Download The Deuterostomes Deuterostomes Why are Echinoderms

Protostome
The Deuterostomes
Echinoderms and Chordates
Ch 35
Deuterostomes
The SECOND opening of
the digestive system (in the
gastrula stage) becomes
the mouth
Deuterostomes
Characteristics:
•  Bilaterally symetrical
•  Coelomate
•  Blastopore fate (anus)
–  Second opening in the gastrula becomes the mouth
–  This developmental similarity between echinoderms and
chordates is supported by DNA sequence data
Phylum Echinodermata
(Gk: echin = spiny; derma = skin)
Why are Echinoderms considered
to be bilaterally symmetrical?
•  5 Classes: Sea stars, brittle stars, sea urchins,
sand dollars, sea cucumbers
•  A thin skin covers an endoskeleton of hard
calcareous plates
•  7,500 species
–  all marine
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Echinoderm larvae are bilateral
Starfish larvae
Adults - Pentaradial
Sea Urchin larva
Sea Cucumber
Some species are commercially harvested for
human food
Echinoderm Features
•  Unique “water vascular system”
•  A network of hydraulic canals used in:
• 
• 
• 
• 
locomotion
feeding
gas exchange
excretion
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Sea Star anatomy
•  Muscle and nervous tissue, but no cephalization
in adults
•  Endoskeleton of hard calcareous plates
•  Complete (but short) digestive system
•  No specialized respiratory, circulatory, or
excretory systems
•  Separate male and female individuals
•  External fertilization
Tube feet of a sea cucumber
Sea Star feeding on a bivalve
Everts stomach into the bivalve to digest
it INSIDE ITS OWN SHELL!!
Sea stars = Keystone predators
•  Keystone species - plays a unique role in the
way ecosystems function – greater than
what their biomass would suggest.
•  Sea stars eat shell fish (urchins, mussels,
etc) that most others do not eat
•  If sea urchins and/or mussels (herbivores)
take over what will happen?
Crown-of-thorns sea star
•  Feed on the living cnidarians that produce coral reefs
•  Population explosions of the crown-of-thorns sea star
can severely damage coral reefs
•  Causes of outbreaks appear to be removal of natural
predators and increased nutrient levels in the water
(both due to human activities)
•  Control efforts include injection of individual starfish with chemical toxins
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Phylum Chordata
Phylum Chordata
This phylum includes the animals that
are most familiar to us - and includes
humans.
•  Divided into 3 subphyla, which include 11
major groups of chordates
•  Some of the 11 groups are defined as “Classes”
by biologists
•  Others of the 11 groups are currently considered
to be “clades”, i.e. formal taxonomic level has
not been agreed upon
Chordates
Fig 35.12
•  Chordate does NOT = Vertebrate
All chordates have four characteristics
(at some developmental stage)
1. Pharyngeal slits
•  Openings to the outside of the body at the
pharynx (region just behind the mouth)
1.  Pharyngeal gill slits
–  Allows water that enters the mouth to exit without
passing though the gut in invertebrate chordates
2.  Dorsal hollow nerve cord
–  Modified for gas exchange or other functions in
vertebrates
3.  Notochord
4.  Post-anal tail
Fig. 34.3
Pharyngeal slits
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2. Dorsal hollow nerve cord
•  Unique to chordates
–  Other animals have a solid nerve cord that is usually
ventrally located
2. Dorsal hollow nerve cord
•  Nerve cord develops into the central nervous
system
–  Brain and spinal cord
Dorsal hollow
nerve cord
Ventral
solid
3. Notochord
•  A flexible rod located between nerve cord and gut
–  Provides skeletal support for muscle attachment
–  Only remnants of embryonic notochord found in most
adult vertebrates
4. Muscular post-anal tail
•  Skeletal elements (notochord) and muscles
•  Lost during embryonic development in many
species
–  Non-chordates have a digestive tract that extends the
length of the body
Notochord
Post-anal tail
Phylum Chordata
Three Subphyla:
1.  Cephalochordata
2.  Urochordata
Subphylum Cephalochordata
•  Called “lancelets” because of their blade-like
shape
•  Chordate characteristics persist into adulthood
•  Globally rare, but some places have high
population density (e.g., Tampa Bay, Florida)
3.  Vertebrates
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Lancelet Anatomy
Suspension
feeders that
use their
pharyngeal
slits to filter
out small food
particles
Typically present buried tail-first in
sand
Subphylum Urochordata
•  Tunicates (“sea squirts”)
Adult tunicates don’t look much
like a “chordate”
•  No notochord, nerve cord or tail
•  Filter feeders with
incurrent and excurrent
siphons
Adult tunicates don’t look much
like a “chordate”
Some tunicate species are
colonial
•  But these features are present in the tunicate
larval form…
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The Vertebrates
Subphylum Vertebrates
•  Chordates with a skull (either bone or
cartilage)
•  Includes all of the animals we call
“vertebrates”
Finally - something we’ve heard of!!
All vertebrates have the same
characteristics as chordates
(at some developmental stage), plus
•  Pronounced cephalization
–  With a skull (cranium)
•  Includes some large and rather obvious
animals that are quite familiar to us
•  But remember that vertebrates represent
less than 5% of the known animal species
on Earth
•  Vertebral column that encloses the nerve cord
–  Replaces the notochord function
•  Endoskeletons that grows with the animal
•  Closed circulatory system
•  Lampreys
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Hagfish (Clade Myxini)
Jawless
Lost vertebrae through evolution (ancestors had
vertebrae)
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