The Pseudocoelomate Body Plan: Aschelminthes

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The Pseudocoelomate Body PlanAschelminthes

• Chapter 11

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Aschelminthes

• Seven different Phyla grouped into either the
  Lophotrochozoan (those that do NOT molt)
• or
• Ecdysozoan (those that DO molt)

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Aschelminthes

• Characteristics
• First animal to posses a body cavity
• Pseudocoelom
• Distinct body cavity with no peritoneal lining
  and mesenteries.
• Visceral (internal) organs lie free in the cavity

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• A pseudocoelom
• Is a body cavity derived from the blastocoel,
  rather than from mesoderm

Pseudocoelomates such as nematodes have a body
cavity only partially lined by tissue derived
from mesoderm.
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Characteristics

• Pseudocoelom acts as an internal hydrostatic
  skeleton
• that functions
• in
• locomotion

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Characteristics

• Most have a complete tubular digestive tract
  (mouth-anus)
• First time mechanical breakdown of food,
  digestion, absorption, and feces formation

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Characteristics

• Show eutely same number of cells for each
  animal and for each given organ in all the
  animals of the species
• Ex. Caenorhabditis elegans (a type of nematode)
  has 959 cells
• Every worm in the species has 80 cells in their
  pharnyx

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• C. elegans
• Nobel Prize (2002)Brenner, Sulston and Horvitz
• Genetic regulation of organ development and
  programmed cell death

http//www.nobel.se/medicine/laureates/2002/press.
html
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Characteristics

• Microscopic to meters in length
• Bilaterally symmetrical
• Unsegmented and cylindrical in cross section
• Most are freshwater
• Osmoregulation through protonephridia (mainly in
  freshwater)

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Characteristics

• Triploblastic
• Most are dioecious
• No separate gas exchanges
• Some cephalization is present
• Cuticle present may bear spines or scales and
  is useful for protection and taxonomic
  identification

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Lophotrochozoan

• Phylum Rotifera
• (rota, wheel fera, to bear)
• Small animals 0.1-3 mm
• Most abundant in freshwater
• About 1,000 cells (3 classes) and organs are
  eutelic

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Phylum Rotifera

• All have a Corona (crown)- ciliated organ for
  locomotion and food gathering

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Phylum Rotifera

• Posterior toes and adhesive glands called
• Pedal gland- secretions aid in temporary
  attachment of foot to a substratum.

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Feeding and Digestion

• Feed on small microorganisms and organic material
• Coronal cilia brings food to mouth
• Food enters the Pharynx that contains a muscular
  organ that grinds food called Mastax

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Feeding and Digestion

• Then food passes through a ciliated Esophagus to
  the ciliated Stomach-complete extracellular
  digestion and absorption of food occurs.

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Feeding and Digestion

• Some species have ciliated Intestine that becomes
• Cloacal bladder- receives water from the
  protonephridia, eggs from ovaries, and digestive
  waste.
• Cloacal bladder opens to the outside via an Anus

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Other Organ Systems

• Protonephridia that empty into the cloacal
  bladder function in osmoregulation
• Nervous System- 2 lateral nerves and bilobed brain

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Reproduction and Development

• Some reproduce sexually
• Many reproduce by parthenogenesis- a type of
  reproduction in which females produce offspring
  from unfertilized eggs

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Reproduction and Development

• Females
• Germovitellarium- ovary and vitellarium fuse
• Males
• Single testis
• Eversible penis-injects sperm into the
  pseudocoelom of female (hypodermic impregnation)

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Reproduction and Development

• Divided into 3 classes

class
order
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Class Seisonidea

• Females produce haploid eggs that must be
  fertilized and develop into either males or
  females.
• ? ? similar in size and form
• Sexually reproduction only

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Class Bdelloidea

• Females are parthenogenetic (asexual)- produce
  diploid eggs that hatch into diploid females.

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Class Bdelloidea

• "The study of rotifers is a study of ladies,
  sometimes beautiful, often capricious, always
  fascinating. (Meglitsch)

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Class Monogononta

• Females produce two kinds of eggs
• Amictic eggs- diploid eggs that have not
  undergone reduction division, cannot be
  fertilized develop only into females.

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Class Monogononta

• Mictic eggs- undergone meiosis and are haploid-
  if
• Unfertilized- develop quickly into males
• Fertilized- they secrete a thick shell and become
  dormant for several months before developing into
  amictic females.

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Phylum Nematoda
Ecdysozoan

• The roundworms

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5 billion per acre
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Phylum Nematoda

• Parasites and free-living
• Lack cilia except in their sensory structures
• Marine, freshwater, and soil habitats
• Triploblastic, bilaterally symmetrical
• Unsegmented vermiform (wormlike organism)

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Phylum Nematoda

• External features
• Noncellular, collagenous cuticle
• Can molt 4 times during maturation
• Maintains internal hydrostatic pressure
• Mechanical protection
• Resists digestion by the host

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Phylum Nematoda

• Longitudinal muscles are used for locomotion
• Thrashing movements (cant crawl like worms)

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Phylum Nematoda

• Sensory organs
• Amphids chemoreceptors along the cuticle
• Phasmids - chemoreceptors near the anus
• Ocelli eyespots found in aquatic nematodes

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Feeding and Digestive System

• Carnivores, herbivores, omnivores,
  saprobes(decomposers), or parasitic
• Complete digestive system
• mouth-gtbuccal cavity-gtpharynx
• -gttubular intestine-gtrectum-gtanus
• Hydrostatic pressure pushes food thru

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Other Organ Systems

• Glandular system
• Aquatic nematodes
• Renettes absorb nitrogenous waste
• Tubular system
• Parasitic nematodes
• Renettes form a canal

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Other Organ Systems

• Nervous system
• Nerve ring from anterior to posterior
• Also have neuroendocrine secretions involved in
  growth, molting, cuticle formation, and
  metamorphisis

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Reproduction

• Sexual
• Dioecious-having separate sexes
• Dimorphic-males are smaller than females
• internal fertilization

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Reproduction

• Males
• One testis
• Bursa- used to insert their sperm into the female
• males have ameboid sperm

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Reproduction

• Females
• Pair of convoluted ovaries
• Oviducts become uterus
• Several hundred to several hundred- thousand eggs
  per day
• Ovovivparity giving birth to larvae that
  hatched from an egg

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Some Important Nematode Parasites of Humans

• Ascaris lumbricoides
• Enterobius vermicularis
• Necator americanus
• Trichinella spiralis
• Wuchereria bancrofti

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Ascaris lumbricoides

• The Giant Intestinal Roundworm- The adult female
  worm can be over 30 cm long and 2-6 mm wide
• 800 million infected- most common parasitic worm
  disease in the world

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Ascaris lumbricoides

• Adults live in small intestines of humans
• Eggs exit with through feces
• Once the eggs are ingested, they hatch in
  intestine and travel to the lungs
• Larvae molt twice, they travel to the trachea
  where they are swallowed

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Ascaris lumbricoides

• Warning those with weak stomachs need not
  continue viewing. Only for the HBO audiences!

When Ascaris becomes a big problem. . .
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Enterobius vermicularis

• Pinworm
• Most common parasite in US
• Adults live in large intestine
• At night females migrate out of the anus and lay
  eggs on skin

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Enterobius vermicularis

• Human ingest eggs, hatch and molt 4 times in
  small intestine migrate to large

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Necator americanus

• Found in Southern US
• Adults live in small intestine with teeth and
  feed on blood tissue fluid
• Females 10,000 eggs daily pass out of body in
  feces

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Necator americanus

• Eggs hatch in warm moist soil and releases a
  small larva, the larva molts and becomes the
  infective filariform larva.

hookworms living in soil
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Necator americanus

• Humans become infected when filariform penetrates
  the skin (usually b/w toes) to reach our
  circulatory system

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Trichinella spiralis

• The Porkworm
• Adults live in mucus of small intestine of humans
  and other mammals

Adult female
Adult male
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Trichinella spiralis

• Females birth young, larvae enter circulatory
  system and are carried to Skeletal (striated)
  Muscles

Larvae entering into Skeletal Muscle cells
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Trichinella spiralis

• Larvae encyst in muscles remains infective for
  many years

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Another host must ingest infective meat to
continue the life cycle
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Trichinella spiralis

• Humans become infected by eating improperly
  cooked pork products
• Larvae encyst in stomach move to small intestine
  molt 4 times, turn into Adults

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Wuchereria bancrofti

• The Filarial worms
• In tropical countries over 250 million human
  infected

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Wuchereria bancrofti

• Thread-like worms that live in the Lymphatic
  System, block the vessels
• This causes enlargement of various appendages
  Elephantiasis

An Adult female Wuchereria bancrofti is about
80-100 mm long and 0.24-0.30 mm in diameter,
whereas a male is about 40 mm long and 0.1 mm in
diameter.
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Elephantiasis

• Warning Pictures not for everyone!

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Wuchereria bancrofti

• Adults copulate produce microfilariae.
• The microfilariae released into the blood stream

A microfilaria is about 240-300 µm (micrometers)
long and 7.5-10 µm thick
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Wuchereria bancrofti

• Mosquito (intermediate host) feeds on human
  (definitive host) ingest microfilariae and larvae
  molts 2 times
• Mosquito bites another human it injects 3rd stage
  larvae into human blood, molts, enters lymphatic
  system

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Other Filarial Worms

• Dirofilaria immitis in US parasite of dogs
• Adult worms live in heart, large arteries, and
  lungs
• Heartworm disease- fatal to dogs, unless give
  preventative medicine

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Phylum Nematomorpha

• Phylum Nematomorpha
• Characteristics
• 250 species
• Horsehair worms
• Adults are free-living
• Juveniles are parasitic in arthropods
• No distinct head
• Dioecious

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