Aschelminthes
(Rotifera, Acanthocephala,
Nemertea)
The Aschelminth body plan
• Aschelminths have a pseudocoel which is
never lined with mesodermal epithelia
• Growth is through eutely (increased cell size
rather than cell number)
• They cannot regenerate lost body parts
Phylum Rotifera
• “wheel bearers”
• Mostly found in freshwater
environments, even in
interstitial spaces
• Mostly carnivorous, some are
parasitic
Body Plan (tube-within-a-tube)
&trophi
Phylum Rotifera
• Rotifers have syncitial epidermis, and never
molt
• Cilia beat metachronally
• These are important food sources for
commercially important fish and crustaceans.
• Utilized as models for research in senescence.
• They can be good pollution indicators.
• Rotifers exhibit parthenogenesis, not by
fission or fragmentation. (Why?)
Phylogenetic relationships
Phylum Rotifera
• Class Seisonidea
• Ectoparasites in marine crustaceans
• All gonochoristic
• Class Bdelloidea
• Free-living and mobile
• Class Monogononta
• Amictic females reproduce by parthenogenesis
• Mictic females can produce haploid males that must do
their jobs quickly; the eggs produced can withstand
unfavorable conditions (cryptobiosis) and become
amictic females
Phylum Acanthocephala
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Greek: Thorny-heads
Parasitic worms
Have bilateral symmetry
1150 species described
Have proboscis used to pierce and hold onto
host’s gut
• Hosts are invertebrates, fishes, amphibians,
birds, and mammals
• Could be highly modified rotifers
Phylum Acanthocephala
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Lack digestive tracts (parasitic existence )
Lack a circulatory system
Have a nervous system
Sexually dioecious
Have complex life cycles involving many hosts
Polymorphus spp. is capable of “brain-jacking”
crustaceans to gain entry to its definitive host,
a duck
Acanthocephalan anatomy
Leptorhynchoides thecatus from the GI tract
of a largemouth bass.
Phylum Acanthocephala
• Acanthocephalans probably descended from
free living marine mud-dwellers who turned
parasitic because of intense predation from
arthropods
Phylum Nemertea
• “Proboscis worms” or “ribbon worms”- w/
muscular, eversible proboscis inside the
• Rhynchocoel, a fluid filled schizocoelous cavity
• Mostly marine and benthic, with some
freshwater species
• Most are carnivorous and
predatory
• Possess a cerebral organ
Phylum Nemertea
Lineus longissimus from
a tidepool
Phylum Nemertea
• Most species have pigmented photoreceptors
and a few have balance organs (statocysts).
• Defend themselves through burrowing,
chemical defenses
• Most are gonochronistic, or protandric
hemaphrodites
Nemertean Anatomy
Phylum Nemertea
• Class Anopla
– Lack stylets, proboscis unarmed, mouth posterior
to brain
• Clas Enopla
– Proboscis has stylets, mouth anterior to brain