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ZLY 103 Animal Diversity
PHYLUM PLATYHELMINTHES The Acoelomates
Introduction•The first triploblastic animals with a third tissue layer, mesoderm; between the endoderm and endoderm.•Due to absence of a coelom or pseudocoel, they are termed acoelomate•They are flat in shape and possess a blind gut (mouth without anus).
•Size varies from a millimeter to several meters.•Some are free-living (Turbellaria) and parasitic.•Majority are bottom-dwellers in aquatic & moist lands.
Introduction• Eumetazoa - animals with tissue–Bilateria - have bilateral symmetry
and are triploblastic.–Protostomia - a group of animals
whose mouth develops from the blastopore, and the mesoderm forms from an area near the blastopore.–Acoelomata - They have a true
mesoderm which fills the original blastocoel between the outer epidermis and digestive tract.
Characteristics• They are triploblastic, acoelomate, non
metamerically segmented metazoans.• They are at organ system level of
organization.• They exhibit bilateral symmetry,• Body contains no internal cavity and
absence of blood system. • Protonephridial excretory organ (Flame
cells) instead of anus.• Nervous system made up of longitudinal
fibers instead of a nerve net.• Dorsoventrally flattened.
Characteristics• They carry out hermaphroditic reproduction.• Free-living forms feed on small animals and
other life forms.• Parasitic forms feed on nutrients derived from
the host tissues.• Some species especially the free-living occur
in major biomes; marine, freshwater & moist lands.
• Central nervous system consists of a brain and two or more ganglionated nerve cord.
• They include tape worms, flatworms, flukes and planarians etc.
• Highly successful, with about 25,000 species.
Characteristics• Shows a degree of cephalization• Epidermis soft and ciliated or covered by cuticle and
with external suckers or hooks or both• Simple sense organs with eyespot in some species.• Parasitic species normally move between different
habitats during their developmental stages.• A few species are commensals deriving harmonious
existence with another larger organism.• Absence of fossil records due to their soft bodies but
they are thought to have evolved around 550my ago.
Triploblastic Condition• The embryo divides into three instead of
two as it is in cnidarians (diploblastic).• With the presence of a (parenchyma)
mesoderm, the platyhelminthes:Have an increase in size,A separation of the alimentary canal from the
body wall, A variety of organs are now formed,There is a movement towards the organ-
system level of organization,Presence of developing muscles, Cilia and
flagella are becoming too irrelevant for the larger sizes evolving.
The Acoelomate Condition• The large size of platyhelminthes with the
emergence of the mesoderm (next slide) created the problems of efficient transport system, hence the acoelomate condition.
• The animals in this group do not have a choice but to be dorsoventrally flattened for it to present a large surface area for diffusion to satisfy the metabolic requirements (flatworms).
Internal section through the gut
Plate 1: Transverse section through a generalized Acoelomate
Epidermis/ECTODERM
gut cavity
no body cavity; region between gut and body wall packed with organs within MESODERM tissue
Acoelomate organism(flatworm)
Lining of gut =ENDODERM
Body plan
Body design in Platyhelminthes
Taxonomy/Classification
• Class Turbellaria • Dugesia
• Class Monogenea –Gyrodactylus
• Class Trematoda –Clonorchis, Fasciola, Schistosoma
• Class Cestoda –Taenia, Dipylidium, Echinococcus
Distinct Features of the Classes in Flatworms
• Proglotids and scolex in Cestoda• Rabdites in Turbellaria• Combination of characteristics-
protonephridia for excretion and acoelomates in all except Cestodes
• Larva of each class are unique–Turbellaria - Planaria–Cestodes - Oncosphere/Cysticerci
(bladder worm)
–Trematodes - Metacercaria/Cerceria
Distinct Features of the Classes in Flatworms
• IntegumentsInteguments- Rhabdites and one cell layer epidermis in Turbellaria and usually ciliated; syncytial tegument in other classes.
• SkeletalSkeletal - hydrostatic• MuscleMuscle - longitudinal, transverse, and
circular muscles are present.• DigestiveDigestive - incomplete with
intracellular and extracellular digestion; no system in Cestoda.
Distinct Features of the Classes in Flatworms
• Excretory Excretory - flame cells, or excretory tubes in Cestodes.
• RespiratoryRespiratory - no system, through diffusion
• CirculatoryCirculatory - none, diffusion.• NervousNervous - anterior ganglia, ventral
ladder-like system (two lateral cords with transverse cords).
Distinct Features of the Classes in Flatworms
• Endocrine Endocrine - hormones produced by nervous system
• ReproductiveReproductive - monoecious in most–Well developed reproductive organs,
mostly internal fertilization.–Two of the parasitic classes have
complex life cycles• Trematoda - miracidium, sporocyst, redia, cercaria, metacercaria (usually snail is secondary host.• Cestoda - oncosphere, cysticercus
Distinguishing Features among the Classes
Turbellaria Trematoda Monogenea CestodaFree living; aquatic
Endoparasitic Ectoparasitic, larva is free-living
Endoparasiticor ectoparasitic
Delicate, soft body
Leaf-like shape
leaf-like/cylindrical in shape
Elongated, proglottides (able to detarch)
Suckers rarely present
Ventral sucker plus sucker on ‘head’ for attachment to host
posterior attachment with hooks, suckers or clamps
Suckers and hooks on ‘head’ plus sucker on ‘scolex for attachment
Enteron present
Enteron present
Enteron absent
No enteron
Cuticle absent(outer surface covered with cilia)
Thick cuticle with spines (not parasitic)
Syncytial teguments, no cilia
Thick cuticle (protection); no cilia in adult
Hermaphroditic/asexual fusion
Indirect development
Direct development
Indirect development
Sense organs in adult
Sense organs only in free-living larval stages
Sense organs only in free-living larval stages
Sense organs only in free-living larval stages
e.g. Planaria, Dugesia, Planocera, Microstomum
e.g. Fasciola (liver fluke) Schistosoma (blood fluke)
e.g. Dactylogyrus, Polystoma, Gyrodactylus
e.g. Taenia (tapeworm)
Turbellaria Trematoda Monogenea Cestoda
Reproduction in Platyhelminthes
Asexual by budding or transverse fission
•RegenerationSexual–Hermaphroditic; complex
reproductive system– Internal fertilization–Direct development, but some
polyclads have free-swimming larvae
Reproductive system of Platyhelminthes
Class Turbellaria• Free-living flatworms, mainly carnivorous.
• Mostly aquatic, a few terrestrial.
• Benthic (bottom dwellers).
• Variable in shape, size and features.
• Best example of cephalization.
• Surprising level of internal complexity
Class Turbellaria
• They are characterized by a soft epidermis ciliated at on the ventral surface.
• The movement of the cilia propels the smaller forms.
• Larger species glide along by muscular waves, usually over mucous beds secreted by special cells.
Turbellarians• Generally divided into five groups based
primarily on differences in the form of the digestive cavity.
• The most primitive are the acoels, (no
digestive cavity). • The ventral mouth, and sometimes a
simple pharynx, lead to an inner mass of nutritive cells.
• Most species measure less than 1/8 in. (3 mm) in length.
Planaria: Tubellaria
Turbellarians
• The rhabdocoels have straight, unbranched digestive cavities.
• Studies show that the rhabdocoels gave rise to both the trematodes and cestodes,
• Several rhabdocoel species exhibit commensal relationships, presumably led to parasitism.
• The allocoels were formerly classified together with the rhabdocoels; the gut can be either saclike or branched.
Diversity in Planarians
• The triclads, also known as planarians, are relatively large flatworms named for their three-branched gut (NEXT SLIDE).
• Most species range from 1/8 in. (3 mm) to about 1 in. (2.5 cm) in length.
• Planarians have more sense organs and a more complex brain than the other turbellarians (Polyclads).
Diversity in intestinal pattern of two orders of Tubellarians
Tricladida Polycladida
Biology of Turbellaria
• The freshwater species Dugesia tigrina has primitive eyes and tactile lobes, or auricles, on the sides of the head.
• Muscular pharynx can be extruded for food capture.
• Dugesia and many other planarians can regenerate entirely new individuals from small pieces cut from the body.
Biology of Turbellaria• The group of turbellarians known as
polyclads tend to be larger (1–2 in./2.5–5 cm) and more oval-shaped than the triclads.
• Their bodies are extremely flat and leaf-like, and the gut is subdivided into numerous branches.
• Many are brightly colored and some have ruffled edges. Some species have numerous eyes scattered over the front end of the body.
Class Trematoda
• Flukes - oral and ventral suckers, no hooks, parasites, body with a syncytial tegument without cilia. Larva stages unique.
• Clonorchis
• Fasciola
• Schistosoma
Class Trematoda
E.g. Flukes • Most important is subclass
Digenea• 11,000 species; second most
diverse group of parasites (Nematoda#1)
• Endoparasites of vertebrates• Some cause debilitating diseases
to livestock and humans
Types of Hosts Often have complex life cycles that
alternate between sexual and asexual stages.
Most require at least 2 different kinds of hosts to complete their life cycle:
1. Definitive host (primary host) Host in which the parasite matures and
reproduces (sexually) Host in which eggs are released
2. Intermediate host Hosts in which larval stages develop and
undergo asexual reproduction Results in increase in the number of the
individuals
Clonorchis
• Human liver fluke - has two intermediate or secondary host snail and fish.
Cerceria
Adult human liver fluke
8.8
Clonorchis Life Cycle
Fasciola
Adult sheep liver fluke
metacercaria on grass.
Fasciola Body Plan
Life cycle of Fasciola
Schistosoma
• Blood Flukes Cerceria usually infect by burrowing through skin.
Schistosomiasis Schistosomiasis (bilharzia) is an infection with
blood flukes and is a major infectious diseases.
More then 200 million people are infected worldwide with these flukes.
They acquire infection by swimming or walking in water domiciled by the intermediate snail host.
Schistosome eggs enter the water when infected people urinate or defecate in or near water.
Schistosomiasis Eggs hatch and the miracidium seeks out a
snail. Within the snail, it develops into a sporocyst
and asexual reproduction takes place. Cercaria are eventually released into the water.
Cercaria swims and avoids UV light which can damage it, but is very sensitive to the scent of humans.
Certain molecules from human skin makes cercaria jerks and releases chemicals that soften the skin and burrows in shedding its tail at the same time (Schistosomula).
8.9a and b
Schistosome Life Cycle
Class Monogenea They were previously classified as an order
of the Trematoda, but recent work suggests they are more closely related to cestodes (tapeworms).
They are small (usually < 2cm) typically external parasites of fish that clamp onto the gills using a hooked organ (often with suckers) called an opisthaptor.
Some also parasitize the urinary bladder or rectum of frogs and turtles; there is a species that parasitizes squid and one that attaches to the eyeball of hippopotomuses.
Class Monogenea Unlike the trematodes, Monogeneans have only a
single host (hence “Mono” in the name). Most feed on the host’s epidermis using their protrusible pharynx, but some are blood feeders.
Monogeneans are hermaphrodites (male organs develop first) and can move around a host in search of a mate (they will also self-fertilize).
The egg hatches into a ciliated larva (an oncomiracidium) which seeks out its host in the water.
8.11
Monogenea Body Plan Gyrodactylus olsoni
Monogenean Fluke on gill of longjaw mudsucker fish
Class Cestoda (Tapeworms)• Have distinct head
structure (Scolex) and reproductive units called proglottids (Strobila).
• Absence of a digestive system.
• Larva are oncospheres and cysticerci (bladder worms) Taenia Dipylidium Echinococcus
Class Cestoda (tapeworms) They are parasites of the vertebrate
digestive tract and about 4000 species are known.
Almost all tapeworms require at least two hosts with the definitive host being a vertebrate, although intermediate hosts can be invertebrates.
They are quite different in appearance from the other members of the Platyhelminthes.
Cestodes Proglottids (Body Plan) & Head
Defini
ng C
hara
cter
istics
Class Cestoda New proglottids form behind the scolex and
the strobila may become extremely long.
Tapeworms live in the intestines where it is immersed in digested food therefore lack a digestive system. Instead they simply absorb food across their tegument.
To facilitate the absorption of food, the tegument has huge numbers of tiny projections called microtriches.
The surface area of the tegument for absorption is greatly increased.
8.12
Cestode: Taenia solium
Class Cestoda Tapeworms are usually monoecious
(have both male and female reproductive organs).
A proglottid is fertilized by another proglottid in the same or a different strobila.
Shell-encased embryos form in the uterus and exit the proglottid via a uterine pore or the entire proglottid may detach and pass out of the host.
8.15
Cestode: Life Cycle
Dipylidium (dog tape)• Juveniles in flea and
louse• Note the two
gential (red arrows) pores and reproductive systems in each proglottid.
Proglottid
Echinococcus• Hydatid cyst (cysticercus)- many scolices and
enlarges to the size of a basketball. • Often called sand.• Only three proglottides
Constraints Associated with Parasitic Living
• Reproduce within the definitive host• Get fertilized eggs out of the host• Contact a new and appropriate host• Obtain entrance into the host• Locate the appropriate environment within
host• Maintain position within the host• Withstand an often anaerobic environment• Avoid digestion or attack by the hosts
immune system• Avoid killing the host, at least until
reproduction is completed
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