1. Deuterostomes Hemichordates Echinoderms Protostomes
Cephalochordates
Urochordates (Ascidians)
Vertebrates (us)
Deuterostomes
Hemichordates
Echinoderms
Protostomes
Spiralia (Platyzoa, Polyzoa, Trochozoa..)
Ecdysozoa (Arthropoda, Nematoda..)

2. Embryonic Development: Early Cleavage
Zygote
Protostomes
Deuterostomes
Radial,
Indeterminate
Cleavage
each cell can
potentially
develop into a
complete embryo
Spiral,
Determinate
Cleavage
fate of cells is
set early in
development
2 cells
4 cells
8 cells
8 cells,
top
view

3. Spiral Cleavage in Protostomes
32 cell stage
- labeled with
Wilson’s coding
system
Annelid cross
Rosette cross
Molluscan cross

4. From Blastula to Gastrula
ectoderm
blasto-
coel
Invagination
endoderm
archenteron
blastopore
Gastrulation = formation of embryonic germ layers
- separates cells fated to interact with the environment
(movement, sensory, protection) from those that
will process food (= the gut)

5. Protostome Deuterostome Fate of Blastopore Coelom Formation
GASTRULA STAGE
Fate of
Blastopore
Coelom
Formation

6. Coelom formation Schizocoely (protostomes) Enterocoely (deuterostomes)
Ectoderm
Blastocoel
future
mesoderm
Archenteron
(larval gut)
Endoderm
Mesoderm forms out of
ball of cells descended
from 4d micromere cell
Mesoderm forms from cells that
pinch off from blastopore

7. Coelom formation via schizocoely
Solid mass of mesoderm, derived from 4d micromere cell;
gives rise to paired coelomic spaces
- associated with segmentation in annelids, arthropods

8. Coelom formation via enterocoely
Pouching of archenteron
production of mesoderm-lined
coelomic spaces
- may results in a 3-part coelom
in deuterostome embryos
(and some lophophorates)
3-part
coelom

9. Evolution of the Bilateria
Ancestor
probably had deuterostome
features as an embryo
Protostomes
Deuterostomes
- spiral cleavage
- blastopore becomes mouth
- mesoderm derived from 4d cell
- ventral nerve chord
- coelom formed by schizocoely,
hollowing out of mesodermal
mass from 4d micromere cell
- radial cleavage
- blastopore becomes anus
- mesoderm from archenteron
- dorsal nerve chord
- coelom forms by enterocoely,
pinching out of gut (with a
3-part arrangement in body)

10. Phylum Echinodermata
~7,000 spp.
Deuterostomes with penta-radial symmetry as adults,
bodies organized along oral-aboral axis (mouth  anus)
- Calcified endoskeleton derived from mesoderm; bony
ossicles or plates filled with living tissue (stroma)
Water vascular system, derived from coelom, powers the
podia (= tube feet) and serves as circulatory system
- Decentralized nerve ring + radial nerves
- Mutable connective tissue that can harden, soften
- Mostly dioecious (separate sexes)
Bilaterally symmetric larvae, such as bipinnaria (starfish)
and pluteus larvae (urchins, brittlestars)

11. Aboral surface Oral surface
Madreporite:
opening to water
vascular system
ambulacral (open on
grooves asteroids)
mouth
central
disc
body rays =
ambulacra
podia (tube feet) line
open grooves

12. Body Orientation in Echinoderms
Ophiuroids
Crinoids
Asteroids (stars)
Echinoids (urchins)
Holothurians (cucumbers)
mouth
ambulacral surface, w/ podia

13. Endoskeleton
Echinoderms have an epidermis covering a mesoderm-derived
dermis layer containing the calcified endoskeleton
- skeleton is composed of ossicles, porous plates of CaCO3
filled with living dermal cells called stroma
stroma tissue
fills these spaces
Under the dermis layer (i.e., beneath the skeleton):
- layer of muscle
- coelom (i.e., components of the water vascular system)

14. Endoskeleton Pisaster ossicles
ossicles may be spread throughout the body, embedded in
connective tissue as in sea stars & sea cucumbers
Pisaster ossicles
ossicles may fuse together to form the solid test (inner shell) & spines of sea urchins, or the arm vertebrae of brittle stars

15. Pedicellariae
Pincer-like structures on aboral surface, formed from ossicles
- have their own muscles, nerves + reflex arcs
- may be hollow & inject toxins
Function in both physical and chemical defense by pinching
and poisoning predators
- also anti-fouling: keep surface clean by pinching any larvae
or algal spores that try to settle on body surface

16. Phylum Echinodermata Class Asteroidea - sea stars
Class Ophiuroidea - brittle stars & basket stars
Class Crinoidea - crinoids
Class Echinoidea - sea urchins & sand dollars
Class Holothuroidea - sea cucumbers

17. Phylum Echinodermata
Class Asteroidea - sea stars

18. Class Asteroidea – sea stars
1,500 species in 5 orders
- 5 or more arms, not distinct
from central disc
- open ambulacral grooves lined with podia that have internal
ampullae (fluid-filled bulbs)
gills found in grooves
& across body surface

19. surface of Astrometis
spines
gills
pedicellariae
podia

20. Water Vascular System each lateral canal ends in a tube foot
- extends around
central disc
regulate
internal
pressure

21. Water Vascular System
Seawater enters through madreporite, mixes with coelomic fluid
and is is pumped through the system by cilia
When an ampulla contracts, it pushes fluid into the tube foot
(1) sucker is pushed against substrate; adheres via secretions
(2) muscles in tube foot contract, pushing fluid back out
(3) muscles pull up sucker of foot, creating vacuum– this
creates the tremendous suction that holds foot to substrate
(4) suction is released when ampulla again contracts, sending
fluid into foot sucker and relieving vacuum

22. Cross-Section of an Asteroid Arm

23. paired digestive glands
run down each arm
everted for
feeding

24. 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

25. Asteroids as keystone predators
Pisaster ochraceous and
P. giganteus function as
keystone predators in rocky
intertidal habitats, preserving
biodiversity
- mussels are competitive dominants for space, so they
crush everything else off the rocks if their populations grow
unchecked  loss of biodiversity
- by consuming mussels, sea stars create free space for other
organisms  promotes biodiversity in rocky intertidal zone

26. Asteroids as keystone predators
maximum foraging distance
- sea stars consume any mussel they can crawl to, open, and
eat during a single high tide
 vertical zonation, with mussels restricted to the tops
of rocks above foraging reach of sea stars, which hang
out near the base where it’s wet and cool

27. Crown-of-Thorns starfish, Acanthaster plancki
eats live coral polyps
in last few decades, major outbreaks have resulted in massive
coral mortality in Australia; human influences suspected

28. Sea stars are famous for regenerating lost arms
- some (Linkia) re-grow whole
body from a dropped arm !
Asteroid larvae can also bud off asexually, the only known
larvae capable of reproduction
Which is healing? which is asexual reproduction?...

29. Phylum Echinodermata Class Asteroidea - sea stars
Class Ophiuroidea - brittle stars & basket stars
Class Crinoidea - crinoids
Class Echinoidea - sea urchins & sand dollars
Class Holothuroidea - sea cucumbers

30. Class Ophiuroidea – brittle stars
2,000 species in 3 orders
Body with 5 or more arms, distinct from central disc
Tube feet have no suckers;
no anus is present
Coelom in arms is reduced,
due to large vertebral ossicles
Ambulacral grooves closed
- Madreporite on oral surface
(underneath)

31. Ophiuroid Cross-section
Central disc Arm
Digestive system
confined to
central disc

32. Ophiuroid External Anatomy
oral surface aboral surface

33. Ophiuroid Oral Surface
bursal slits, openings that draw
in water for gas exchange with
coelomic fluid-filled sinuses
mouth
ventral
arm shield
arm spines

34. Ophiuroid Arm Cross-section

35. Ophiuroids are predominantly deposit
or suspension feeders
- food particles are trapped in
mucous or passed
podia-to-podia to the mouth
Basket stars use multi-branched arms
to suspension feed or catch larger prey;
incomplete gut