Ctenophora
Did you know?
• Tapeworms can be in sizes longer than a bus.
• Largest one found is 90 feet.
• Flatworms can stick its pharynx into their prey, like a vacuum cleaner hose
and suck out the prey’s insides.
• The deep-sea species of comb jellies are so fragile that, they cannot be
collected for examination.
Ctenophora
• Commonly known as sea walnuts
or comb jellies
• Also called sea gooseberries.
• Transparent, gelatinous
unsegmented animals.
• Formerly included under
Coelenterata, along with
cnidarians.
Habitat
Exclusively marine.
Habit
• All are solitary and free-swimming
animals.
Diploblastic
Tissue level of organization.
Symmetry
Radial
• Most of the structures are radially
arranged but tentacles are paired
(biradial).
Colloblasts
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Lasso cells
Present on the surface of the tentacles
Secrete a sticky substance
Facilitate prey capture
Ctenophores lack cnidoblasts
Comb plates
•
The body bears eight external rows of ciliated comb
plates, which help in locomotion.
Digestion
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The gastrovascular cavity consists of a pharynx,
stomach, and a system of gastrovascular canals.
The gastrovascular cavity ends in two aboral anal
pores.
Digestion is both extracellular and intracellular.
Respiration
Occurs by diffusion through the body wall.
Excretion
•
Occurs by diffusion through the body wall.
Bioluminiscence
The property of a living organism to emit
light is well-marked in ctenophores.
Coordination
• Thy have a nerve net system
similar to that of cnidarians.
• Statocyst is present.
Reproduction
• Reproduction takes place only by
sexual means.
• Sexes are not separate.
• Fertilization is external.
• Development is indirect
• Larva is cydippid larva.
• Regeneration is common.
Classification
Ctenophora
Tentaculata
Adults possess two
aboral tentacles
Examples:
Pleurobrachia
Ctenoplana
Hormiphora
Nuda
Tentacles are absent
Example:
Beroe
Affinities of Ctenophores
Differences with Cnidaria
Similarities with Cnidaria
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Diploblastic condition
Tissue level of organisation
Presence of coelenteron
Extracellular & intracellular digestion
External fertilization
Indirect development
• Absence of cnidocytes
• Monoecious nature
• Absence of asexual reproduction
Unique Features
• Comb plates
• Lasso cells
Platyhelminthes
Common Forms
Platyhelminthes
(Flatworms)
Turbellaria
(Planarians)
Trematoda
(Flukes)
Cestoda
(Tapeworms)
Dugesia
Fasciola
Taenia
Body Form
They have dorso-ventrally flattened body, hence are called flatworms.
• Greek platys = flat; helmins = worm
Eggs
Proglottid
Transverse Section
Eggs
Habits
These are mostly endoparasites found in animals including human beings.
• Planarians are free-living.
Level of Organisation
Flatworms are bilaterally symmetrical, triploblastic and acoelomate animals with
organ level of organisation.
Symmetry
Flatworms are bilaterally symmetrical, triploblastic and acoelomate animals with
organ level of organisation.
Body Covering
Planarians
Flukes
Tapeworms
Ciliated epidermis with mucus
producing rhabdites
Tegument
Tegument
Acoelomate Condition
Flatworms are bilaterally symmetrical, triploblastic and acoelomate animals with
organ level of organisation.
• They have a solid bauplan.
• Space between the gut and the body wall is filled with parenchyma.
Organs of Anchorage
Hooks and suckers are present in the parasitic forms.
Planarians
Flukes
Tapeworms
Free-living
Parasitic
Parasitic
No hooks and suckers
Suckers
Hooks & suckers,
rostellum
Organs of Anchorage
• In flukes, two suckers are present
• Oral sucker
• Ventral sucker (acetabulum)
Organs of Anchorage
• In tapeworms, scolex bears
• Rostellum
• Hooks
• Suckers
Digestion
Incomplete gut (blind sac body plan).
• Mouth is used for ingestion and
egestion.
• Anus is absent.
Some of them absorb nutrients from
the host directly through their body
surface.
• Tapeworms absorb nutrients through
the tegument (parasitic mode of
nutrition).
Respiration and Circulation
• Respiratory and circulatory systems are absent.
• Parenchyma helps in circulation of substances.
Excretion
Specialised cells called flame cells help in osmoregulation and excretion.
• Protonephridia with flame cells are useful primarily for osmoregulation and
secondarily for excretion.
• Ammonotelic
Nervous Coordination
• Cerebral ganglia constitute the brain.
• Longitudinal nerve cords are joined by
transverse commissures at regular intervals
giving a ladder-like appearance.
• Sense organs are common in turbellarians
• Sense are greatly reduced in trematodes
and cestodes (parasitic forms).
Reproduction and Development
Sexes are not separate.
• Schistosoma haematobium is unisexual.
Fertilisation is internal. (copulatory organ is present)
Development is through many larval stages.
• Polyembryony is common in flukes.
Planarians
Muller’s larva
Flukes
Miracidium, Cercaria, etc.
Tapeworms
Hexacanth, Cysticercus (bladderworm)
Life cycle of Fasciola
(digenetic parasite)
• Miracidium larva (forms in sheep;
infective stage to intermediate host)
• Sporocyst (parthenogenetic)
• Redia (parthenogenetic)
• Cercaria larva
• Metacercaria, (forms in snail; infective
stage to primay host)
• Adult stage.
Life cycle of Taenia
(digenetic parasite)
Regeneration
Some members like Planaria possess
high regeneration capacity.
In Planaria (flat worms), we observe
true regeneration, i.e., a fragmented
organism regenerates the lost part of
its body and becomes, a new
organism (NCERT; Living World).
• Many turbellarians reproduce
asexually.
Metazoan features that appeared first in Platyhelminthes
Triploblastic condition
Unidirectional movement
Mesoderm
Cephalization (moderate)
True muscle tissue
Digestive system
Organ/organ-system level
Excretory system
Bilateral symmetry
Brain
Examples
Dugesia
Planarian
Fasciola hepatica
Liver fluke
Fasciolopsis buski
Intestinal fluke
Schistosoma haematobium
Blood fluke
Taenia solium
Pork tapeworm
Taenia saginata
Beef tapeworm
Echinococcus granulosus
Dog tapeworm
Dugesia
Fasciola hepatica
Fasciola hepatica
Taenia solium
Schistosoma haematobium
Echinococcus granulosus