Flatworms
Phylum: PLATYHELMINTHES
VS.
Class Turbellaria
includes: Planarians
Class Cestoda
Includes: Tapeworms
Class Trematoda
Includes: Flukes
II. Body Plan/Structure
• Flatworms demonstrate a bilaterally
symmetrical body plan
• They are dorsoventrally flattened and lack
a skeleton
II. Body Plan/Structure con’t
• They have a highly branched gastrovascular
cavity .
• There is only one opening which serves the
function of both the mouth and anus
• They have three germ layers:
– Ectoderm: Outside
– Endoderm: Inside
– Mesoderm: Middle layer of tissue between the
ectoderm and the endoderm
• Though Flatworms have three germ layers
they are acoelomates and have no body cavity
• Flatworms do have true organs and primitive
organ systems that are used for digestion and
excretion
• Flatworms are also the most primitive
organisms to show cephalization.
• The head region of Flatworms contains a
concentration of nerve tissue called
ganglia (singular=ganglion) that resembles
a primitive brain.
Cephalization
• They have two nerve cords that run from the
ganglia in the head region along the ventral
side of the worm to the tale region
• In the head region of Flatworms are two eyespots
• They also have lobes on the side of their head
called auricles
Auricles
Feeding
• Flatworms have a mouth/anus which is
connected to the gastrovascular system
through a long muscular pharynx.
• Small invertebrates or the remains of dead
animals are taken into the mouth/anus by the
muscular pharynx
• The food is then digested in the highly
branched gastrovascular cavity
• The nutrients moves from the gut into the
body cells by diffusion
Respiration
• Respiration occurs by diffusion
Internal Transport
• Nutrients and oxygen in the gut are simply
absorbed into the body cells by the process of
diffusion
Excretion
• Most undigested food is released directly out
of the anus/mouth: Pharynx
Nerve Response
• Flatworms are able to sense and respond to at least three
forms of stimuli:
– Sense and respond to light: The eye-spots can detect light
and allow the Flatworms to respond to it
– Sense and respond to chemicals: Pits on the side of their
head regions can sense chemicals in the water and allow
the Flatworms to respond (like “smelling”)
– Sense and respond to touch: The auricles on either side of
the head region can sense touch and allow the Flatworms
to respond
• The ganglia in the head region relay messages
from the sensory organs down the nerve
cords to the rest of the body. The nerve cords
can control muscles in the body which allow
the Flatworms to move or eat.
Auricles
Movement
• The flatworms move across a surface using
cilia on their ventral surface
• They can also move by contracting circular
and longitudinal muscles that lay just below
the ectoderm. These muscles are controlled
by the nerve cords.
Reproduction
• Asexual Reproduction:
– Flatworms can asexually reproduce through a
process called fission. The anterior and posterior
ends hold a surface and the midsection constricts.
This results in two new flatworms, one from the
anterior end of the original flatworm and the
other from the posterior end of the original
flatworm.
Reproduction
• Flatworms can also regenerate parts
they have lost.
• Sexual Reproduction:
– Flatworms are hermaphroditic
– After two flatworms have copulated they release
sacs of fertilized eggs and attach them to a
surface
Parasitic Flatworms: Tapeworms
• The tapeworm has an anterior end called a
scolex with complicated hooks for attaching to
the intestines of its host.
• The tapeworm does not have a mouth or
digestive system. Instead they bath in the predigested fluids of their host and absorb
nutrients directly into their body cells
Hooks
Scolex
Suckers
Proglottids
Lifecycle of a Tapeworm