The Pseudocoelomate Body Plan:
Aschelminthes Chapter 11
Aschelminthes• Seven different Phyla grouped into either the Lophotrochozoan (those that do NOT molt)
orEcdysozoan (those that DO molt)
AschelminthesCharacteristics
–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
• A pseudocoelom– Is a body cavity derived from the blastocoel, rather
than from mesoderm
Pseudocoelom
Muscle layer(from mesoderm)
Body covering(from ectoderm)
Digestive tract(from ectoderm)
Pseudocoelomates such as nematodes have a body cavity only partially lined by tissue derived from mesoderm.
Characteristics
Pseudocoelom – acts as an internal hydrostatic skeleton
that functions
in locomotion
Characteristics
• Most have a complete tubular digestive tract (mouth-anus)
First time mechanical breakdown of food, digestion, absorption, and feces formation
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
http://www.nobel.se/medicine/laureates/2002/press.html
Sydney Brenner- C. elegans since 1963
C. elegans Nobel Prize (2002)
Brenner, Sulston and Horvitz– “Genetic regulation of
organ development and programmed cell death”
Characteristics–Microscopic to meters in length–Bilaterally symmetrical–Unsegmented and cylindrical in cross section
–Most are freshwater–Osmoregulation through protonephridia (mainly in freshwater)
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
LophotrochozoanPhylum 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
Phylum Rotifera
• All have a Corona (crown)- ciliated organ for locomotion and food gathering
Phylum Rotifera
Posterior toes and adhesive glands called:Pedal gland- secretions aid in temporary attachment of foot to a substratum.
Feeding and DigestionFeed 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
Feeding and Digestion
Then food passes through a ciliated Esophagus to the ciliated Stomach-complete extracellular digestion and absorption of food occurs.
Feeding and DigestionSome 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
Other Organ Systems
Protonephridia that empty into the cloacal bladder function in osmoregulation
Nervous System- 2 lateral nerves and bilobed brain
Reproduction and Development
Some reproduce sexually
Many reproduce by parthenogenesis- a type of reproduction in which females produce offspring from unfertilized eggs
Reproduction and Development
FemalesGermovitellarium- ovary and vitellarium fuse
MalesSingle testisEversible penis-injects sperm into the pseudocoelom of female (hypodermic impregnation)
Divided into 3 classes:
Reproduction and Development
class
order
Class Seisonidea
Females produce haploid eggs that must be fertilized and develop into either males or females.
♀ & ♂ similar in size and formSexually reproduction only
Class Bdelloidea
Females are parthenogenetic (asexual)- produce diploid eggs that hatch into diploid females.
Class Bdelloidea"The study of rotifers is a study of
ladies, sometimes beautiful, often capricious, always fascinating.“ (Meglitsch)
Class MonogonontaFemales produce two kinds of eggs:–Amictic eggs- diploid eggs that have not undergone reduction division, cannot be fertilized & develop only into females.
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.
Phylum Nematoda
The roundworms
Ecdysozoan
5 billion per acre
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)
Phylum NematodaExternal features
•Noncellular, collagenous cuticle–Can molt 4 times during maturation–Maintains internal hydrostatic pressure
–Mechanical protection–Resists digestion by the host
Phylum Nematoda• Longitudinal muscles are used for
locomotion– Thrashing movements (can’t crawl like
worms)
Phylum Nematoda•Sensory organs
–Amphids – chemoreceptors along the cuticle
–Phasmids - chemoreceptors near the anus
–Ocelli – eyespots found in aquatic nematodes
Feeding and Digestive System
Carnivores, herbivores, omnivores, saprobes(decomposers), or parasitic
Complete digestive systemmouth->buccal cavity->pharynx ->tubular intestine->rectum->anus
Hydrostatic pressure pushes food thru
Other Organ Systems
Glandular system–Aquatic nematodes–Renettes – absorb nitrogenous waste
Tubular system–Parasitic nematodes–Renettes form a canal
Other Organ Systems•Nervous system
–Nerve ring from anterior to posterior
–Also have neuroendocrine secretions involved in growth, molting, cuticle formation, and metamorphisis
ReproductionSexual–Dioecious-having separate sexes–Dimorphic-males are smaller than females
–internal fertilization
Reproduction–Males
»One testis»Bursa- used to insert their sperm into the female
»males have ameboid sperm
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
Some Important Nematode Parasites of
Humans• Ascaris lumbricoides• Enterobius vermicularis• Necator americanus• Trichinella spiralis• Wuchereria bancrofti
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
Ascaris lumbricoidesAdults live in small intestines of
humansEggs 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
Ascaris lumbricoidesWarning: those with weak stomachs need
not continue viewing. Only for the HBO audiences!
When Ascaris becomes a big problem. . .
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
Enterobius vermicularis
Human ingest eggs, hatch and molt 4 times in small intestine & migrate to large
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
Necator americanusEggs hatch in warm moist soil
and releases a small larva, the larva molts and becomes the infective filariform larva.
hookworms living in soil
Necator americanusHumans become infected when
filariform penetrates the skin (usually b/w toes) to reach our circulatory system
Trichinella spiralisThe PorkwormAdults live in mucus of small
intestine of humans and other mammals
Adult female Adult male
Trichinella spiralisFemales birth young, larvae
enter circulatory system and are carried to Skeletal (striated) Muscles
Larvae entering into Skeletal Muscle cells
Trichinella spiralisLarvae encyst in muscles &
remains infective for many years
Another host must ingest infective meat to continue the life cycle
Trichinella spiralisHumans become infected by
eating improperly cooked pork products
Larvae encyst in stomach move to small intestine molt 4 times, turn into Adults
Wuchereria bancroftiThe Filarial wormsIn tropical countries over 250 million human infected
Wuchereria bancroftiThread-like worms that live in the
Lymphatic System, block the vesselsThis 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.
ElephantiasisWarning: Pictures not for
everyone!
Wuchereria bancroftiAdults 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
Wuchereria bancroftiMosquito (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
Other Filarial WormsDirofilaria immitis in US parasite of
dogsAdult worms live in heart, large
arteries, and lungsHeartworm disease- fatal to dogs,
unless give preventative medicine
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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