Download Chapter 14 - Angelo State University

Acoelomate
Chapter 14
Bilateral
Acoelomates
• No internal body space other than
digestive tube.
• Spaces between organs filled with a loose
connective tissue with more cells than
mesoglea = parenchyma.
• Solid construction is a diagnostic
character of the phylum.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
General Features
• Cephalization
– Sessile animals survive well with radial
symmetry
– Concentrating the sense organs on the head is
an advantage to active animals that seek food
– This provides an anterior and posterior end
and bilateral symmetry
Development of a Centralized
Nervous System
• Protozoa are unicellular and lack nerves
• Nerve Net
– This is the simplest pattern of nervous
system found in sea anemones, jellyfish,
hydra and comb jellies.
– The nerve net is an extensive network in
and under the epidermis.
– c.Impulses starting in one part are
conducted in all directions; synapses do not
direct one-way impulses.
– There are no sensory, motor or
interneurons.
– This type of system survives in advanced
animals as a nerve plexus that governs
intestinal movement.
CNS continued
• Bilateral Nervous Systems
– Flatworms represent the
simplest bilateral nervous
system.
– They have two anterior
ganglia leading to two main
nerve trunks that run
posteriorly.
– Lateral branches form a ladder
appearance.
– This is the simplest system to
have a peripheral nervous
system extending to all parts
of the body, and a central
nervous system concentrating
nerve cell bodies.
Position and Biological
Contributions
Phylum
Platyhelminthes
• Simplest animals with primary bilateral
symmetry
• Solid body without a coelom
• Organ-system level of organization
• Protostomes with spiral cleavage
• Triploblastic with well defined mesoderm
• Simple nervous, excretory, and osmoregulatory
systems appear
General Characteristics
• Dorsoventrally flattened animals with primary
bilateral symmetry.
• Without coelom, anus, circulatory, respiratory, or
skeletal systems.
• With flame-bulb protonephridia.
• With a cellular mesodermal connective tissue
filling all spaces between organ- systems.
• The phylum includes the free living flatworms
and the parasitic flukes and tapeworms.
Water and Osmotic Regulation
• Animals that must live within a narrow salinity
range are stenohaline.
• Organisms that can tolerate the wide variations
found in estuaries are euryhaline.
• A hyperosmotic regulator maintains body fluids
in higher concentration than surrounding water.
– Maintaining a higher concentration by excreting excess
water requires specialized excretory organs
Homeostasis: Water and Osmotic Regulation
• Invertebrates Meet Problems of Salt and
Water Balance
– Most marine invertebrates are in osmotic equilibrium
with their seawater environment.
– With body surfaces permeable to water and salts, the
internal and external concentrations are equal.
– Such animals that cannot regulate osmotic pressure of
their body fluids are called osmotic conformers.
– This functions for open ocean organisms because the
open ocean is stable.
Protonephridium
• This tubular structure is the
most common design to
maintain osmotic balance.
• The flame cells system or
protonephridium is the simplest
arrangement.
• Planaria and other flatworms
have a highly branched duct
system to all parts of the body.
• Fluid enters the system through
specialized “flame cells” and
passes through tubules to exit
the body.
Protonephridia
• Rhythmical beating of a flagellar tuft creates a
negative pressure that draws fluid into the tubes.
• In the tubule, water and metabolites are
recovered by reabsorption; wastes are left to be
expelled.
• Nitrogenous wastes, mainly ammonia, diffuse
across the surface of the body.
• Flatworms have no circulatory system so the
flame cell system must branch throughout the
animal.
• This system is a closed system since the fluid
must pass across flame cells.
Bilateral Symmetry
• All remaining animal phyla are bilaterally
symmetrical.
• Correlated with motility. Anterior end meets
environment first and bears most of the sense
organs.
• Cephalization results.
– Right and left sides each contact environment in same
way so virtually identical, but upper and lower halves
differ as do front and back.
• Of all bilaterally symmetrical animals these are
probably the most primitive.
Appearance
• Range in size from microscopic worms no larger
than protozoans to the extreme elongation seen
in tapeworms which can be 10 to 15 m, but most
are small to moderate size.
• Most are white or colorless or owe their color to
the food they ingest; Free-living worms are often
gray, brown or black (when not white), but some
are brightly colored.
Germ Layers
• Three distinct germ layers; Ectoderm (outer),
(endo)mesoderm(middle) and endoderm (inner).
• Triploblastic).
• Ectoderm Æ surface layer (skin, hair, etc.) And
nervous system.
• Endoderm Æ gut lining and internal organs.
• Mesoderm Æ everything else (muscles, blood,
bones, connective tissue, etc.).
Appearance
• May have clearly recognizable head at anterior
end separated from body by neck-like
constriction, but anterior is organized as a head
even when the constriction is absent.
• Can tell it is head because the main nerve center
is there, the presence of numerous sensory organs,
and because it is directed forward during
locomotion.
• In parasitic flatworms head usually bears hooks
and suckers to attach to host (but these may be
elsewhere on body also).
Appearance
• Ventral surface on which locomotion
occurs (often ciliated), and which also
bears mouth and genital pores, differs
from dorsal surface in turbellarians.
• Difference is less marked in trematodes
and virtually absent in cestodes.
Body Wall
• Ciliated epidermis one layer thick in f.L. For
locomotion.
• Tegument in parasites.
– Cells of tegument lie under muscle layers and extend
up as a syncytial layer of cytoplasm over surface.
• Beneath surface layer generally find circular,
longitudinal and often also diagonal muscle
fibers of mesodermal origin.
Hydrostatic Skeleton
• Many invertebrates use their body fluids as an
internal hydrostatic skeleton.
• Muscles in the body wall contract against the
coelomic fluids that are incompressible.
• In flatworms, if body fluid is lost, the worm
cannot continue to move.
– Annelids “solved” this problem by dividing the body
into compartments by septa so that each surviving part
can still develop pressure and move.
Nervous System
• Nervous system of the most primitive flatworms
derivable directly from cnidarians. = Epidermal
nerve net and a slight massing of nervous tissue
representing a brain.
• In most, the nervous system has sunk below
epidermis and muscle layer and the main nerve
net shows concentration into several longitudinal
ganglionated cords - 2 ventral ones are either
most conspicuous ones or only ones.
Nervous System
• System primarily radial becomes bilateral
by elimination of some of the radial cords
(around bell of medusa).
• Brain = main nervous center.
• Ladder type of nervous system when only
two nerve cords present because joined
periodically by transverse connections.
Sense Organs
• Stimuli
– Sense organs are specialized receptors designed to detect
environmental status and change.
– Sense organs are the first level of perception and are channels
for bringing information to the brain.
– A stimulus is some form of energy: electrical, mechanical,
chemical or radiant.
– The sense organ are biological transducers that must
transform the energy form of the stimulus into nerve
impulses.
• usually respond to only one kind of stimulus.
– Muller (1830s) detected that animals perceived different
sensations only because impulses originating from one sense
organ arrive at a particular sensory area: the “law of specific
nerve energies.”
Chemoreception
• This is the oldest and most universal sense in the
animal kingdom.
• Protozoa use contact chemical receptors to locate
food and oxygenated water, and to avoid harmful
substances.
• Chemotaxis is an orientation behavior toward or
away from a chemical source.
• Most metazoans have specialized and sensitive
distance chemical receptors or a “sense of
smell.”
Specific Sense Organs
Digestive System
• Turbellarians richly supplied with sensory
organs - greatly reduced in parasitic
classes chemoreceptors - widespread.
• Similar to anthozoans. Has mouth, pharynx and
blind intestine, no anus.
• Mouth is located at about the center of the ventral
surface. Primitive.
• Pharynx varies in phylum. Usually is a strong
muscular tube which can sometimes be extruded.
• Intestine varies from a simple sac to complicated
branchings; Missing completely in some primitive
turbellarians and in cestodes (tapeworms).
– tangoreceptors (for sensing touch)
widespread.
– statocysts in some.
– ocelli - widespread. Also in larval flukes and
some adult groups.
Reproduction
• Reproductive system is most complicated
system (in tapeworms there is little else).
• Almost all are monoecious.
• Reciprocal fertilization is accomplished
by a complicated copulatory apparatus.
Reproduction
• Fertilization is internal.
– Fertilized, shelled eggs, often already containing
embryos are shed to exterior or fastened to objects by
adhesive secretions.
• Asexual fission in some turbellarians. Reduced
population density triggers asexual strategy.
• Asexual reproduction in many parasitic stages.
Important mechanism to increase reproductive
potential.
Turbellaria
• Free-living flatworms. Live in fresh
water, in springs, streams, ponds and
lakes; Or in the ocean, chiefly along
shores; Or in moist terrestrial habitats,
mostly in tropical or subtropical regions.
– A few Turbellarians are commensal or
parasitic.
Trematode Life Cycle
(Generalized scheme)
Ecology
• Either free-living or ecto-or endocommensals or
parasites.
• Class Turbellaria are mainly free living.
• Classes Trematoda and Cestoda are wholly
parasitic, chiefly endoparasitic.
– Adult stage parasitizes vertebrates (exceptions rare).
– Larval stages often occur in invertebrates, usually
snails.
– Hosts may be fresh-water, marine or terrestrial.
Trematoda
• Includes parasitic flukes
– Several subclasses—most important is
Digenea, the digenetic flukes
• Many are serious parasites in humans.
• Have a complicated life cycle with at least
two different hosts.
Miracidia
• Egg is enclosed within an oval shell with a
lid, deposited in the gut of the host and
passed to the outside with the host's feces.
• Miracidium. = free swimming ciliated
larva which hatches from eggs.
– Enters molluscan host (eaten by it or
penetrates host's epidermis).
– Usually host is a snail.
The Miracidium is inside the egg.
www.path.cam.ac.uk/~schisto/Pictures
/ Eggs/S.mansoni.egg.gif
www.volny.cz/schistosomes/
miracidia.htm
Sporocyst
• Miracidium looses its cilia after
penetration of host and becomes
a sporocyst.
• Inside the hollow sporocyst,
germinal cells give rise to a
number of embryonic masses.
• Each mass develops into a
daughter sporocyst or another
develop mental stage, called a
redia.
Redia
• Redia is also a chambered
form. Germinal cells within
the redia again develop into
daughter rediae or more
usually into a number of
larvae called cercariae.
• If there are daughter
sporocysts the redia stage is
skipped and cercariae develop
inside of daughter sporocysts.
www.path.cam.ac.uk/.../Flukes_Gen/ Fluke_Life3.html
Cercaria
• The cercaria possesses a digestive tract,
suckers and a tail.
• It leaves the host and is free-swimming.
• If it comes in contact with a second
intermediate host, an invertebrate (commonly
an arthropod) or a vertebrate, it penetrates the
host and encysts.
• The encysted stage is called a metacercaria
Metacercaria
• This is the usual resting stage in the
second intermediate host.
• Sometimes there is an intermediate
stage called a mesocercaria capable
of inhabiting a wide range of hosts
and becoming a metaceria when
the host is eaten.
Some cercariae
• www.path.cam.ac.uk/.../Flukes_Gen/
Fluke_Life3.html
Metacercaria
• Can encyst on vegetation or in a host.
• Matures into final adult stage when host is
eaten.
• Vegetarians not immune since can eat plant
on which metacercariae have encysted.
Adult
• If the host of the metacercaria is eaten by
the final vertebrate host, the metacercaria
actively escapes from its cyst, migrates and
develops into the adult form within a
species specific location in the host.
• Can stay in the intestine or must penetrate
gut wall and migrate to final site.
Cestoda
• Segmented entoparasites in digestive tract
and associated ducts of vertebrates.
• Have no epidermis, mouth or digestive
tract.
Cestode Morphology
• Neck region.
• Situated immediately posterior to scolex.
• Unsegmented, poorly differentiated area
generally narrower than scolex and
strobila proper.
• Continuously differentiating zone which
gives rise to new proglottids (body
segments).
Cestode Morphology.
• 3 body regions.
• Scolex (holdfast organ) located at
the anterior end and provided with
suckers and/or hooks for adhering
to the host. Can be quite
complicated. Small when compared
to a mature proglottid.
Cestode Morphology.
Cestode Morphology
(Segmentation)
• Metamerism (segmentation) is a serial repetition
of body parts along anterior-posterior axis.
• Differs from "typical" segmentation
(metamerism) of annelids, arthropods and
chordates.
• If above definition is true, this is segmentation.
• Strobila.
• Main bulk of body.
• Linearly arranged
segments called
proglottids form a
chain.
• Newest segments at
neck and oldest at tail
end.
– Big difference is in direction.
– In cestodes go from scolex --> young segments. --> Old
segments. In others get new segments from segment
before end.
Web site
Cestode Body Wall
• Several layers thick.
• Outermost is tegument (nonciliated cytoplasmic
syncytium overlying muscle layers).
• The syncytium is formed from extensions of cells
located in the parenchyma.
• Differs from tegument of trematodes in having
surface microthrix (= microvilli; Surface folds)
which increase the surface area through which
food can be absorbed. Pinocytosis and exocytosis
(excretion) can occur here.
Reproduction
• Monoecious. Self fertilization within single proglottid and
between proglottids, and copulation between 2 worms if
they bump into each other in the gut of the host all occur in
monoecious.
• Selfing between different proglottids of the same worm is
probably deleterious. When one worm per mouse
(Hymenolopus diminuta) found an increase in frequency of
abnormal larvae. Also a decrease in egg viability. No selfed
strain can be maintained beyond 5th generation so it is
probable that cross fertilizaion to provide hybrid vigor is
essential at least occasionally. Usually male organs mature
before female so fertilization of same segment is not
possible.
Other Cestode features
• Respiration is largely anaerobic, but have
enzymes for aerobic respiration also.
• Nervous system is typical, but no
specialized sense organs.
• Reproduction- mono and dioecious.
Reproduction
• Dioecious. Cross fertilization is necessary. Only
one dioecious order.
• Complete reproductive system in each proglottid.
Basic structure similar to digenetic trematodes.
• Fertilized eggs usually stored in blind (no exit)
uterus. Terminal proglottids packed with eggs
break off and eggs are freed and pass out through
intestine in feces or proglottids.
Life cycles
Life cycles
• Usually at least 1 or 2 intermediate hosts
(arthropods and vertebrates).
• "Typical" life cycle. Egg --> shelled larva
--> cysticercus --> adult.
• Larva penetrates intestinal wall and reaches
coelom, liver, spleen, muscles or other parts while
gradually developing scolex with adhesive organs
typical of the species and have young tapeworm,
but no strobilation. Encysts in host tissue until
host eaten by final host.
• In intestine of final host larva attaches to the wall
and develops into mature tapeworm.
• Produces many proglottids each day.
Life Cycles
• Cysticercus. Some use a vertebrate host instead of
arthropod to eat larvae and grow cysticercus = bladder
worm.
• Humans can eat and get adult tapeworms. Usually from raw
or incompletely cooked pork.
• Can get tapeworms from beef as well. Adult tapeworms not
too bad. Symptoms absent or mild.
• Much more serious to eat embryonated eggs of pork or
beef tapeworm. Bladder worms develop in various organs,
including the brain. Symptoms similar to tumor, and can get
serious reactions to liberated toxins.
Phylum Nemertea
•
•
•
•
•
•
•
•
Ribbon worms
Long muscular proboscis
General body plan similar to Turbellarians
Ciliated epidermis with many gland cells
Excretory system with flame cells
Mostly dioecious
Mouth to anus digestive system
Most free-living, few commensal or parasitic