Download Bilateral Symmetry

Bilateral Symmetry
• The remaining phyla are all bilaterally
symmetrical or at least have primary bilateral
symmetry
• Also called Bilateria
• the development of bilateral symmetry is one
of the most important traits in higher animals
• it implies that a single line or plane will divide
the body into equal halves or mirror images
Bilateral Symmetry
• bilateral symmetry has allowed several other
important changes in body structure
• Cephalization- concentration of the nerve
tissue into a head; which allow for a single
organ to direct the functions of the animal
• allow for greater organ development
• allows for greater differentiation of structure;
such as appendages
Bilateral Symmetry
• Advantages of bilateral symmetry:
• better coordinated movement
• much quicker and more precise
response to stimulation; since it is
directed by a central nerve center.
Acoelomates
• Includes phyla Platyhelminthes and
Nemertea
• those organisms that do not have a
true coelom or body cavity
• are entirely solid except for the
gastric cavity or coelenteron
PHYLUM
PLATYHELMINTHES
THE FLATWORMS
CHARACTERISTICS OF PHYLUM
• bilaterally symmetrical
• triploblastic; 3 distinct tissue or germ
layers
– Ectoderm
– Mesoderm
– Endoderm
• Dorsoventrally flattened
• lack an anus; incomplete digestive tract
CHARACTERISTICS OF PHYLUM
cont.
• coelom- solid mesenchyme
(mesoderm); first phylum to show a
definite cellular mesoderm
• have no true respiratory system or
circulatory system
– Have cutaneous respiration
• first phylum to show distinct excretory
system; get rid of nitrogenous waste
• are usually hermaphroditic
PLATYHELMINTHES
• They consist of 4 classes of
flatworms; 3 of which are entirely
parasitic, the other free-living
– Turbellaria- free-living
– Monogenea- parasitic, flukes
– Trematoda- parasitic, flukes
– Cestoda- parasitic, tape worms
CLASS TURBELLARIA
CLASS TURBELLARIA
• consists of all of the free-living
flatworms, but some are found on
aquatic hosts as ectoparasites or
commensals
• few live in freshwater (ie. Planaria),
marine as well as moist terrestrial
habitats
Morphology
• the outer surface of
turbellarians consists of
ciliated epidermis
• usually best developed on
the ventral surface and
function in locomotion
• epidermis also has a large
number of mucous glands
that secrete mucous that is
used by the cilia in
locomotion - Rhabdites
Morphology
• Important
Structures
• Eyespots- for
light reception;
phototaxis
• Mouth with
pharynx
• Incomplete
digestive tract
• No anus
Locomotion/Movement
• Below epidermis are a
series of muscles
– Dorso-ventral or oblique
muscles (=parenchymal)
– circular muscles
– longitudinal muscles
• Movement is combination of
these muscles contracting
Feeding
• Turbellarians primarily carnivorous
• capture of prey is done by wrapping
themselves around it and entangling
it with mucous
• they ingest the whole prey or
• suck its juices through a hardened
stylet (modified pharynx)
• in many species the pharynx is
completely eversible and can
envelope the entire prey
• there is no anus so ingestion and
egestion are through the mouth
Excretion/Osmoregulation
• done by
specialized cell
called flame cells
or protonephridia
– first group with
specialized
excretory
structures
Nervous System and
Sensory Structures
• Ladder-like arrangement
• show a beginnings of a
well developed central
nervous systemcepahlization
• a variety of sensory cells
and glands; most are
chemo- or tactile
receptors
– 2 eye spots or ocelli,
which can discriminate
varying light intensities
Reproduction
• the most complex organ system
• most are hermaphroditic
• cross fertilization is the most common
mode of reproduction
• some self fertilization can and does
occur
• usually the eggs and sperm are produced at
different times in the individual
Reproduction cont.
• fertilization is internal
• fertilized eggs are usually deposited in
clusters
• winter eggs have a hard outer covering that
can survive desiccation and freezing
• in freshwater forms eggs hatch into miniature
adult forms; direct development
• in marine forms (not all) a larva is produced
called a Müller's larva which is free swimming
Asexual Reproduction
• fragmentation and regeneration
• many studies have been done on
regeneration in Planaria
Life as a Parasite
• Loss of sensory structures
– Eye spots; tactile sensors
• Loss of some organ systems
– Digestive system
• Increased reproductive potential
– Insures survival of species
Parasites and Hosts
• Host- organism parasite lives on or in
– Definitive or Primary host- where parasite has
sexual reproduction
– Secondary or Intermediate hosts- parasite has
asexual reproduction
• Endoparasite/Ectoparasite- in or on host
• Facultative/Obligatory parasites
– Facultative- on host only part time; e.g., leech
– Obligatory- on or in host entire life; e.g., fluke
Classes Trematoda and Monogenea
(Flukes)
• all are parasitic (endoparasitic and
ectoparasitic)
– most parasitic to vertebrates; especially
fish
– most have intermediate hosts as
invertebrates
– many species are economically and
medically important
Characteristics
• Have many of the same
characters as turbellarians
• usually have an anterior
sucker around mouth and
a posterior sucker- used to
attach to host tissue
– suckers best developed
in Monogenea where
they are called
Opisthaptors
• body does not have
ciliated epidermis as
turbellarians
of Flukes
Reproduction of Flukes
• Sexual in definitive host
– is generally through copulation with cross
fertilization; sometimes self fertilization
occurs
• Asexual in intermediate hosts
Typical Life Cycle
•
•
•
•
•
•
•
Egg
Miracidium larva
Sporocysts
Redia
Cercaria
Metacercaria
Adult
• Usually 2 but as
• many as 4 hosts
– Often intermediate
host is a snail
Two intermediate hosts
Another Example: this one with one intermediate host
CLASS CESTODA
(tapeworms)
• All are endoparasites
• the body is covered by a cuticle like the
trematodes
• they differ from all of the other flatworms
in that they do not have a digestive tract
Characteristics of Tapeworms
• Features similar to other
flatworms
– E.g. respiration, and
excretion
• Morphological features
include:
– Scolex
• Rostellum
• Suckers and hooks
– Proglottids (repeated
segments)
• Immature; sexually immature
• Mature; sexual structures
present
• Gravid; filled with fertilized
eggs
Typical Life Cycle
• Egg
• Onchoshere
larva
• Cysticercus
larva in cysts
• Adult
• Usually only 2
hosts
• Intermediate
host usually
warm blooded
Hydatid cysts can form when larva is ingested by wrong host;
example here is in liver of human
Example here is dog tapeworm hydatid cysts in brain of human,
which are often inoperable and fatal
PHYLUM NEMERTEA
THE PROBOSCIS OR RIBBON WORMS
NEMERTEA
• Almost all are marine, one freshwater
genus and one terrestrial genus
• most are free-living, bottom dwellers
PHYLUM NEMERTEA
• closely related to
flatworms but differ in a
number of ways
• have a circulatory
system
• have tubular
complete (**first
phylum with
complete gut) gutmouth and anus
• have an eversible
proboscis
PHYLUM NEMERTEA
• the most diagnostic feature is the eversible
proboscis
• used to capture small prey or for browsing on
dead and decaying organisms; are
carnivorous
• lies in a fluid filled cavity (rhyncocoel)
• in some species the proboscis is armed with
barbs or spines and may inject a toxin
• once food is captured the food is passed into
the mouth and gut
REPRODUCTION
• are dioecious and fertilization is
external
– reproductive organs are simple
masses of mesenchymal cells
– eggs and sperm are released to
outside through them
– Produce a free living larva- Pilidium
• in some species fragmentation is
common especially when worms
are disturbed
– usually only anterior end can
regenerate to produce a new
posterior end
Pilidium larva
OTHER FEATURES
•
•
•
•
•
Excretion- Flame cells
Respiration- osmosis
Locomotion- gliding
Feeding- proboscis, ingestion of prey
Regeneration