adhesive glands.
Larger polyclads and terrestrial turbellarians crawl with muscular undulations like snails.
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Marine Turbellarian
Figure 14.15 Pseudobiceros hancockanus, a marine polyclad turbellarian.
©Diane R. Nelson
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Class Trematoda
All are parasitic flukes and most adults are endoparasites of vertebrates.
Mostly leaflike with one or more suckers, but lack opisthaptor of monogenean flukes.
Adaptations for parasitism include:
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Trematode Diversity
Trematodes share many characteristics with ectolecithal turbellarians.
Subclass Aspidogastrea is least well-known.
Subclass Digenea is largest and most well-known.
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Subclass Digenea
Complex life cycle.
Some species need a 2nd or 3rd intermediate host in the life cycle.
Parasitize almost all kinds of vertebrate hosts and can inhabit a wide variety of body parts within the hosts.
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General Digenean Life Cycle
Larva passes from definitive host in excreta and must reach water to develop.
Hatches into a free-swimming ciliated larva, the miracidium.
Miracidium penetrates tissues of a snail and is transformed into a sporocyst.
Sporocyst reproduces asexually to form more sporocysts or rediae.
Rediae reproduce asexually to form more rediae or cercariae.
Single egg therefore can produce a multitude of infectious progeny.
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Life Cycle of a Digenean
Cercariae emerge from the snail and penetrate a final host, a 2nd intermediate host, or encyst on aquatic vegetation.
Cercaria then develop into metacercariae (juvenile flukes).
Metacercariae are eaten by definitive host and move to final infection sites and grow into adults.
Numerous infectious digenean parasites impact humans and domesticated animals.
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Sheep Liver Fluke
The sheep liver fluke (Fasciola hepatica) was first digenean life cycle described.
Infects sheep and other ruminants.
Adult flukes live in liver bile passage; eggs released in feces.
Miracidia hatch and penetrate snails to become sporocysts.
After two generations of rediae, the cercaria encyst on vegetation and await being eaten.
When eaten, metacercariae develop into young flukes and live in the liver of hosts.
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Human Liver Fluke
Clonorchis sinensis is most important human liver fluke; also infects cats, dogs, and pigs.
Common in China, Japan, and Southeast Asia.
Adult fluke is 10 to 20 mm long with an oral and ventral sucker.
Digestive system includes pharynx, muscular esophagus, and two long unbranched intestinal ceca.
Excretory system has two protonephridial tubules with branches lined with flame cells that form a bladder and open to the outside.
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Liver Fluke Structure
Nervous system has two cerebral ganglia and longitudinal cords with transverse connectives like other flatworms.
Reproductive system is hermaphroditic with 80% of body devoted to it.
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Clonorchis Life Cycle
Adults live in bile passageways of humans and other fish-eating mammals.
Eggs containing a complete miracidium are shed into water with feces.
Eggs hatch only when ingested by snails of specific genera.
Miracidium enters snail tissue and transforms into a sporocyst.
Sporocyst produces one generation of rediae, which begin development.
Rediae pass into the snail liver and develop into tadpole-like cercariae.
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Life Cycle of Clonorchis
Cercariae escape into water and make contact with fish of the family Cyprinidae.
Bore under scales and into fish muscles where they shed tail and encyst as metacercariae.
A mammal eats raw fish and cyst dissolves, releasing young flukes to migrate up bile duct.
Heavy infection can destroy the liver and result in death.
Control of parasites requires the removal of snails or thorough cooking of fish.
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Life Cycle of Clonorchis sinensis
Figure 14.16 Life cycle of Clonorchis sinensis.
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Schistosoma – Blood Flukes
Over 200 million people infected with schistosomiasis.
Common in Africa, South America, West Indies, and the Middle and Far East.
Sexes are separate (dioecious) with males being broader, heavier and have large ventral groove called gynecophoric canal that is posterior to the ventral sucker.
Gynecophoric canal wraps around long and slender female during mating session.
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Schistosomiasis
Three species account for most human schistosomiasis:
Control is best achieved through proper hygiene and avoidance of contaminated areas.
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Schistosoma Life Cycle
Eggs discharged in human feces or urine.
In water, eggs hatch as ciliated miracidia and search for snail.
In snail, transform to sporocysts.
Sporocysts produce daughter sporocysts that produce cercaria.
Cercariae escape snail and swim until they contact human skin where cercariae pierce the skin and shed their tails.
Enter blood vessels and migrate to the hepatic portal blood vessels so as to develop in the liver.
No redia or metacercariae stages.
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Life Cycle of Schistosoma
After development in liver, migrate to target sites.
As females release eggs, they are extruded through venous walls and gut or bladder lining to exit with feces or urine.
Eggs that do not get extruded flow back to the liver in blood and can form centers of inflammation.
Eggs cause most of the ill effects in the human host.
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Life Cycle of Schistosoma mansoni
Figure 14.17 (A) Adult male and female Schistosoma mansoni in copulation.
(B) Life cycle of Schistosoma mansoni.
©Larry S. Roberts
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Liver Damaged by Schistosoma
Figure 14.18 This cut surface of a liver shows schistosomal hepatic fibrosis.
©Larry Roberts/McGraw-Hill Education
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Schistosome Dermatitis
Schistosome dermatitis is known as swimmer’s itch.
Various species can cause rashes and dermatitis when the cercariae penetrate an unsuitable host, like humans.
Severity of the rash increases with increasing number of contacts or sensitization as the cercariae are attacked by the hosts’ immune system and release allergenic substances.
It affects many tourists at contaminated vacation sites and infested lakes.
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Paragonimus—Lung Flukes
Paragonimus westermani is a lung fluke that parasitizes a variety of mammals.
Eggs are coughed, then swallowed, and eliminated in feces.
Zygotes develop in water and miracidia penetrate a snail host.
In snail, miracidia produce sporocysts, which become rediae.
Cercariae form in rediae and are shed into the water or ingested by freshwater crabs that prey on infested snails.
Metacercariae develop in crabs and human infection occurs by eating uncooked crabmeat.
Infection causes respiratory issues like breathing difficulties and chronic cough that can lead to fatalities.
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Other Trematodes
Fasciolopsis buski lives in human and pig intestines.
Leucochloridium lives in snails and birds.
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Paragonimus
Figure 14.19 Lung fluke Paragonimus westermani.
©Natural History Museum, London/SPL/Science Source
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Class Monogenea
Originally placed in Trematoda but now in different classes.
Some now argue they are sister taxa, both having a posterior attachment with hooks.
Monogeneas are all external parasites of many fish, especially on gills, but a few are found in bladders of frogs and turtles.
Generally cause little harm or damage to host but can become a problematic pathogen in crowded fish farming areas.
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Monogenean Features
Have direct life cycle in a single host.
Egg hatches to produce ciliated larvae called oncomiracidium that attaches to host by posterior hooks.
Posterior hooks may become the posterior attachment organ of the adult, called the opisthaptor.
Opisthaptors vary widely as hooks, suckers, clamps, and a combination of forms to withstand the force of water flow while attached to the gills and skin of fish.
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Structure of Monogenean
Figure 14.20 A monogenetic
fluke Gyrodactylus cylindriformis, ventral view.
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Class Cestoda
Tapeworms have long flat bodies.
Tapeworms lack a digestive system but have well-developed muscles.
Excretory and nervous systems similar to other flatworms.
Lack sensory organs except for modified cilia that are sensory endings on the tegument.
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External Anatomy of a Tapeworm
Figure 14.21 A tapeworm, showing strobila and scolex. The scolex is the organ of attachment.
©Science Photo Library RF/Getty Images
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Tapeworm Sensory Ending
Figure 14.22 Schematic drawing of a longitudinal section through a sensory ending in the tegument of Echinococcus granulosus.
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Form of Cestoda
Cestodes, like trematodes and monogeneans, have no external motile cilia.
However, entire surface of cestodes is covered with small projections called microtriches similar to microvilli seen in the vertebrate small intestine.
Microtriches increase the surface area for food absorption since tapeworms are parasitic and attach to the intestines of the hosts.
Subclass Eucestoda has the most species.
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Subclass Eucestoda
Main body of tapeworm is a chain of proglottids called the strobila.
Proglottids originate in the germinative zone just behind the scolex.
Some practice self-fertilization, although the norm is cross-fertilization.
Each proglottid contains a complete male and female reproductive system.
Shelled embryos form in the uterus and either expelled through uterine pore or the entire proglottid is shed from the worm.
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Tapeworm Diversity
Proglottid formation is not “true” segmentation since replication of sex organs is not equivalent to metamerism in annelids and others.
Nearly all cestodes require two hosts and the adult is parasitic in the digestive tract of the vertebrates.
Over 1000 species of tapeworms known, infecting almost all vertebrates and having intermediate invertebrate host.
Most tapeworms do little harm to host.
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Taenia saginata—Beef Tapeworm
Lives as an adult in the alimentary canal of humans while juveniles mostly form in the intermuscular tissue of cattle.
Mature adults can reach over 10 meters in length with over 2000 proglottids.
Scolex has four suckers but no hooks and a short neck connecting to strobili.
Proglottid has muscles and parenchyma with repeating reproductive and excretory systems and complete male and female organs like those in trematodes.
Human infection very common; can be avoided by eating only thoroughly cooked beef since much of the beef supply (20%) is not inspected by the USDA.
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Beef Tapeworm Features
Excretory canals run from scolex along entire body and connect to excretory duct.
Nerve cords from a nerve ring in the scolex run along proglottids.
Contains vitellaria in a single compact vitelline gland posterior to ovaries.
Gravid proglottids break off and usually crawl out of feces and attach to vegetation.
Proglottids rupture as they dry; embryos are viable for five months and are picked up by grazing animals.
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Life Cycle of Beef Tapeworm
Cattle swallow shelled larvae; these hatch as oncospheres and use hooks to burrow through intestinal wall to blood or lymph.
Reach voluntary muscle and encyst to become bladder worms (juveniles called cysticerci).
When the infected meat is eaten, the cyst wall dissolves and the scolex evaginates to attach to intestinal mucosa.
New proglottids develop in 2 to 3 weeks to form mature worm.
Infected individuals expel numerous proglottids daily either in feces or by crawling out of the anus.
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Beef Tapeworm Life Cycle
Figure 14.23 Life cycle of beef tapeworm, Taenia saginata.
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Taenia solium—Pork Tapeworm
Adults live in small intestines of humans while juveniles live in muscles of pigs.
Most common mode of infection occurs when pigs consume infected human fecal material containing fertilized eggs.
If human ingest fertilized tapeworm eggs, can develop cysticercosis.
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Brain Damage from Cysticercosis
Figure 14.25 Section through the brain of a person who died of cerebral cysticercosis, an infection with cysticerci of Taenia solium.
©Ana Flisser
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Diphyllobothrium latum—Fish Tapeworm
Adults found in intestines of humans, dogs, cats, and other mammals.
Immature stages found in crustaceans and fish.
Largest cestode to infect humans, reaching up to 20 meters in length.
Fish tapeworm infections can occur anywhere people eat raw fish and is quite common in the Great Lakes region of the USA.
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Echinococcus granulosus—Unilocular Hydatid
A dog tapeworm that causes hydatidosis.
Adults parasitize dogs and other canines; juveniles infest many mammal species.
Humans may serve as dead-end host.
Juveniles form a special cysticercus, a hydatid cyst, that grows up to 20 years and form large masses that can affect major body parts.
Treatment is chemotherapies or surgical removal.
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Dog Tapeworm Proglottids
Figure 14.24 Mature proglottid of Taenia pisiformis, a dog tapeworm.
©NHPA/M. I. Walker
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Dog Tapeworm
Figure 14.26 Echinococcus granulosus, a dog tapeworm. (A) Early hydatid cyst or bladder-worm stage found in cattle, sheep, hogs, and sometimes humans that produces hydatid disease. (B) The adult tapeworm lives in the intestine of a dog or other carnivore.
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Phylum Gastrotricha
Gastrotrichs are small ventrally flattened animals.
Look similar to rotifers but lack corona and mastax and have bristly scaly body.
Usually found gliding along substrates via their ventral cilia.
Found in fresh, brackish, and salt water with many species being cosmopolitan.
About 450 species, only a few of which can be in both fresh and marine habitats.
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Gastrotrich Form
Have convex dorsal surface bearing bristles, spines or scales, and a ventral flattened ciliated surface.
Head region is lobed and ciliated while tail region may be elongated and forked.
Has partial syncytial epidermis beneath cuticle with a dual-gland system for attachment and release.
No specialized respiratory or circulatory system; uses simple diffusion.
No body cavity.
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Gastrotrich Function
Gastrotrichs have extracellular digestion with complete digestive system including mouth, muscular pharynx, stomach-intestine region, and anus.
Protonephridia with solenocytes rather than flame cells.
Nervous system has brain near pharynx with a pair of lateral nerve trunks.
Generally lack eyespots; some have pigmented ocelli in brain.
Some sensory bristles on the head used for tactile response.
Pestle organ on head may be chemoreceptor.
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Gastrotrich Reproduction
Gastrotrichs are typically hermaphroditic.
Produce thin-walled eggs but also develop thick-shelled dormant eggs that can withstand harsh environments for many years.
Development is direct with growth and maturation being rapid and juveniles reach sexual maturity within days.
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External and Internal Gastrotrich Anatomy
Figure 14.27 (A) Live Chaetonotus simrothic, a common gastrotrich. (B) Dorsal surface. (C) Internal structure, ventral view.
©Perennou Nuridsany/Science Source
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Structure of a Gastrotrich
Figure 14.28 Gastrotrichs in order Macrodasyida. (A) Macrodasys. (B) Turbanella.
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Clade Gnathifera
Consists of 4 lophotrochozoan phyla.
Ancestors possessed complex cuticular jaws with homologous microstructure.
Living gnathiferans vary in the number of pairs of jaws.
Most gnathiferans are free-living aquatic animals.
Rotifers and acanthocephalans are presumably sister taxa.
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Phylum Gnathostomulida
Jaw worms are found in a variety of areas around the world.
Can endure very low oxygen.
Live in association with a variety of other small forms like ciliates, tardigrades, and worms.
Glide, swim in loops and spirals and bend the head from side to side with many sensory cilia on the head.
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Gnathostomulid Features
Feed by scraping bacteria and fungi from the substrate with paired jaws on the pharynx.
Ciliated epidermis, but only 1 cilium per epidermal cell (unusual in lophotrochozoans).
Body is acoelomate with no circulatory system; probably use diffusion for excretion and gas exchange.
Not much known about mating behavior and reproductive system.
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Structure of a Gnathostomulid
Figure 14.29 (A) Gnathostomula jenneri is a tiny member of the interstitial fauna between grains of sand or mud.
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Phylum Micrognathozoa
One known species, Limnognathia maerski.
Tiny animals living interstitially, using cilia to move.
Two-part head, thorax and abdomen leading to short tail.
Epidermis has dorsal plates but no ventral ones.
Have a ventral ciliary adhesive pad that produces glue.
Have three pairs of complex jaws with mouth leading to simple gut and anus.
Two pairs of protonephridia.
Reproductive system is not well understood; only female organs have been found.
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Structure of a Micrognathozoan
Figure 14.30 (A) Limnognathia maerski, a micrognathozoan. (B) Detail of complex jaws. (C) A living specimen.
©Martin V. Sorensen
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Phylum Rotifera
Have ciliated crown (corona) that beats like rotating wheels.
About 2000 species, most between 100 and 500 µm.
Inhabit freshwater lakes and ponds; usually benthic, living on vegetation and between sand grains (meiofauna).
Pelagic forms common in surface waters; some are epizoic (live on body of another animal) and some parasitic.
Can have bizarre shapes ranging from globular and saclike to elongated and vase-like with thick outer epidermis (lorica).
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Features of Rotifera
Rotifers can endure long periods of dryness (desiccation).
Other species can survive extreme cold temperature (−272 Celsius) and be successfully revived.
Can have a variety of locomotory forms ranging from free-floating, creeping and swimming, to sessile forms.
Some are colonial while others are solitary.
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Structure of a Rotifer
Figure 14.31 (A) Live Philodina sp., a common rotifer. (B) Structure of Philodina sp.
©John Walsh/Science Source
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External Features of Rotifers
Rotiferan body has a head, trunk, and tail (foot); except for the corona, it is nonciliated and covered in cuticle.
Corona can have sensory bristles (papillae), a midventral mouth and coronal cilia use for swimming and feeding.
Trunk may be elongated or saclike with sensory antennae.
Foot may have 1 to 4 toes with pedal glands for attachment.
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Examples of Rotifers
Figure 14.32 Variety of form in rotifers.
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Internal Features of Rotifers
Syncytial epidermis secretes cuticle and bands of subepidermal muscles around the body.
Large pseudocoel filled with fluid, muscles, and mesenchymal ameboid cells.
Digestive system is complete with pharynx (mastax) fitted with hard jaws (trophi) for sucking and grinding of food,
Salivary and gastric glands likely secrete enzymes for extracellular digestion; absorption occurs in the stomach.
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Rotifer Function
Excretory system has pair of protonephridial tubules with flame cells that empty to common bladder.
Pulsating motion drains bladder into cloaca; intestines and oviduct also empty into cloaca.
Protonephridia may be important in osmoregulation.
Has bilobed brain dorsal to the mastax region and has paired eyespots, sensory bristles, papillae, and antennae.
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Rotifer Reproduction
Dioecious with males smaller than females.
Females in Bdelloidea and Monogononta have combined ovaries and yolk glands, called germovitellaria.
In Bdelloidea, all females are parthenogenetic.
In Seisonidea, females produce haploid eggs.
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Reproduction in Class Monogononta
Most of the year, diploid females produce diploid amictic eggs.
Environmental factors like crowding, diet, and photoperiod may induce amictic eggs to develop into diploid mictic females.
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Monogonont Reproductive Structures
Males have single testis and ciliated sperm duct leading to genital pore.
End of sperm duct forms copulatory organ which can penetrate any part of female body and inject sperm into pseudocoelom where fertilization occurs.
Females hatch with complete adult features and mature quickly.
Males often do not grow and are sexually mature at hatching.
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Monogonont Life Cycle
Figure 14.33 Reproduction of some rotifers is parthenogenetic during the part of the year when environmental conditions are suitable.
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Phylogeny of Rotifera
Traditionally rotifers are split into three classes:
Recent molecular work has challenged these classifications; they are subject to debate and possible revision.
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Phylum Acanthocephala
Spiny-headed worms have distinctive cylindrical invaginable proboscis with rows of recurved spines.
Adults are endoparasites of birds, fish, and mammals while larvae live in crustaceans and insects.
Females are usually larger than males.
Bilaterally flattened body with many transverse wrinkles.
Variable sizes and cosmopolitan in distribution, with over 1100 species.
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Acanthocephalan Form
Syncytial body wall with many minute crypts (depressions) to increase body surface area.
Tegument has lacunar system of fluid-filled canals to enhance diffusion across body wall.
No heart but muscular body wall forms tubes connected to lacunar system and collectively function like heart that uses lacunar fluid as a circulatory system.
Proboscis can be inverted into a proboscis receptacle.
Two elongated hydraulic sacs, lemnisci, are attached to neck; function is unknown.
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Acanthocephalan Function
No respiratory system.
When present, excretory system has simple protonephridia with flame cells.
Nervous system and sense organs are reduced.
No digestive tract; absorb nutrients through tegument.
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Acanthocephalan Reproduction
Acanthocephalans are dioecious.
Unique embryo selective apparatus system in uterine bell.
Shelled embryos are released in the feces of host and await entry to intermediate host.
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Hosts of Acanthocephalans
Acanthocephalans are not normally parasitic to humans, though they do infect pigs and other mammals.
One common species uses soil-inhabiting beetle larvae as intermediate host.
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Structure of Acanthocephalans
Figure 14.34 Details of the Acanthocephalan worm, Polymorphus botulus.
(a-c): ©Wayne Lord and Inga Sidor
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Phylogeny of Acanthocephala
Largely organized by shape and structure of spines on the proboscis.
Traditionally divided into three classes.
New molecular data suggests that acanthocephalans may be a class of highly derived rotifers, possibly sister taxon to Bdelloidea.
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Phylum Mesozoa
Mesozoa were considered a “missing link” between unicellular eukaryotes and metazoan.
Usually minute, ciliated, and wormlike animals that live as parasites or symbionts in marine invertebrates.
Arranged in two layers of 20 to 30 cells; layers are not homologous to germ layers of metazoans.
Two classes, Rhombozoa and Orthonectida, are so different that some authorities place them in separate phyla.
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Rhombozoans
Rhombozoans live in kidneys of benthic cephalopods.
Adults are called vermiforms (or nematogens) and are long and slender.
Inner, reproductive cells give rise to vermiform larvae.
When overpopulated, reproductive cells develop into gonad-like structures producing male and female gametes.
Zygotes grow into ciliated infusoriform larvae which are shed with host urine into the seawater.
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Structure of Rhombozoans
Figure 14.35 Two methods of reproduction by mesozoans. (A) Asexual development of vermiform larvae. (B) Under crowded conditions in the host kidney, gametes that produce infusoriform dispersal larvae in the host urine.
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Orthonectids
Orthonectids parasitize a variety of invertebrates like brittle stars, molluscs, and worms.
Reproduce sexually and asexually.
Asexual stage is quite different from rhombozoans.
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Structure of Orthonectids
Figure 14.35 (A) Female and, (B) male orthonectid (Rhopalura).
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Phylogeny
Evolutionary relationships are still in flux for these groups.
Most current phylogenies place members of Acoelomorpha as sister taxon of all Bilateria.
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Phylogenetic Uncertainty
We depict a lophotrochozoan clade called Platyzoa (Platyhelminthes, Gastrotricha, and Gnathifera), but not all phylogenies support this grouping.
Within Platyhelminthes, class Turbellaria is clearly paraphyletic.
Within Gnathifera, clade Syndermata (Acanthocephala and Rotifera) emerges repeatedly from phylogenetic studies repeatedly.
Mesozoans are identified as lophotrochozoan protostomes based on molecular data, but are not placed in Platyzoa.
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Accessibility Content: Text Alternatives for Images
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Body Plans - Text Alternative
All have an outer layer of ectoderm and a central gut cavity lined with endoderm. Acoelomate body plan has mesoderm filling the area between the ectoderm and endoderm. Pseudocoelomate body plan has layer of mesoderm just inside the ectoderm, and then a cavity the pseudocoel between the mesoderm and the endoderm. Coelomate body plan has mesoderm lining the entire body cavity between the ectoderm and the endoderm.
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Structure of an Acoelomorph - Text Alternative
This elongated oval worm has a proboscis sheath and statocyst at one end and a gonopore at the other. The center of the worm contains a large gut cavity. To the sides of the gut are the reproductive structures - testes and ovaries.
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Platyzoan Relationships - Text Alternative
The platyzoan clade is united by molecular characters. The clade splits into three groups. One is Phylum Platyhelminthes, one is phylum Gastrotricha, and one is clade Gnathifera. Gnathiferans are united by cuticular jaws, and split into four groups. The gnathiferans include phylum Gnathostomulida, Micrognathozoa, Rotifera, and Acanthocephala. The last two phyla are united in clade Syndermata.
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Anatomy of a Planarian - Text Alternative
A planarian has two ocelli at the anterior end, an extendable pharynx in the center of the body, and a branching intestine. A cross section shows the pharynx in the middle with an intestinal branch on either side. Circular and longitudinal muscles underlie the epidermis. The ventral surface contains dual-gland adhesive organs, while the dorsal surface has scattered rhabdites.
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Structure of the Tegument - Text Alternative
Within the parenchyma are tegumentary body cells, containing typical eukaryotic organelles such as mitochondria and Golgi. The cytoplasm of these tegumentary cells extends upward, through a muscle layer, to a layer of distal cytoplasm on the surface of the trematode.
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Structure of Monogenean - Text Alternative
This elongated fluke has a circular opisthaptor at one end, covered in hooks. The pharynx is at the opposite end, leading to a Y-shaped digestive tract. Reproductive structures are in the center.
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Tapeworm Sensory Ending - Text Alternative
Sensory endings are present in the tegument of tapeworms. The nerve process extends from the body through the longitudinal and circular muscle layers, into the distal cytoplasm of the tegument. There are mitochondria present in the sensory ending, which enlarges as it reaches the distal cytoplasm.
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Structure of a Gnathostomulid - Text Alternative
Gnathostomulids are small, wormlike, and relatively colorless. Bristly extensions are present at both anterior and posterior ends of the body. Jaws are visible inside the body, just behind the anterior end, leading into the gut. Reproductive organs are in the center of the body, while the stylet is in the posterior.
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