Pseudocoelomate Body Plan - Aschelminths
Chapter 11
Aschelminths GENERAL CHARACTERISTICS
The aschelminths are the first assemblage of animals to possess a distinct body cavity, but they lack the peritoneal linings and membranes, called mesenteries, found in more advanced animals. As a result, the various internal (visceral) organs lie free in the cavity. Such a cavity is called a pseudocoelom or pseudocoel and the animals are called pseudocoelomates
Aschelminths 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
Most phylum's contain a complete tubular digestive tract
Aschelminths Eutely – Same number
of cells for each animal and for each given organ
Ex. Caenorhabditis elegans (a type of nematode) has 959 cells
Every worm has 80 cells in their pharnyx
Aschelminths Microscopic to meters in length Bilaterally symmetrical Unsegmented and cylindrical in cross
section Most are freshwater Osmoregulation through protonephridia
(mainly in freshwater) 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
Phylum Rotifera Phylum Rotifera
Characteristics Corona – ciliated organ
around the head used for locomotion and food gathering
0.1 to 3 mm in length Most are freshwater (less
than 10% marine) 2,000 species Usually solitary, free
swimming animals although there are a few colonical members
Phylum Rotifera Characteristics (cont.)
Triploblastic, bilateral, unsegemented, pseudocoelomate
Complete digestive sys. Post. End with toes and
adhesive glands Developed cuticle Protonephridia Pathogenesis common,
males reduced in this phylum
Phylum Rotifera External features
Epidermally secreted cuticle used for protection
Lorica – thickened cuticle that makes an encasement used for protection and support
Epidermis is synctial Head has a mouth,
brain, sensory organs Foot has 1-2 toes
Foot has pedal glands
Phylum Rotifera Phylum Rotifera (rotifer-
ah) A cilated corona surrounding a mouth; muscular pharynx (mastax) present with jawlike features; nonchitinous cuticle; parthenogenesis
Class Monogononta (mono-go-nontah) Rotifers with one ovary; mastax not designed for grinding; produce
Class Seisonidea (syson-ide-ah) A single genus of marine rotifers that are commensals of crustaceans; large and elongate body with reduced corona. Seison.
Class Bdelloidea (del-oide-ah)
Anterior end retractile and bearing two trochal disks; mastax
Phylum RotiferaBody parts
Head - Anterior Corona Mouth Buccal field
Trunk Middle
Foot Toes Adhesive glands
Phylum RotiferaDigestion
Most are free-living Small microorganisms Organic material
Mastax jaw that grinds food
Phylum Rotifera
Other organs Protonephridia with flame
cells Functions for
osmoregulation
Phylum Rotifera Reproduction
Some perform sexual reproduction (several use pathogenesis)
Class Seisonida 2 Species Marine Haploid eggs that must be fertilized Males have no mouth, digestive
tract, or bladder Class Bdelloidea
All females are parthenogenic No males present
Class Monogononta Amitic eggs – diploid eggs Mictic eggs – haploid eggs, can
become amitic Small sized males
Phylum KinorhynchaPhylum Kinorhyncha
Characteristics < 1mm in length Marine Bilateral symmetry Burrowers – mud and
sand 150 species Dioecious
Phylum Kinorhyncha Composed of 13 or 14 zonites
Definite units called zonites Zonite 1 can retract into zonite 2 Spines line most zonites
Pair of lateral spines and one dorsal spine
Protonephridia in Zonite 11 Brain and ventral nerve cord with
a ganglion in each zonite The body wall consists of a cuticle, epidermis,
and two pairs of muscles: dorsolateral and ventrolateral.
The pseudocoelom is large and contains amoeboid cells.
Phylum Nematoda Phylum Nematoda
Characteristics These are the
roundworms Up to 5 billion per
acre of land 16,000 to 500,000
species exist Microscopic to meters
long
Phylum NematodaParasites and free-
livingLack cilia except in
their sensory structures
Marine, freshwater, and soil habitats
Triploblastic, bilaterally symmetrical
Unsegmented vermiform (wormlike organism)
Phylum NematodaExternal features
Noncellular, collagenous cuticle
May have spines, bristles, papillae, or warts
Maintains internal hydrostatic pressure
Mechanical protection Resists digestion by the
host Longitudinal muscles
are used for locomotion
Phylum NematodaSome nematodes have
lips, some have spines or teeth on those lips
Sensory organs Amphids – chemoreceptors
along the cuticle Phasmids - chemoreceptors
near the anus Ocelli – eyespots found in
aquatic nematodes
Phylum NematodaDigestion
All have a complete digestive tract
Carnivores Herbivores Omnivores Saprobes -
decomposers Parasites
Phylum Nematoda Other organs
Glandular system Aquatic nematodes Renettes – absorb
nitrogenous waste Tubular system
Parasitic nematodes Renettes form a canal
Nervous system Nerve ring from anterior to
posterior Also have neuroendocrine
secretions involved in growth, molting, cuticle formation, and metamorphisis
Phylum Nematoda 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
ReproductionDioeciousMales are
smaller than females
MalesOne
testisBursa
Phylum NematodaTransmitted through
feces contactOnce the eggs are
ingested, they hatch and travel to the lungs
After two moltings, they travel to the trachea where they are swallowed
Important ParasitesAscaris lumbricoides
“The Giant Intestinal” roundworm
800 million infectedLive in small
intestines of humans
Phylum Nematoda
Phylum NematodaEnterobius
vermicularis Pinworm Staple size (1-2mm) Live in rectum At night females migrate
out of the anus and lay eggs on skin
Phylum Nematoda Necator americanus
Hookworm Penetrates skin, usually feet Females may lay 10,000 eggs/day
Phylum Nematoda
Phylum Nematoda Trichinella spiralis
Porkworm Transmission through
eating uncooked pork Can cause loss of
muscle control or death
Now, less than 12 cases per year
skeletal muscle
Phylum NematodaWuchereria
Filarial worms Infect the lymph
vessels which are responsible for returning fluid to the circulatory system
Cause trichinosis or elephantiasis
Heartworms in dogs
Phylum NematomorphaPhylum Nematomorpha
Characteristics 250 species Horsehair worms Adults are free-living Juveniles are parasitic No distinct head Dioecious
Phylum Acanthocephala Phylum Acanthocephala
Characteristics Spiny-headed worm Proboscis – anterior region covered
with spines This structure is retractable and is
responsible for attachment 1,000 species Most are < 40 mm long Parasites Dioecious Macracanthorhynchus hirudinaceus
80 cm long As many as 10 million eggs
Phylum LoriciferaPhylum Loricifera
Characteristics Discovered in 1983 Live in spaces between marine
gravel 14 species Spiny-head called an introvert
which is retractable
Phylum PriapulidaPhylum Priapulida
Characteristics 16 species Live buried in mud in the
seafloor 2 mm to 8 cm Cold marine climates
VINIA M. BADATO
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