Chapter 16

The Echinoderms

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

• Characteristics1. Calcareous endoskeleton (ossicles)2. Adults with pentaradial symmetry3. Water-vascular system4. Complete digestive tract5. Hemal system6. Nervous system consisting of nerve net, nerve ring,

and radial nerves

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

Figure 16.1 Evolutionary relationships of the echinoderms to other animals.

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).

Figure 16.2 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)

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

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

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

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)

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

Figure 16.5 Internal structure of a sea star.

Regeneration, Reproduction, and Development

• Regeneration– 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 16.4 -16.5)– External fertilization and planktonic larval

development (figure 16.6)

Figure 16.6 Development of a sea star.

Class Asteroidea

• Sea Daisies– Previously class

Concentricycloidea– Highly modifies

Asteroidea• Lack arms• 1 cm diameter• Digestion and

absorption of decaying organic matter

Figure 16.7 A sea daisy (Xyloplax medusiformis).

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.

Figure 16.8 Class Ophiuroidea. (a) A brittle star (Ophiopholis aculeata). (b) A basket star (Gorgonocephalus arcticus).

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.

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

Regeneration, Reproduction, and Development

• Regeneration– 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.

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)

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

(a)

(b)

Class Echinoidea

• Water-vascular system– Radial canal runs along inner body wall.– Tube feet with ampullae and suction

disks–Madreporite opens at aboral surface.

• Spines– Locomotion and burrowing

Figure 16.11 (a)Internal anatomy of

a sea star. (b) Aristotle’s lantern.

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)

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

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.

Figure 16.12 Class Holothuroidea (Parastichopus californicus).

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.

Figure 16.13 Internal structure of the sea cucumber, Thyone.

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

Class Holothuroidea

• Reproduction and Development– 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

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

Figure 16.14 A sea lily (Ptilocrinus).

Figure 16.15 A feather star (Neometra).

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

Class Crinoidea

• Reproduction and Development– 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

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

Figure 16.16 Evolutionary relationships among the echinoderm classes.