Download Annelids are segmented worms

33-16-05
Barb
Teeth of radula
-the hardest material
(magnetite) of
biological origin
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• Most mollusks have separate sexes
– However, many snails are outcrossing
hermaphrodites.
• The life cycle of many marine mollusks
includes a ciliated larvae, the trophophore.
– This larva is also found in marine annelids
(segmented worms) and
some other lophotrochozoans.
Fig. 32.9
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A trochophore larva.
• The basic molluscan body plan has evolved in
various ways in the eight classes of the phylum.
– The four most
prominent are the
Polyplacophora
(chitons),
Gastropoda
(snails and slugs),
Bivalvia (clams,
oysters, and other
bivalves), and
Cephalopoda
(squids, octopuses,
and nautiluses).
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Polyplacophora - chitons
• Marine; oval shapes and shells divided into
eight dorsal plates.
• Muscular foot grips the rocky substrate tightly
and creep.
• Grazers;
use radulas
to scrape and ingest
algae.
Fig. 33.17
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• Gastropoda - > 40,000 species; mostly marine,
but also many freshwater species.
– Garden snails and slugs have adapted to land.
• Distinctive characteristic - During embryonic
development, gastropods undergo torsion in
which the visceral mass is rotated up to 180
degrees, such that
the anus and mantle
cavity are above
the head in adults.
Fig. 33.18
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Diagrammatic Illustration of Torsion in Gastropod Molluscs
• Most gastropods are protected by single,
spiraled shells into which the animals can
retreat if threatened.
– Other species have lost their shells entirely and may
have chemical defenses against predators
(nematocysts from anemones).
Shell-less nudibranchs
(sea slugs)
Fig. 33.19
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• Many gastropods have distinct heads with eyes
at the tips of tentacles.
Conch
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• Gastropods are among the few invertebrate
groups to have successfully populated the land.
• In place of the gills found in most aquatic
gastropods, the lining of the mantle cavity of
terrestrial snails functions as a lung –
pulmonate snails
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• Class Bivalvia -- clams, oysters, mussels, and
scallops.
• Bivalves have shells divided into two halves.
– The two parts are hinged at the mid-dorsal line, and
powerful adductor muscles close the shell tightly to
protect the animal.
– When the shell is open,
the bivalve may extend
its hatchet-shaped foot
for digging or anchoring.
Fig. 33.20
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• The mantle cavity of a bivalve contains gills
that are used for feeding and gas exchange.
• Most bivalves are suspension feeders, trapping
fine particles in mucus that coats the gills.
– Cilia convey the
particles to the mouth.
– Water flows into mantle
cavity via the incurrent
siphon, passes over the
gills, and exits via the
excurrent siphon.
Fig. 33.21
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• Cephalopods - rapid movements to dart toward
their prey which they capture with several long
tentacles.
– Squids and octopuses use beaklike jaws to bite their
prey and then inject poison to immobilize the
victim.
• A mantle covers the
visceral mass; shell is
reduced and internal
in squids, missing in
many octopi. Only
nautilus has external
shell.
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Fig. 33.22
• Fast movements by a squid occur when it
contracts its mantle cavity and fires a stream of
water through the excurrent siphon.
– By pointing the siphon in different directions, the
squid can rapidly move in different directions.
• The foot of a cephalopod (“head foot”) has
been modified into the muscular siphon and
parts of the tentacles and head.
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• Unique among mollusks, cephalopods have a
closed circulatory system to facilitate the
movements of gases, fuels, and wastes through
the body.
• They have a well-developed nervous system
with a complex brain and well-developed sense
organs.
– This supports learning and complex behavior.
– Squid giant axon located on inside of mantel.
Innervates mantel and causes large contraction
leading to forcing a jet of water through syphon.
– Giant axon used as model axon to work out the
biophysics of nerve conduction. Large size
electrode could be inserted directly into nerve.
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• Highly developed eye in cephalopods
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
6. Phylum Annelida: Annelids are
segmented worms
• All annelids (“little rings”) have segmented
bodies.
• ~ 15,000 species ranging in length from less than
1 mm to 3 m for the giant Australian earthworm.
• Annelids live in the sea, most freshwater habitats,
and damp soil.
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The phylum Annelida - three classes:
Oligochaeta, Polychaeta, and Hirudinea.
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• OligochaeteE.g. earthworm
Coelom is
partitioned by
septa, but the
digestive tract,
longitudinal
blood vessels,
and nerve cords
penetrate the
septa and run the
animal’s length.
Fig. 33.23
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• Digestive system - a pharynx, an esophagus,
crop, gizzard, and intestine.
• The closed circulatory system carries blood
with oxygen-carrying hemoglobin through
dorsal and ventral vessels connected by
segmental vessels.
• In each segment is a pair of excretory tubes,
metanephridia, that remove wastes from the
blood and coelomic fluid.
– Wastes are discharged through exterior pores.
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• A brainlike pair of cerebral ganglia lie above and
in front of the pharynx.
• Earthworms are cross-fertilizing hermaphrodites.
– Two earthworms exchange sperm and then separate.
• Some earthworms can also reproduce asexually
by fragmentation followed by regeneration.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Polychaete - Each segment of a polychaete
(“many setae”) has a pair of paddlelike or
ridgelike parapodia (“almost feet”) that function
in locomotion.
– Each parapodium has several chitinous setae.
– In many polychaetes, the
rich blood vessels in the
parapodia function as gills.
Fig. 33.24b
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• Most polychaetes are marine.
• Polychaetes include carnivores, scavengers, and
planktivores. Many live in tubes with only the
tentacles protruding from the tube.
– The brightly colored
fanworms trap plankton
on feathery tentacles.
– Many build tubes and
Thus called tube worms.
Fig. 33.24c
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Chaetopterus
Scale Worms
• Some benthic polychaetes crawl around on
the ocean bottom and are covered with large
protective scales.
• Hirudinea - leeches.
• Many leeches feed on other
invertebrates, but some bloodsucking parasites feed by attaching
temporarily to other animals,
including humans.
Fig. 33.24d
– Some parasitic species use bladelike
jaws to slit the host’s skin, while others
secrete enzymes that digest a hole
through the skin.
– The leech secretes hirudin, an
anticoagulant, into the wound, allowing
the leech to suck as much blood as it
can hold.
– Medicinal use of leech.
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Prostostomia: Ecdysozoa
Major Phyla:
1. Nematoda – round worms
2. Arthropoda – crabs, insects
spiders
Ecdysozoa - Introduction
• Undergo ecdysis  shedding of exoskeleton.
• Ecdysozoa grouping based mainly on molecular data.
1. Phylum Nematoda
• ~ 90,000 described species, and ~10x undescribed.
• 1 mm to 1m - Ubiquitous habitats. Even in the extreme
cold soil of the Dry Valleys of Antarctica.
• Many parasites, including human pathogens –
elephantiasis, trichinosis, onchocerciasis (river blindness)
Wuchereria bancrofti
Scottnema
lindsayae
(Antarctic soil)
Ascaris
lumbricoides
(human parasite)
Potato cyst
nematode
Elephantiasis
(Wuchereria
bancrofti)
Nematoda Characteristics
• Cylindrical bodies, chitinous cuticle.
• Complete digestive tract.
• No circulatory system; use pseudocoelom fluid to
transport nutrients.
• Their thrashing motion by
contraction of longitudinal
muscles.
• Sexual reproduction.
Fig. 33.24d
Name the most famous nematode ?
3 shared 2002Nobel prize in medicine,
using this worm as a model system for studies
in genetic regulation of organ development
and programmed cell death
Caenorhabditis elegans
At hatching:
558 cells
+ hermaphrodite
560 cells
Adult:
+ hermaphrodite 1090 cells
Programmed death 131 cells
959 cells
Programmed death
1179 cells
148 cells
1031 cells
The worm Noble laureatesm (2002):
Sydney Brenner, H. Robert Horvitz, John E. Sulston
Berkeley
MIT
Sanger,UK
2. Arthropoda:
•
•
•
•
~ 1 billion billion (1018) individuals.
~ 1 million arthropod species described ~67% of spp.
Ubiquitous habitats.
The diversity and success due to: body segmentation, a
hard exoskeleton,
and jointed appendages
 specialization for
functions.
Fig. 33.26
Arthropoda Characteristics
• Cuticle (exoskeleton) - protein and chitin:
– thick and inflexible in some regions; protection.
– thin and flexible in others; joints.
– attachment points for muscles.
– relatively impermeable to waterland habitats.
– must molt (ecdysis) to grow  danger of predation.
• Well-developed sense organs:
– eyes, olfactory receptors, antennae.
– cephalization; sense organs anterior.
• Open circulatory system.
• Coelom much reduced.
Arthropod Respiration
• Respiratory system specialized for different habitat.
• Aquatic species:
– External gills
• Terrestrial species:
– Internal organs (book lungs in arachnids)
– Tracheal system in insects.
Alternate taxonomy:
4 major lineages
Taditional
Taxonomy:
Phylum:
Class:
(trilobites –extinct)
Arachnida
Phylum
Arthropoda
(spiders, scorpions,
mites)
Diplopoda
Trilobita
Chelicerata
(jaw-like
chelicerae, no
antennae, simple
eyes)
(millipedes)
Chilopoda
Uniramia
(centipedes)
(jaw-like
mandibles, 1
pair antennae,
complex eyes)
Insecta
(insects)
Crustacea
Crustacea
(Crabs, lobsters, (jaw-like mandibles,
2 pairs antennae,
crayfish, shrimps)
complex eyes)
SuperPhylum
Arthropoda
• Trilobita
- Earliest arthropods.
- Cambrian (540 mya) to Permian. Extinct ~250 mya.
- Pronounced segmentation; uniform segments and
appendages
- Subsequent arthropod
evolution towards fewer,
more specialized
segments and appendages.
Fig. 33.27
• Chelicerata
• Anterior cephalothorax, posterior abdomen.
– appendages more specialized than trilobites
– most anterior appendages modified as chelicerae
(pincers or fangs).
• Marine chelicerates – 4 species.
Horse shoe crab
-living fossil
Sea spider
• Terrestrial chelicerates – majority in Arachnida.
– Scorpions, spiders  venomous predators
– Ticks  blood sucking parasites
– Mites detritus eater; parasites of animals & plants
scorpion
mite
tick
Fig. 33.29
• Arachnid characteristics
• Cephalothorax has 6 pairs of appendages.
– 4 pairs of walking legs.
– 1 pair pedipalps  sensing or feeding.
– Chelicerae usually for feeding. Glands contain
poison.
– Spiders inject poison:
immobilize prey
spills digestive juices
on prey; sucks up liquid
meal.
Fig. 33.30x
• Spider Anatomy
• book lungs - stacked plates  large surface area for gas
exchange
• Spider silk/web – unique prey capturing feature.
Fig. 33.30b
spider spinning cribellate net
Wool-like cribellate silk
Sticky silk
microcrystalline arrays of
glycine-rich regions
beta-sheets of
alanine-rich regions