Bilateral Symmetry

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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
Bilateral Symmetry

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Slide 1

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

Slide 2

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

Slide 3

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.

Slide 4


  • 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

Slide 5



Slide 6


  • bilaterally symmetrical

  • triploblastic; 3 distinct tissue or germ layers

    • Ectoderm

    • Mesoderm

    • Endoderm

  • Dorsoventrally flattened

  • lack an anus; incomplete digestive tract

Slide 7


  • 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

Slide 8


  • 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

Slide 9


Slide 11


  • 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

Slide 12


  • 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

Slide 13


  • Important Structures

  • Eyespots- for light reception; phototaxis

  • Mouth with pharynx

  • Incomplete digestive tract

    • No anus

Slide 14


  • Below epidermis are a series of muscles

    • Dorso-ventral or oblique muscles (=parenchymal)

    • circular muscles

    • longitudinal muscles

      • Movement is combination of these muscles contracting

Slide 15


  • 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

Slide 16


  • done by specialized cell called flame cells or protonephridia

    • first group with specialized excretory structures

Slide 17

Nervous System and Sensory Structures

  • Ladder-like arrangement

  • show a beginnings of a well developed central nervous system- cepahlization

  • a variety of sensory cells and glands; most are chemo- or tactile receptors

    • 2 eye spots or ocelli, which can discriminate varying light intensities

Slide 18


  • 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

Slide 19

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

Slide 20

Asexual Reproduction

  • fragmentation and regeneration

  • many studies have been done on regeneration in Planaria

Slide 21

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

Slide 22

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

Slide 23

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

Slide 24

Characteristics of Flukes

  • 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

Slide 25

Reproduction of Flukes

  • Sexual in definitive host

    • is generally through copulation with cross fertilization; sometimes self fertilization occurs

  • Asexual in intermediate hosts

Slide 26

Typical Life Cycle

Two intermediate hosts

  • Egg

  • Miracidium larva

  • Sporocysts

  • Redia

  • Cercaria

  • Metacercaria

  • Adult

  • Usually 2 but as

  • many as 4 hosts

    • Often intermediate host is a snail

Slide 27

Another Example: this one with one intermediate host

Slide 28

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

Slide 29

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

Slide 30

Typical Life Cycle

  • Egg

  • Onchoshere larva

  • Cysticercus larva in cysts

  • Adult

  • Usually only 2 hosts

  • Intermediate host usually warm blooded

Slide 31

Hydatid cysts can form when larva is ingested by wrong host; example here is in liver of human

Slide 32

Example here is dog tapeworm hydatid cysts in brain of human,

which are often inoperable and fatal

Slide 33



Slide 35


  • Almost all are marine, one freshwater genus and one terrestrial genus

  • most are free-living, bottom dwellers

Slide 36


  • closely related to flatworms but differ in a number of ways

  • have a circulatory system

  • have tubular complete (**first phylum with complete gut) gut- mouth and anus

  • have an eversible proboscis

Slide 37


  • 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

Slide 38


Pilidium larva

  • 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

Slide 39


  • Excretion- Flame cells

  • Respiration- osmosis

  • Locomotion- gliding

  • Feeding- proboscis, ingestion of prey

  • Regeneration

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