Transcript The Triploblasitc, Acoelomate Body Plan

Triploblastic – have three primary germ
layers
2. Acoelomate – without a coelom
3. Classified into three phyla –
1.
›
›
›
Phylum Platyhelminthes (flatworms)
Phylum Nemertea (unsegmented)
Phylum Gastrotricha (bottom dwellers)






aka – Flatworms
Most common =
planarian
Contains over 34,000
species
Currently, no uniquely defining characters
(synapomorphies) in this phylum
Adult size from 1 mm or less to 25 m
Live in marine, freshwater, and damp
terrestrial habitats
Bilateral Symmetry
 Cephalization (have a so
called head)
 Mesoderm tissue includes a
loose tissue (parenchyma)
that fills spaces between
specialized tissues, organs,
and body wall.

› May provide skeletal support, nutrient storage,
motility, transport of materials, oxygen storage, etc.
Most cells are close to external
environment ∴ materials can pass easily
into and out of their bodies (via diffusion)
 Rely on diffusion for respiration,
excretion, and circulation
 Other functions vary among species
(free-living or parasitic)


Free-living flatworms:
› Class Turbellaria

Parasitic Species:
› Class Monogenea
› Class Trematoda
› Class Cestoidea



Mostly free-living
bottom dwellers in
freshwater and
marine environments
Crawl on stones,
sand, or vegetation
Named for the
turbulence that their
beating cilia create
in the water
Over 3,000 species
 Few terrestrial species live in humid
tropics and subtropics
 Less than 1 cm long (rare
terrestrial/tropic = up to 60 cm long)
 First group of bilaterally symmetrical
animals to appear

Carnivores (small invert.),
scavengers (dead), some
herbivores (algae)
 Sensory cells (chemoreceptors) on their
heads help detect food far away
 Digestive cavity has a single opening (or
mouth) through which food and waste
pass


Pharynx – muscular
tube that extends
out of the mouth and
pumps food into the
digestive cavity or gut
› highly branched gut transports food to all
parts of the body (this is lacking in many
parasitic species)
Cilia on epidermal cells help to glide
through the water and over the bottom
of a stream or pond
 A layer of mucus is laid down to aid in
adhesion and help cilia gain traction


Muscle cells controlled by the nervous
system allow them to twist and turn so
that they are able to react rapidly to
environmental stimuli
› Dorsoventral muscles essential for
maintaining flatness (ie-for diffusion)

Do not have respiratory organs ∴ done
via diffusion
› Respiratory gases (CO2 and O2)
› Metabolic wastes (ammonia)

Depends on environment
› Marine = osmotic equilibrium
› Freshwater = hypertonic


Protonephridia is a network of fine tubules for
excretion of metabolic wastes.
Flame cells are ciliated and induce currents to
push fluids through tubules.

Tubules eventually merge and open to
the outside of the body wall through a
minute opening called a nephridiopore.

Subepidermal nerve plexus (resembles
cnidarians but depends on species)
› Mechanoreceptor (excited by pressure) at
anterior end detects body position(due to
gravity)
› Cerebral ganglia – more centralized nerve
net (“brain”)
› Longitudinal nerve cords –
ladderlike appearance
(evolutionary
advancement
to a nervous system)
Auricles – sensory
lobes on side of
head aid in food
location
(chemoreceptor)
 Ocelli – eye spots;
orient in direction
of light
(photoreceptor)


See how it works…



Asexual reproduction
Budding or fission
Regeneration

Can reproduce either asexually or sexually
› Asexual = planarian detaches its tail end and each half
regrows the lost parts (referred to as zooids).
› Sexual = each Planaria gives and receives sperm.
› Benefits to sexual over asexual?


Have both testes and ovaries (hermaphroditic)
Eggs develop inside the body and are shed in
capsules called cocoons
› Weeks later the eggs hatch and grow into adults.

Can also reproduce by regeneration (fragmentation)
› If it is cut into two halves, both halves may become two new
Planaria.
What type of
symmetry?
 How do they
feed?
 What type of
sensory organs?
 How do they
reproduce?

› Class Monogenea
› Class Trematoda
› Class Cestoidea




External parasites
(ectoparasitic)
Commonly found on
the gills, skin or fins of
fishes and lower
aquatic invertebrates
Have a series of hooks
that enable them to
attach while feeding
Have one generation
in their life cycle from
egg to adult




8,000 parasitic
species (aka – flukes)
Internal parasites
(endoparasitic)
Complex life cycles
specialized in
parasitism in animal
or human tissues
One or more suckers
around anterior end
(oral sucker)
Life cycle involves two - four hosts and a
number of developmental stages,
including two types of free-living larvae
(most complex in the animal kingdom)
 The definitive (final) host of adult is
always a vertebrate
 Snails are common intermediate hosts

1.
2.
3.
4.
5.
6.
7.
8.
Eggs reach freshwater
and Miracidium
(ciliated larva) swims
out
Finds host (snail)
Penetrates snail, loses
cilia, develops into a
sporocyst (contain
embryonic cells)
Develops into
daughter sporocysts
(hundreds can form
from one miracidium)
Hundreds of next
larval stage are
produced (cercariae)
Cercariae leave the
snail and find 2nd or
final host
Penetrates host and
becomes a
Metacercaria
When the definitive
host eats the 2nd
intermediate host, it
becomes an adult
Dicrocoelium dendriticum and Opisthorchis sp. liver flukes of mammals
blood flukes, Schistosoma spp. are among most widespread and
serious parasites of humans
aka – tapeworms
 Most highly specialized class
of flatworms
 Intestinal parasites
 No digestive tract ∴ reside in digestive
system of vertebrates
 Absorb nutrients across body wall
 Adults range from 1mm to 25 m in length


The body consists of an anterior scolex solely for
attachment to the host's gut and a string of
proglottids, each of which possesses both male and
female organs
aka – ribbon worms
or proboscis worms
 ~ 900 species
 More closely related
to other
acoelomates than to
any flatworm
 Longest of any
invertebrate (30 m but 2x

as long when fully extended)
Unsegmented
 Proboscis tipped
with barbs to capture prey (see it in action!)
 Complete digestive tract with an anus
 Closed circulatory system (w/o a heart)

~ 500 species
 Bottom dwellers in both marine &
freshwater
 Range from 0.1 mm to 4 mm
 Dorsal contains scales, bristles,
or spines
 Have a forked tail
 Development is direct
(there is no larval stage)
