1. Chapter 16
The Echinoderms

2. Evolutionary Perspective
Flourished in 400 million year-old seas
Many were attached suspension feeders.
12 of 18 classes now extinct
Remain a major component of marine ecosystems
Characteristics
Calcareous endoskeleton (ossicles)
Adults with pentaradial symmetry
Water-vascular system
Complete digestive tract
Hemal system
Nervous system consisting of nerve net, nerve ring, and radial nerves

3. Evolutionary Perspective
Deuterostomes
Closest relatives
Hemichordata
Chordata
Evolved from bilaterally symmetrical ancestors?
Larval stages are bilaterally symmetrical.
Some extinct forms bilaterally symmetrical.
Evolved from radially symmetrical ancestors?
Arkarua tentatively identified as oldest extinct echinoderm.
Pentaradially symmetrical

4. Figure 16.1 Evolutionary relationships of the echinoderms to other animals.

5. Echinoderm Characteristics
Pentaradial symmetry
Body parts arranged in fives around an oral-aboral axis.
Some secondarily bilateral
Evolution of the skeleton may account for pentaradial form
(figure 16.2).

6. Figure 16. 2 Pentaradial symmetry
Figure Pentaradial symmetry. (a) Body parts are arranged in fives around an oral-aboral axis.
(b) Arrangement of the body in fives means skeletal joints are not directly opposite one another. This arrangement may make the skeleton stronger than if joints were opposite one another.
(a)
(b)

7. Echinoderm Characteristics
Water vascular system (figure 16.3)
Water-filled canal and tube feet
Ring canal opens to outside via stone canal and madreporite.
Polian vesicles function in water storage.
Tube feet
Muscular ampulla
Often suction cup at distal end (may also be blunt or pointed)
Functions
Locomotion
Attachment
Feeding
Exchanges of respiratory gases and wastes
Sensory functions
Hemal system
Distributes nutrients and large molecules

8. Figure 16.3 The water-vascular system of a sea star.

9. Class Asteroidea Sea stars Hard or sandy substrates
Moveable and fixed spines roughen body surface.
Dermal branchiae (papulae)
Gas exchange
Pedicellariae
Pincerlike
Clean and protect body surface
Tube feet with suction disks

10. Class Asteroidea Maintenance Functions Predators and detritus feeders
Ingest whole prey
Many are bivalve predators
Internal transport of gases, nutrients, and metabolic wastes by diffusion and hemal system
Gas exchange and excretion by diffusion across dermal branchiae
Nervous system
Nerve ring, radial nerves, nerve net
Sensory receptors
Widely distributed over body surface
Photoreceptors at tips of arms (specialized tube feet)

11. Figure 16.4 Body wall and internal anatomy of a sea star.

12. Figure 16.5 Internal structure of a sea star.

13. Regeneration, Reproduction, and Development
Broken arm replaced
Entire sea star from portion of central disk
Asexual reproduction in some
Regeneration after division of central disk
Sexual reproduction
Dioecious
Two gonads per arm (figures )
External fertilization and planktonic larval development (figure 16.6)

14. Figure 16.6 Development of a sea star.

15. Class Asteroidea Sea Daisies Previously class Concentricycloidea
Highly modifies Asteroidea
Lack arms
1 cm diameter
Digestion and absorption of decaying organic matter
Figure A sea daisy (Xyloplax medusiformis).

16. Class Ophiuroidea Basket stars and brittle stars
Arms long, sharply set off from central disk (highly branched in basket stars)
No dermal branchiae or pedicellariae
Tube feet lack suction disks.
Madreporite on oral surface
Muscles and articulating ossicles produce snake-like movements of arms.
Water vascular system is not used in locomotion.

17. Figure 16. 8 Class Ophiuroidea
Figure Class Ophiuroidea. (a) A brittle star (Ophiopholis aculeata). (b) A basket star (Gorgonocephalus arcticus).

18. Class Ophiuroidea Maintenance functions Predators and scavengers
Arms sweep substrate.
Basket stars are suspension feeders.
Wave arms and trap plankton on mucus-covered tube feet
Coelom confined to central disk.
Distribution of nutrients, gases, wastes
Ammonia lost by diffusion across tube feet and bursae.

19. Figure 16.9 Oral view of the disk of the brittle star Ophiomusium.

20. Regeneration, Reproduction, and Development
Lost arms
Autotomy common
Fission across central disk
Sexual Reproduction
Dioecious
Gonads associated with bursa
Gametes released into bursa
Eggs may be retained and fertilized within bursa or released for external fertilization.
Development
Within bursa or as planktonic larvae
Ophiopluteus is planktonic and metamorphoses to adult.

21. Class Echinoidea Sea urchins, sand dollars, heart urchins
Sea urchins—hard substrates
Sand dollars and heart urchins—sand or mud just below surface
Sea urchin skeleton
Test of 10 sets of closely fitting plates
Ambulacral plates with openings for tube feet
Interambulacral plates articulate spines
Pedicellaria with long stalk (may be venomous)

22. Figure 16.10 (a) A sea urchin (Strongylocentrotus).
(b) A sand dollar.
(a)
(b)

23. Class Echinoidea Water-vascular system Spines
Radial canal runs along inner body wall.
Tube feet with ampullae and suction disks
Madreporite opens at aboral surface.
Spines
Locomotion and burrowing

24. Figure 16.11
Internal anatomy of a sea star.
(b) Aristotle’s lantern.

25. Class Echinoidea Maintenance Functions
Feed on algae, bryozoans, animal remains
Aristotle’s lantern (figure 16.11b)
Complete digestive tract (figure16.11a)
Circulation
Coelomic fluids
Gas exchange
Diffusion across gill membrane surrounding mouth (figure 16.11a)

26. Reproduction and Development
Dioecious
Gonads on internal body wall
One gonopore in each of 5 genital ossicles
Sand dollars have 4 gential ossicles and gonopores.
External fertilization and planktonic larvae
Metamorphosis to adult

27. Class Holothuroidea Sea cucumbers
Hard and soft substrates in all oceans
Elongate along oral-aboral axis
One side flattened and “ventral”
Secondary bilateral symmetry
Oral tube feet enlarged and modified as tentacles.
Body wall thick and muscular with microscopic ossicles.

28. Figure 16.12 Class Holothuroidea (Parastichopus californicus).

29. Class Holothuroidea Water-vascular system Madreporite internal
Filled with coelomic fluid
Ring canal encircles oral end.
1-10 Polian vesicles
Radial canals run between oral and aboral poles.
Tube feed with ampulae and suction cups
3 of 5 rows on flattened “ventral” surface used for attachment.

30. Figure 16.13 Internal structure of the sea cucumber, Thyone.

31. Class Holothuroidea Maintenance Functions
Feed on particulate organic matter by sweeping substrate with mucus-covered tentacles
Long, looped intestine
Circulation
Coelomic fluid
Gas exchange
Respiratory trees attach to rectum.
Defense
Toxins in body wall
Evert Cuverian tubules

32. Class Holothuroidea Reproduction and Development Asexual Dioecious
Single gonad and single gonopore
Fertilization external
Planktonic larvae
Eggs may be trapped in female’s tentacles and transferred to body surface for larval brooding.
Asexual
Transverse fission and regeneration

33. Class Crinoidea Sea lilies and feather stars
Most primitive living echinoderms
Extensive fossil record
Sea lilies
Attach permanently to substrate by stalk
Crown
Calyx and arms with pinnules
Mouth and anus open to upper (oral) surface.
Feather stars
Lack stalk
Swimming and crawling
Cling to substrate by cirri when at rest

34. Figure 16.14 A sea lily (Ptilocrinus).

35. Figure 16.15
A feather star (Neometra).

36. Class Crinoidea Maintenance functions Arms used in suspension feeding
Plankton trapped by tube feet
Transported with cilia to mouth
Original function of water-vascular system (?)
Cup-shaped nerve mass with radial nerves to arms
Lack nerve ring

37. Class Crinoidea Reproduction and Development Development Regeneration
Many dioecious
Others monoecious
Protandry common
Gametes released through ruptures in walls of arms.
Development
Planktonic larvae metamorphose to adults.
Some brood larvae on outer surface of arms
Regeneration
As in other echinoderms

38. Further Phylogenetic Considerations
Crinoids most closely resemble oldest fossils.
Mouth-up suspension feeding probably original orientation and function of water-vascular system
Calcium carbonate endoskeleton may have evolved to support filtering arms.
Mobile, mouth-down, free-living lifestyle probably secondarily derived
Ampullae, suction disks, tentacles, and secondary bilateral symmetry adaptations for this mobile life style

39. Figure 16.16 Evolutionary relationships among the echinoderm classes.