44.0 Introduction

1. Echinoderms Exhibit Bilateral and Radial Symmetry at Different Times fig 44.1
1. Part of the Coelom Becomes the Water Vascular System
2. Some Have an Endoderm of Dermal Plates

44.1 The embryos of deuterostomes develop quite differently than those of protostomes

1. Protostomes and Deuterostomes
1. Comparing Two Distinct Kinds of Coelomate Animals
1. Protostomes
1. Include mollusks, annelids, arthropods
2. Mouth (stoma) develops from or near blastopore fig 44.2a
3. Same pattern seen in all noncoelomates
4. Anus develops in another region of embryo
5. Original state was characteristic of common ancestor of all eumetazoans

2. Deuterostomes
1. Includes echinoderms, chordates, few other related phyla
2. Anus forms at or near blastopore fig 44.2b
3. Mouth develops from another region of blastula
4. Derived from protostome pattern of development

2. Other Fundamental Differences Between Deuterostomes and Protostomes
1. Present two different cleavage patterns
1. Protostomes exhibit spiral cleavage
1. New cell buds off at oblique angle
2. Produces closely packed array of cells
3. Line drawn through sequence of divisions produces a spiral from axis

2. Deuterostomes exhibit radial cleavage
1. Cells divide parallel to and at right angles to polar axis
2. Produces loosely packed array of cells
3. Line drawn through sequence of division produces a radius outward from axis

2. Differences in developmental fate of cells
1. Protostome cell fate is fixed when that cell first appears
1. Individual cells will not develop into complete animal if separated
2. Chemicals controlling developmental signals are localized early

2. Deuterostome cell fate is fixed later in development
1. Daughter cells from early divisions are totally identical
2. Cells from early stages can become complete individuals
3. Commitment to developmental pathway occurs later

3. Differences in development of coelom from mesoderm
1. Occurs simply and directly in protostomes
1. Cells move away from one another
2. Coelomic cavity expands within mesoderm

2. Complex development in deuterostomes
1. Groups of cells move around forming new tissue associations
2. Coelom produced from invagination of archenteron
3. Cavity opens outward via blastopore, becomes gut

4. Deuterostomes clearly derived from protostomes early in their evolution

44.2 Echinoderms are deuterostomes with an endoskeleton

1. The Echinoderms
1. Echinoderms Were the First Deuterostomes
1. Ancient group of exclusively marine animals, well-represented in fossil record
2. Name "spiny skin" refers to hard endoskeleton just beneath delicate skin fig 44.4
1. Endoskeleton composed of calcium-rich plates
1. Are totally encased in living skin when first formed
2. Fuse forming hard shell in adults

3. Possess a unique water vascular system
1. Hydraulic system to aid movement
2. Central ring with five radial canals

4. Include sea stars, brittle stars, sea urchins, sand dollars, sea cucumbers
5. Echinoderms exhibit radial symmetry as adults
1. Other animals are radially symmetrical
2. Lack complex organ systems of adult echinoderms

6. Origin of phylum is unclear
1. May have evolved from bilateral ancestors since embryos are bilateral
2. Radial symmetry develops only in adult form
3. Symmetry may be related to mobility
1. Bilateral symmetry is important to highly motile organisms
2. Early forms were probably sessile, radial symmetry is valuable

4. Early echinoderms attached to the sea floor with a stalk

2. Echinoderm Body Plan
1. Body Plans Shift During Development
1. Echinoderms exhibit secondary radial symmetry
1. Are bilaterally symmetrical as larva
2. Become radially symmetrical as adults

2. Axis of sea cucumbers is horizontal, radial symmetry not as obvious
3. Have a five-part body plan
4. Adults have no head or brain
1. Nervous system composed of circular nerve ring and its branches
2. Capable of complex response patterns, but lack centralized functions

2. Endoskeleton
1. Have epidermis stretched over an internal skeleton
2. Endoskeleton composed of ossicles: Movable or fixed calcium plates
3. Plates enclosed within living tissue when first formed
1. Bear spines as indicated by phylum's name
2. Have perforations through which tube feet extend

3. The Water Vascular System
1. Five radial canals extend from a ring canal around the esophagus
2. Radial canals determine basic five-part symmetry fig 44.5
1. Water enters through madreporite, a sievelike plate
2. Flows to ring canal through the tubular stone canal
3. Radial canals extend out into hollow tube feet fig 44.6
4. Some echinoderms have suckers at end of tube feet, others do not
5. Each tube foot has a muscular fluid-filled ampulla at its base

3. Operation of the water vascular system
1. Ampulla contracts, fluid can't enter ring canal due to one way valve
2. Fluid forced into tube foot, extends it
3. Foot can attach to substrate
4. Longitudinal muscles contract and shorten tube foot
5. Water forced back into ampulla
6. Repeated movement results in locomotion

4. Special adaptations in some classes
1. Sea cucumbers fig 44.3b
1. Have five rows of tube feet along body for locomotion
2. Tube feet around mouth are modified for feeding

2. Sea lilies
1. Tube feet arise from branched arms
2. Take food from surrounding water

3. Brittle stars
1. Tube feet pointed
2. Specialized for feeding fig 44.33d

4. Body Cavity
1. Large coelom
1. Interconnected with complicated system of tubes
2. Helps provide for circulation and respiration

2. Respiration and waste removal occur through skin gills, projections near spines
3. Digestive system is simple but complete: Mouth, gut and anus

5. Reproduction
1. Capable of extensive regeneration
1. Sea and brittle stars may drop parts when attacked
2. May reproduce asexually by splitting into parts

2. Sexual reproduction and fertilization is external
1. Sexes are separate, but difficult to distinguish externally
2. Develop into free-swimming, bilaterally symmetrical larvae fig 44.7
3. Significantly different from annelid/mollusk trochophore larvae
4. Larvae are free swimming, metamorphose into sessile in adults

44.3 The six classes of echinoderms are all radially symmetrical as adults

1. Diverse Extinct and Living Groups
1. Twenty Extinct Classes
2. Six Living Classes
1. Crinoidea includes sea lilies and feather stars
2. Asteroidea includes sea stars
3. Ophiuroidea includes brittle stars
4. Echinoidea includes sea urchins and sand dollars
5. Holothuroidea includes sea cucumbers
6. Concentricycloidea includes recently discovered sea daisies

2. Class Crinoidea: The Sea Lilies and Feather Stars
1. Basic Biology
1. Mouth and anus located on upper surface in an open disk
2. Simple excretory and reproductive systems
3. Extensive water vascular system
4. Large numbers of highly branched arms located around central disk
1. Species may have 5 to 200 arms
2. Smaller pinnules branch from each arm

5. Filter feeders, food collected by mucus from tube feet on pinnules
6. Common ancestor of echinoderms may have resembled crinoids
1. Sessile, sedentary radial animals
2. Fossils abundant, ten times more than living species

2. Sea Lilies
1. One of two basic crinoid body plans
1. Flower-shaped body attached to a substrate by a stalk fig 44.8
2. Stalk is usually 15 to 30 cm, may be as long as 1 meter

2. May move slowly with featherlike arms if detached from stalk
3. All species found deeper than 100 meters
4. Only fully sessile living echinoderms

3. Feather Stars
1. Second of two basic body plans
1. Disks detach from stalks early in development fig 44.9
2. Attach to substrate by claw-like structures on branched arms

2. May swim short distances and move along substrate
1. Found in shallower waters
2. Abundant, with sea cucumbers, in warm waters and coral reefs in Pacific Ocean

3. Sexes separate, external fertilization
1. Sex organs within cavities on arms and pinnules
2. Females occasionally brood young

3. Class Asteroidea: The Sea Stars
1. General Biology
1. Most familiar echinoderms, the "starfish"
1. Active, important marine predators
2. Abundant in intertidal zone, also found at great depths

2. Arms prominent merge gradually with disk
1. Generally five in number, or multiples of five
2. Body flattened, flexible, covered with pigmented epidermis

2. Endoskeleton
1. Calcium containing plates found beneath epidermis
1. Called ossicles
2. Bound together with connective tissue

2. Spines project from ossicles
1. Minute pincerlike pedicellariae surround base of spines
2. Possess tiny muscle-operated jaws
3. Keep body free of debris, may help capture food particles

3. Water Vascular System
1. Underside of each arm has deep groove running along its length
1. Bordered by rows of locomotive tube feet
2. Radial canal connects tube feet to ring canal in central disk

2. Unique hydraulic system
1. Ampulla contracts and forces water into tube foot
2. Extends podium by force of water
3. Muscles in tube foot contract and force water back into ampulla
4. Small muscles at end of tube foot contract forming suctioncup structure

3. Tube feet contracting or extending in unison move arm along surface

4. Feeding
1. Mouth located in center of the lower surface
2. Often feed on bivalve mollusks fig 44.11
1. Grasp shell with tube feet
2. Extrude stomach into opening between shells as small as 0.1 mm
3. Secretes enzymes, digests soft parts of bivalve

5. Reproduction
1. Sexes separate, external fertilization
2. Pair of gonads located within ventral regions of each arm
3. Some species brood young in special cavities or underneath adult
4. Larvae possess conspicuous bands of cilia

4. Class Ophiuroidea: The Brittle Stars fig 44.33d
1. General Ecology
1. Largest class by number of species, most abundant fig 44.12
2. Found in shallow water and deep in seas
1. One of most abundant animals in deep seas
2. Secretive, avoid light, active in dark

2. General Biology
1. Slender arms more sharply set off from central disk than sea stars
2. Move by active movement of their arms along the substrate
1. Arms may be covered with spines that aid in movement
2. May use arms to swim through water

3. Capture suspended particles with tube feet, long spines or arms
1. Tube feet are important sensory organs
2. Tube feet may help move food to mouth

4. Arms detach readily, helping protect animals from predators
5. Distinctly different from sea stars
1. Lack pedicellariae
2. Groove on arm is closed and covered with ossicles
3. Tube feet lack ampullae and suckers, used for feeding not locomotion

6. Reproduction
1. Separate sexes, gametes released into water, external fertilization
2. Some may brood young and release larval stages
3. Larvae are free-swimming with bands of cilia

5. Class Echinoidea: The Sea Urchins and Sand Dollars
1. Comparison to Other Echinoderms
1. Lack distinct arms, still have five-part body plan fig 44.13
2. Five rows of tube feet protrude from plates of skeleton
3. Possess distinct openings for mouth and anus
4. Endoskeletons are made up of fused calcareous plates
1. Sea urchins are globular in shape
2. Sand dollars are flattened

2. General Biology
1. Walk along substrate with tube feet or movement of spines
2. Feed on algae, debris scraped off the surfaces by triangular teeth
3. Reproduction similar to other echinoderms
1. Some brood young, others have free-swimming larvae
2. Larvae have cilia on long arms, unlike those of other classes

6. Class Holothuroidea: The Sea Cucumbers
1. Comparison to Other Echinoderms
1. Soft, sluglike organisms with tough, leathery outer skin fig 44.14
2. Most lie on sides at the bottom of the ocean
3. Mouth is located on one end, anus on other
1. Mouth surrounded by tube feet modified into tentacles
2. Tentacles secrete mucus to trap food particles

2. General Biology
1. Calcareous skeleton reduced to widely separated, microscopic plates
2. Have highly branched respiratory trees that originate from cloaca
1. Water brought into and out of cloaca by muscular contractions
2. Gas exchange occurs across the respiratory trees

3. Most have separate sexes, some forms are hermaphroditic
4. Have tube feet on body, may be restricted to five grooves
1. Move by tube feet or wriggling of the entire body
2. Most are sluggish, some swim actively

5. May eject portion of intestines when irritated

7. Class Concentricycloidea: The Sea Daisies
1. Most Recently Discovered Echinoderms
1. Disc-shaped animals less than 1 cm diameter
2. Two species found in deep waters off New Zealand fig 44.15

2. General Biology
1. Five-part radial symmetry, lack arms
2. Tube feet located along edge of disk, not along radial lines
3. Unusual digestive systems
1. One species has saclike stomach no intestine or anus
2. Other species lacks digestive tract, absorbs nutrients via membrane on mouth

44.4 Arrow worms and acorn worms are two minor phyla

1. Arrow Worms
1. General Ecology of Phylum Chaetognatha
1. Abundant predators in marine plankton fig 44.16a
2. Most common in warm, shallow seas
3. Dart rapidly in both directions
4. Prey on copepods, medusae, fish larvae

2. General Biology
1. Small translucent animals, up to 7 cm long as adults
2. Bilaterally symmetrical
1. Divided into head, trunk and tail by transverse septa
2. Have powerful jaws, head ringed with many sharp hooks fig 44.16b

3. Embryology is deuterostomic
1. Lack any close relatives
2. Virtually unchanged from first appearance in fossil record

2. Acorn Worms
1. Phylum Hemichordata Means "Half Chordate" fig 44.17
2. Deuterostomes that Share Features with Echinoderms and Chordates
1. Ciliated larvae resemble those of sea stars
2. Possess dorsal and ventral nerve cords
1. Part of dorsal cord is hollow
2. Feature found only in chordates

3. Has pharynx with perforations called pharyngeal slits like chordates
4. Soft-bodied tubular shaped animals
1. Live in burrows in sand or mud
2. Range between 2.5 cm and 2.5 meters long
3. Body divided into proboscis, collar and trunk