Acoelomates, p. 1
ACOELOMATES
(chapter 8)
GENERAL CHARACTERISTICS
Note chapter summary on p. 146 (Position in the Animal Kingdom)
1) body symmetry is bilateral
included is a distinct anterior and posterior
some species have a specialized anterior, cephalization
2) three germs layers present = triploblastic:
endoderm, ectoderm and mesoderm
presence of mesoderm allows for more tissue specialization
3) Mesoderm is completely filled with cells
no cavity = acoelomate
tissue present = mesenchyme
much of tissue without specialized cells = parenchyma
4) various tissues organized into organs & organ systems
ACOELOMATE PHYLA
Phylum Platyhelminthes - flatworms
large group of freeliving & parasitic organisms
flatworms, flukes, tapeworms
Phylum Nemertea - ribbon worms
have a long tongue-like proboscis
PHYLUM PLATYHELMINTHES
MORPHOLOGY & ANATOMY
Flatworm due to general flat shape & bilateral symmetry
flattening is dorsoventrally
oral & genital openings define the ventral surface
The digestive system = gastrovascular system (GVC)
oral opening typically not at far anterior end
Acoelomates, p. 2
Nervous system present
anterior region of nerve cluster = ganglion
longitudinal nerves within mesenchyme
Include both freeliving & parasitic species
Most species are monoecious
CLASS TURBELLARIA (p. 153)
Body covered by cilia, aids in movement
mesenchyme contains many muscles (circular, longitudinal, diagonally)
Mostly freeliving individuals
mouth present near middle of ventral surface (fig. 8.7c)
many species are carnivorous, eating small animals
most have a highly branched GVC (3 functions)
Excretion and osmoregulation handled by special organs (fig. 8.7a)
small cavities on body surface pull waste out of body
moved from by with aid of flame cells
system = protonephridia
Nervous system developed best in this group (fig. 8.7b)
many have eyespots which detect the presence of light
CLASS TREMATODA (pp. 153-155)
The parasitic flukes
almost all species have adult forms as parasites in a vertebrate’s bile duct
(associated with liver)
Numerous adaptations to parasitic life style
1) well developed attachment organs, oral & ventral suckers
2) digestive system complexity related to host, obtain predigested food (intestine, blood)
3) reproductive system well developed
Reproduction -- hermaphroditic & complex
Male system of 2 testes, 2 vasa efferentia, 1 vas deferens, and seminal receptacle
Female system of 1 ovary, 1 oviduct, 1 uterus
Acoelomates, p. 3
Self fertilization common/typical
eggs released and flow out of body
Eggs hatch when ingested by snails
develop into several larval forms:
sporocyst (early larval form)
rediae (late larval form)
transforms into cercariae form (little tadpole), swim out of snail
swim to and lodge into muscle under scale of fish
form a cyst and become dormant as metacercaria
when eaten by vertebrate it hatches & gets into blood, then to liver
Schistosomiasis - a human blood fluke (fig. 8.10)
one of major infections of world - 200 million
causes ulcers in infected tissue
genus Schistosoma (common in much of the tropics)
three species in humans of venules of small or large intestine, urinary bladder
lacks rediae & metacercarial forms
cercaria burrows directly into humans from snail (fig. 8.10, p. 155)
CLASS MONOGENEA (p. 156)
The monogenetic flukes
historically part of the Tremetoda
Most are parasites in gills of fish
highly modified attachment organ = opisthaptor
only become a problem when fish in high density (fish farm)
Life cycle: simple, only one host
Acoelomates, p. 4
CLASS CESTODA (pp. 156-158)
The tape worms
Great exaggeration of the reproductive system
produce many repeating reproductive units = proglottids
also have the holdfast = scolex
Lack GVC and protonephridia
Scolex (holdfast): combination of hooks & suckers
Proglottids (reproductive units):
squared unit with male & female reproductive organs
testes, sperm duct
ovary, vagina, uterus
the two come together at a genital pore
Life cycle:
adults in intestine of mammals (humans, dogs)
proglottids break off & released into soil
herbivores eat units (cow, deer, pigs)
larval forms burrow from intestine into blood
finally migrate to muscle & form a cyst
eating of raw or uncooked meat allows into carnivores (Table 8.2, p. 158)
dog tapeworms with an intermediate host of a flea
PHYLUM NEMERTEA
Ribbon worms, proboscis worms: about 650 species (pp. 159-161)
MORPHOLOGY & ANATOMY
Thread to ribbon shaped worms generally less than 20 cm long
one species does get to 30 m
Body plan very similar to the turbellarians
1) ciliated epithelium
2) flame cells
Acoelomates, p. 5
Differences:
1) eversible proboscis (protrude by inversion)
2) presence of mouth & anus (complete digestive system)
3) small body cavity present = rhynchocoel (coelomate??)
4) circulatory system present (circulation by body moving)
REPRODUCTION
Most species are dioecious, some are hermaphroditic
eggs or sperm are released into the water