Transcript Chapter 11

Chapter 11
The Psuedocoelomate Body Plan:
Aschelminthes (Lophotrochozoan
and Ecdysozoan Phyla)
Evolutionary Perspective
• 7 Phyla linked as the aschelminths for
convenience
• Fossil record is meager
• 2 Ideas of possible relatedness
1st Hypothesis
• The phyla are related based on the
presence of the following structures: a
pseudocoelom, a cuticle, a muscular
pharynx and adhesive glands.
2nd Hypothesis
• Contends that the various aschelminth
phyla are not related to each other; thus,
are probably polyphyletic.
Conclusions?
• The absence of any single unique feature
found in all groups strongly suggests
independent evolution of each phylum.
• The similarities found among these phyla
may simply be the result of convergent
evolution as these animals adapted to
similar environments
General Characteristics
• 1st animals to possess a distinct body
cavity
• Lack peritoneal linings found in more
advanced animals
• Organs lie free in cavity
• Cavity is called a pseudocoelom or
pseudocoel
• Thus AKA Psuedocoelomates
General Characteristics (cont.)
• Pseudocoel often fluid-filled or may
contain a gelatinous substance with
mesenchyme cells
– Helps in circulation
– Aids in digestion
– Acts as an internal hydrostatic skeleton that
functions in locomotion
General Digestion
• Most aschelminths have a complete
tubular digestive tract that extends from an
anterior mouth to posterior anus
– Exceptions:
• Acanthocephalans
• Nematomorphs
Eutely
• A condition in which the number of cells is
constant both for the entire animal and for
each given organ in all animals of that
species
– Ex. Caenorhabditis elegans
• Somatic cells = 959
• Cells in pharynx = 80
Shapes
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Most are microscopic
Some can be over a meter in length
Bilaterally symmetrical
Unsegmented
Triploblastic
Cylindrical in cross section
Other Stuff
• Protonephridia present
• Mostly diecious
• Cuticle may bear spines, scales or other
forms of ornamentation that protect the
animal are useful to taxonomists
• Some aschelminths shed their cuticle in a
process called molting or ecdysis in order
to grow
Location, Location, Location
• Most aschelminths are freshwater animals;
very few marine species
• The nematomorphs & acanthocephalans
and many nematodes are parasitic
• The rest of the aschelminthes are mostly
free-living
• Some rotifers are colonial
Aschelminthes That Do Not Molt
(Lophotrochozoan Phyla)
• Phylum Rotifera
• Phylum Acanthocephala
• Some scientists combine them into a
single Phylum Syndermata
Phylum Rotifera
• Pseudocoelomates with jaws, crowns of cilia and
complete digestive tracts
• Possess a pair of corona, ciliated organs that aid
in locomotion and food gathering
• Cilia of the corona do not beat in synchrony;
instead, each cilium is at a slightly earlier stage
in the beat cycle that the cilium next to them
– Gives the appearance of spinning wheels
– Once called “Wheel animalicules.”
Phylum Rotifera (cont.)
• 0.1 to 3 mm in length
• Mostly freshwater
– Less than 10% marine
• 2000 species divided into 3 classes
• Body has approx. 1000 cells
– Organs eutelic
• Usually solitary, free-swimming animals
– Some colonial forms known
Characteristics of the Phylum
Rotifera
• Triploblastic, bilateral, unsegmented,
pseudocoelomate
• Complete digestive system, regionally
specialized
• Anterior end often has a ciliated organ called a
corona
• Posterior end with toes and adhesive glands
• Well-developed cuticle
• Protonephridia with flame cells
• Males generally reduced in number or absent;
parthenogenesis common
Feeding and Digestion
• The coronal cilia create a current that
brings food particles to the mouth
• The pharynx contains a unique structure
called a mastax (jaws)
• The mastax is a muscular organ that
grinds food
Class Seisonidea
• A single genus (Seison) of marine rotifers
that are commensals of crustaceans; large
and elongate body with reduced corona.
Only 2 Species.
Class Bdelloidea
• Anterior end retractile and bearing 2
trochal disks
• Mastax adapted for grinding
• Paired ovaries; cylindrical body
• Males absent
• About 590 species
• Adineta, Philodina, Rotaria
Class Monogononta
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Rotifers with 1 ovary
Mastax not designed for grinding
Produce mictic and amictic eggs
Males appear only sporadically
1400 species
Conochilus, Collotheca, Notommata
Phylum Acanthocephala
• Akantha = spine or thorn
• Kephale = head
• “Spiny headed worms”
Phylum Acanthocephala (cont.)
• Endoparasites in the intestinal tract of
vertebrates (especially fishes)
• 2 hosts per life cycle
• Juveniles found in crustaceans and
insects
• About 1000 species
Size
• Usually less than 40 mm
– Exception
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Macracanthoryncus hirudinaceus
Found in pigs
Can be up to 80 cm long
Can hold up to 10 million eggs
Looks
• Body of an adult is elongate and
composed of a short anterior proboscis, a
neck region, and a trunk.
– Proboscis covered with re-curved spines
– Proboscis provides a means of attachment in
the host’s intestine
• Females larger than males
Aschelminths that Molt
(Ecdysozoan Phyla)
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Phylum Nematoda
Phylum Nematomorpha
Phylum Kinoryncha
Phylum Loricera
Phylum Priapulida
Characteristics of Phylum
Nematoda
• Triploblastic, bilateral, vermiform (resembling a
worm in shape; long and slender),
unsegmented, pseudocoelomate
• Body round in cross section and covered by a
layered elastic cuticle; molting usuallu
accompanies growth in juveniles
• Complete digestive tract, mouth usually
surrounded by lips bearing sense organs
• Most with unique excretory system comprised of
1 or 2 renette cells or a set of collecting tubules
• Body wall has only longitudinal muscles
Phylum Nematoda
• Nematodes or Roundworms
• 16,000 species
• 2 Classes
– Secernentea
– Adenophorea
Class Secernentea
• Formerly known as Phasmidea
• Possess paired glandular or sensory structures
called phasmids in the tail region
• Poorly developed amphids on anterior
• Free living and parasitic
• 5000 species
• Ascaris, Enterobius, Rhabditis, Tubatrix,
Necator, Wuchereria.
Class Adenophorea
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Formerly known as Aphasmidia
Phasmids absent
Most free-living, some parasitic
3000 species
Dioctophyme, Trichinella, Trichuris.
External Features
• Body is typically slender, elongate,
cylindrical and tapered at both ends.
• Amphids are anterior depressions in the
cuticle that contain modified cilia and
function in chemoreception.
• Phasmids are near the anus and also
function in chemoreception
• Paired ocelli (eyes) are present in aquatic
nematodes
Important Nematode Parasites of
Humans
• High reproductive potential
• Life cycles that increase the likelihood of
transmission from one host to another
• Enzyme resistant cuticle
• Resistant eggs
• Encysted larvae
Ascaris lumbricoides
• The Giant Intestinal Roundworms of
Humans
• 800 million people may be infected
worldwide
• Live in small intestine of humans
Ascaris lumbricoides life cycle
• Human ingests embryonated eggs
• Eggs hatch in intestine
• Larvae penetrate intestinal wall and
carried via circulation to the lungs
• Larvae molt twice in lungs
• Migrate up the trachea and are swallowed
• Attain sexual maturity in the intestine,
mate, and begin egg production
Enterobius vermicularis
• The Human Pinworm
• Most common roundworm parasites in the
US
• Lives in lower region of large intestine
E. vermicularis life cycle
• At night, gravid females migrate out of the
rectum to the perianal area, where they
deposit eggs containing the 1st larval stage
and then die
• The females and eggs produce an itching
sensation
• When a person scratches the itch, the
hands and the bedclothes become
contaminated
E. vermicularis life cycle (cont.)
• If the hands touch the mouth and the eggs
are swallowed, the eggs hatch
• The larvae molt 4 times in the small
intestine
• Larvae leave the small intestine for the
large intestine
• Adults mate and females begin egg
production
Necator Americanus
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The New World Hookworm
Found in SE US
Adults live in small intestine
Feed on blood and tissue fluids
Females can produce up 10,000 eggs
daily
– Passed through feces
Necator Americanus life cycle
• Humans become infected when the filari-form
larva penetrates the skin, usually between the
toes
• Outside defecation and subsequent walking
barefoot through the immediate area maintains
the life cycle in humans
• The larvae burrows deep into the skin to reach
the circulatory system
• The rest of the life cycle is similar to that of
Ascaris
Trichinella spiralis
• The Porkworm
• Live in the mucosa of the small intestine of
humans and other carnivores and
omnivores
• In the intestine, the adult females give
birth to young larvae that then enter into
the circulatory system and are carried to
striated skeletal muscle
Trichinella spiralis (cont.)
• The young larvae encyst in the skeletal muscle
and remain ineffective for many years
• The disease this nematode causes is called
trichinosis
• Another host must ingest infective meat (muscle)
to continue the life cycle
• Humans often become infected by eating
improperly cooked pork products
• Larvae encyst in stomach; move to small
intestine; molt 4 times and develop into adults
Wuchereria spp.
• The Filarial Worms
• 250 million human infected worldwide
• 2 Examples
– Wuchereria bancrofti
– Wuchereria malayi
Wuchereria spp. (cont.)
• Elongate, threadlike nematodes live in the
lymphatic system, where they block the
vessels
• Because lymphatic vessels return tissue
fluids to circulatory system, when the
filarial nematodes block these vessels,
fluids and connective tissue tend to
accumulate in peripheral tissues
– Elephantiasis
Elephantiasis
• In the lymphatic vessels, filarial nematode adults
copulate and produce larvae called microfilariae
• The microfilariae are released into the
bloodstream of the human host and migrate to
the peripheral circulation at night
• Mosquito ingests microfilariae
• Molt 2 times in mosquito stomach
• Injected back into humans
Heartworm Disease
• Dirofilaria immitis
• A parasite of dogs
• Live in the heart and large arteries of the
lungs
• Prevention with medicine
Phylum Nematomorpha
• 250 species
• Horsehair worms or Gordian worms
– The hairlike nature of these worms is so striking that
they were formerly thought to arise spontaneously
from the hairs of a horses tail in drinking troughs or
other stock-watering places
• No distinct head
• Body wall has a thick cuticle, a cellular
epidermis, longitudinal cords, and a muscle
layer of longitudinal fibers
• Two separate sexes
Phylum Kinorhynca
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1 mm in length
Bilaterally symmetrical
Exclusively marine, in mud and sand
No arms or cilia
– Burrow through the sand with their snouts
• 150 species
Phylum Kinorhynca
• 13 or 14 definitive units called zonites
• Zonite 1 bears the mouth, and oral cone, and
spines
• Zonite 2, represented by the neck, contains
spines called scalids and plates called placids
• The trunk consists of the remaining 11 or 12
zonites and terminates wi the anus
• Zonite 11 contains protonephridia
• Diecious
Phylum Loricifera
• Recently described (1983)
• Example Species
– Nanaloricus mysticus
• 14 species
Phylum Priapulida
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16 species
2mm to 8cm
Marine worms found in cold water
Priapos = Phallus
– Priapos the Greek god of reproduction
• Separate sexes but not superficially
distinguishable