Protostomes
Lophotrochozoa
Ecdysozoa
Deuterostomes are different from protostomes
due to basic features of how the zygote forms
a multicellular embryo
Deuterostomes
Embryonic Development: Early Cleavage
Protostomes
Spiral,
Determinate
Cleavage
Zygote
2 cells
- developmental
fate is set early
4 cells
- each cell has
specific destiny
8 cells
view
from
above
Deuterostomes
Radial,
Indeterminate
Cleavage
- each cell can
develop into a
complete embryo
Protostome
Fate of
Blastopore
1st opening of
embryo becomes
either the larval
mouth or anus
Coelom
Formation
ball of mesoderm
cells hollows out
versus
space pinches off
from embryo’s gut
Deuterostome
GASTRULA STAGE of embryo
Comparison of 2 major animal lineages
Protostomes
Deuterostomes
- spiral cleavage
- radial cleavage
- cell fate is determinant (fixed)
- cell fate is indeterminant
(flexible; twins possible)
- blastopore becomes mouth
- blastopore becomes anus
- coelom develops from
- coelom develops from
hollow ball of mesoderm
- ventral nerve chord
pinched-off gut space
- dorsal nerve chord
Phylum Echinodermata
7,000 species
Deuterostomes with penta-radial (5-fold) symmetry as adults
- body organized along oral-aboral axis (mouth-to-anus)
- larvae are still bilaterally symmetric
Endoskeleton: hard parts inside of soft tissue (like our bones)
- bony ossicles (chunks) or plates develop from mesoderm
Water vascular system develops from coelom
- powers movement, often using external tube feet
- important in local marine ecosystems as herbivores (urchins)
and keystone predators (sea stars)
Aboral (top) surface
Madreporite, opening to water
vascular system
central
disc
body ray
Oral (bottom) surface
arm grooves (open on sea stars)
mouth
tube feet line
open grooves
Endoskeleton
Echinoderms have an epidermis covering a calcium carbonate
endoskeleton (= inner skeleton), derived from mesoderm
- skeleton is composed of ossicles,
porous chunks of calcium carbonate
filled with living tissue
Ossicles fuse together in sea urchins to form
protective inner shell and spines
Phylum Echinodermata
Class Asteroidea - sea stars
Class Ophiuroidea - brittle stars & basket stars
Class Holothuroidea - sea cucumbers
Class Echinoidea - sea urchins & sand dollars
Class Asteroidea – sea stars
- Body has 5 or more arms that flow into a central disc
- Arm grooves open, lined with tube feet  provide suction for
holding onto rocks, opening bivalves
- feathery gills anywhere on
body surface
Water Vascular System
each tube foot has its
own bulb of fluid
extends around
central disc
-may produce coelomocytes
Water Vascular System
Seawater enters through madreporite (opening on top),
mixes with coelom fluid
- fluid gets pushed into a tube foot when its bulb contracts
(1) sucker is pushed flat against substrate
(2) muscles in tube foot contract, pushing fluid back out
(3) muscles pull up against the sucker, creating a vacuum
 this creates the suction which holds the foot to a rock
(4) suction is released when fluid is once again pushed into
the tube foot’s sucker, relieving the vacuum
Sea stars use their tube feet to pull open shells of bivalves
such as mussels, clams
- then turn their stomach
inside-out, into the shell
- digest the soft bivalve
tissue inside its own shell
sea star hunched over a mussel,
ready to start pulling its shell open
Stomach of the bat star Asterina
pushed out against glass
Ecological Role of Sea Stars
Pisaster sea stars are important local keystone predators in
intertidal habitats
- eat mussels, which are dominant competitors for space
- maintain biodiversity by preventing mussels from taking up
all available space & crowding everything else off the rocks
Crown-of-Thorns starfish, Acanthaster planci
- eats live coral polyps
- in recent decades, major outbreaks have resulted in massive
coral loss in Australia; human influences suspected
Regeneration
Sea stars are famous for regenerating lost arms
- some can re-grow an entire
body from one dropped arm !
Which is healing? which is asexual reproduction?...
Phylum Echinodermata
Class Asteroidea - sea stars
Class Ophiuroidea - brittle stars
Class Holothuroidea - sea cucumbers
Class Echinoidea - sea urchins & sand dollars
Class Ophiuroidea – brittle stars
5 arms attached to central disc at flexible joints
- tube feet lack suckers; watch
how they move using their arms
- hide under rocks; snake-like
arms reach out to grab food
- ossicles form a “spine” of
vertebrae down each arm
- arms bend side-to-side but
not up/down: “brittle”stars
because arms break off
Phylum Echinodermata
Class Asteroidea - sea stars
Class Ophiuroidea - brittle stars
Class Echinoidea - sea urchins & sand dollars
Class Holothuroidea - sea cucumbers
Class Echinoidea – Sea Urchins
1,000 species
Ossicles fuse into a solid shell, under epithelium (= skin)
- moveable spines also covered in epithelium
- holes in shell let tube feet poke out [look for these in lab !]
- mouth on bottom; anus opens on top
Unique 5-sided tooth called
Aristotle’s Lantern used for
scraping algae  important
herbivores (feed on kelp)
Pluteus larva has bilateral
symmetry, 8 arms
“purps”
Strongylocentrotus purpuratus
spines are a key adaptation: physical defense against predation
Urchin spines
In most urchins, spines are sharp
and may contain toxins
 spines of
Tripneustes
spine
epithelium
pencil urchin
In pencil urchins,
spines are fat and
blunt... urchin uses
them to jam itself
into holes so it can’t
be pulled out
shell
Ecology of Urchin Barrens
Urchins eat kelp and other macroalgae
Urchins are eaten by sea otters, which are keystone predators
healthy ecosystem with
plenty of algae left
lots of food for other animals
Ecology of Urchin Barrens
When predators are hunted
down, urchin populations
grow out of control
- quickly eat all available
drift kelp falling onto
sea floor; get hungry....
- urchins mobilize, march out of hiding places across sea floor
- graze down all available kelp, creating urchin barrens where
there is no algae for other consumers  lose biodiversity
removal of a keystone predator like the otter has
cascading negative effects on diversity of an ecosystem
Diadema is a large,
long-spined urchin
- over 90% in Caribbean
wiped out by mystery
plague in 80’s
Corals became overgrown
by algae in many places
following Diadema die-off
- algae grew too fast for
coral to compete, without
the big urchins to graze
down the algae
Diadema is a very
long-spined urchin
- over 90% in Caribbean
wiped out by mystery
plague in 80’s
Shrimp swarms used to hide
among Diadema spines
- switched to aggressive
damselfish (“farming fish”)
when urchins died off
Phylum Echinodermata
Class Asteroidea - sea stars
Class Ophiuroidea - brittle stars
Class Echinoidea - sea urchins & sand dollars
Class Holothuroidea - sea cucumbers
Class Holothuroidea – Sea Cucumbers
1,100
species
Only echinoderm where body lies on its side
- tube feet on ventral (belly) side only
Feeding tentacles around mouth used to filter feed or eat sand
- organic matter digested, clean sand pooped out
rows of tube feet
Class Holothuroidea – Sea Cucumbers
1,100
species
Only echinoderm where body lies on its side
- tube feet on ventral (belly) side only
Feeding tentacles around mouth used to filter feed or eat sand
- organic matter digested, clean sand pooped out
rows of tube feet
Cucumber Defense
No spines –
defend against predators by:
a) shooting defensive strings out of anus
b) spitting out intestines (regrow them later)
 also breathe through their anus!
Cucumber Commensalism
Sea cucumbers may have commensal organisms, including
a pearl-fish and a crab, living in their anus!
commensal = doesn’t hurt, but doesn’t help
Echinoderm larvae
Pluteus larvae of urchins and
brittlestars have bilateral symmetry
- show that echinoderms evolved
from a typical bilaterian
ancestor, but adult stage evolved
a weird radial symmetry
4 pairs of cilia-covered arms
- catch one-celled algae, pass
them along to mouth
- cilia also used to swim
pluteus
sea star larva
larva