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CHAPTER 22
Chaetognaths,
Echinoderms and
Hemichordates
22-1
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22-2
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Phylum Chaetognatha: Diversity and Characteristics
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Diversity
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Evolutionary position is still much debated
Pelagic marine predators commonly called
arrow worms
Highly specialized for planktonic existence
Several benthic genera
Horizontal fins bordering the trunk used in
flotation rather than in active swimming
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Phylum Chaetognatha: Diversity and Characteristics
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Form and Function
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Body is unsegmented and includes a head, trunk,
and postanal tail
Head is a large vestibule with teeth flanked by
chitinous spines for seizing prey
Voracious feeders, living on planktonic forms,
especially copepods
Thin cuticle and epidermis covers body
Complete digestive system
Well-developed coelom
Nervous system with a nerve ring
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Phylum Chaetognatha: Diversity and Characteristics
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Ciliary loop may detect water currents or may be
chemosensory
Vascular, respiratory, and excretory systems
absent
Hermaphroditic with either cross- or selffertilization
Embryogenesis
 Suggests deuterostomes affinitities
 However, coelom is formed by a backward
extension from the archenteron rather than by
pinched-off coelomic sacs
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Phylum Chaetognatha: Diversity and Characteristics
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New studies indicate that cleavage planes in fourcell embryos are similar to crustaceans and
nematodes
Some phylogenies based on nucleotide
sequences place chaetognaths within Ecdysozoa,
However, there are no reports of molting
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Clade Ambulacraria
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Superphylum that contains two deuterostome
phyla
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Members share a three-part (tripartite) coelom,
similar larval forms, and an axial complex
Xenoturbella is the sister taxon to Ambulacraria
Body form
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Echinodermata and Hemichordata
Blind gut: coelom and excretory structures absent
No cephalization but does have a diffuse netlike
nervous system
Muscles are present
No structured gonads, but sexual reproduction
occurs
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Phylum Echinodermata: Diversity and Characteristics
Characteristics
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All members have a calcareous skeleton
Spiny endoskeleton consists of plates
Unique water-vascular system
Possess pedicellariae and dermal branchiae
Pentaradial symmetry in adults
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Phylum Echinodermata: Diversity and Characteristics
Diversity
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Ancient group extending back to Cambrian
period
Likely descended from bilateral ancestors
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Larvae are bilateral
Perhaps evolved radiality as an adaptation to
sessile existence
Body plan is derived from crinoid-like
ancestors transformed into free-moving
descendants
Lack ability to osmoregulate
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Restricts them to marine environments
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Phylum Echinodermata: Diversity and Characteristics
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None are parasitic
Few are commensals
Asteroids or sea stars
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Ophiuroids or brittle stars
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Mostly predators
Move by bending their jointed muscular arms
May be scavengers, browsers, or commensal
Holothurians or sea cucumbers
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Mostly suspension or deposit feeders
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Phylum Echinodermata: Diversity and Characteristics
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Echinoids or sea urchins
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Found on hard bottoms while sand dollars prefer
sand substrate
Feed on detritus
Crinoids
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Sessile and flower-like as young and detach as
adults
Suspension feeders
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Phylum Echinodermata: Diversity and Characteristics
Ecology, Economics, and Research
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Due to spiny structure, echinoderms are not
often preyed upon
A few fish and otters are adapted to feed on
sea urchins
Sea stars feed on molluscs, crustaceans, and
other invertebrates
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May damage oyster beds
Artificial parthenogenesis was first described
for sea urchin eggs
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Develop without fertilization if treated with
hypertonic seawater or subjected to other stimuli
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Class Asteroidea
Diversity
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Common along shorelines and may
aggregate on rocks
Some live on muddy or sandy bottoms, or
among coral reefs
Approximately 1500 living species
Range from a centimeter across to about a
meter across and may be brightly colored
Asterias is common on the east coast of the
U.S.
Pisaster is common on the west coast
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Class Asteroidea
Form and Function
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External Features
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Have a central disc with tapering arms extending
outward
Body is flattened and flexible, with a pigmented
and ciliated epidermis
Mouth is on the underside or oral surface
Ambulacrum runs from the mouth to the tip of
each arm
Usually there are 5 arms but there may be more
Ambulacral groove bordered by rows of tube feet
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Class Asteroidea
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Radial nerve located in center of each ambulacral
groove
Under the nerve is an extension of the coelom and
the radial canal of the water-vascular system
In all except crinoids, ossicles or other dermal
tissue covers these structures
Aboral surface is spiny
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At base of spines are groups of pincer-like pedicellariae
 Keep the body surface free of debris
Papulae (dermal branchiae or skin gills) are soft
projections lined with peritoneum
 Function in respiration
On the aboral side is a circular madreporite
 Sieve leading to the water-vascular system.
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Class Asteroidea
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Endoskeleton
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Under the epidermis is the mesodermal
endoskeleton
 Small calcareous plates or ossicles
 Bound together by an unusual form of mutable
collagen termed catch collagen
 Ossicles penetrated by meshwork of spaces
filled with fibers and dermal cells, the steroem
Muscles in the body wall move rays and partially
close ambulacral grooves
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Class Asteroidea
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Coelom, Excretion, and Respiration
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Spacious body coelom filled with fluid is one
coelomic compartment
Fluid contains amebocytes (coelomocytes)
Ciliated peritoneal lining of coelom circulates fluid
around the cavity and into papulae
Respiratory gases and ammonia diffuse across
the papulae and tube feet
Some wastes are picked up by coelomocytes,
which migrate to exterior
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Class Asteroidea
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Water-Vascular System
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This system is another coelomic compartment
and is unique to echinoderms
Consists of system of canals, tube feet, and
dermal ossicles
Functions in locomotion, food-gathering,
respiration, and excretion
Opens to outside at madreporite on aboral side
Madreporite leads to stone canal, which joins
ring canal that encircles the mouth
Radial canals diverge from ring canal and extend
into each ray
4 or 5 pairs of Tiedemann’s bodies attach to ring
canal and may produce coelomocytes
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Class Asteroidea
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Polian vesicles may also be attached
 Function in fluid storage and regulation of
internal pressure of water vascular system
Inner end of each tube foot called an ampulla
 Lies within the coelom
Outer end of each tube foot bears a sucker
Water-vascular system operates hydraulically
 Valves in lateral canals prevent backflow
Muscles in ampulla contract forcing fluid into and
extending the podium
Small lateral canals, each with a one-way valve,
connect radial canal to tube feet
Contraction of longitudinal muscles in tube foot
retracts it, forcing fluid back into the ampulla
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Class Asteroidea
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Small muscles in end of the tube foot raise the
middle section, creating suction
Sea star can move while being firmly adhered to
the substrate
Tube feet are innervated by a central nervous
system
Move in one direction but not in unison
Cutting a radial nerve ends coordination in one
arm
Cutting a circumoral nerve ring stops all
movement
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22-28
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Class Asteroidea
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Feeding and Digestive System
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Mouth leads through a short esophagus to large
central stomach
Lower cardiac part of stomach can be everted
through the mouth during feeding
Upper stomach is smaller and connected by ducts
to a pair of pyloric ceca in each arm
Anus is inconspicuous
Some species lack an intestine and anus
Consume a wide range of food
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Sea urchins
Molluscs
 Sea stars pull valves apart and evert stomach through
crack
Small particles carried up ambulacral grooves to mouth
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Class Asteroidea
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Hemal System
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System of tissue strands enclosing unlined
sinuses
 System itself enclosed in perihemal channels
Hemal system may play a role in distributing
nutrients
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Class Asteroidea
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Nervous System
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The oral system of a nerve ring and radial nerves
coordinate the tube feet
Hyponeural system aboral to oral system forms
ring around anus and extends into roof of each ray
Epidermal nerve plexus coordinates responses of
dermal branchiae to tactile stimulation
Tactile organs are scattered over the surface and
an ocellus is at tip of each arm
 React to touch, temperature, chemicals, and light
intensity
 Mainly active at night
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Class Asteroidea
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Reproductive System, Regeneration, and
Autonomy
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Sexes separate in most sexes
Pair of gonads in each interradial space
Fertilization is external
Eggs and sperm are shed into the water in early
summer
Regenerate lost parts
 Cast off injured arms and regenerate new ones
 An arm can regenerate a new sea star if at least
one-fifth of central disc is present
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Class Asteroidea
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Development
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Quite different among different sea star lineages
In most cases, embryonating eggs are dispersed
in water and hatch to free-swimming larvae
Embryogenesis shows typical primitive
deuterostome pattern
Coelomic compartments, called somatocoels,
arise from posterior blastocoel
Left hydrocoel becomes water-vascular system
Left axocoel becomes stone canal and perihemal
channels
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Class Asteroidea
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Free-swimming larva, bipinnaria, has cilia
arranged in bands
Ciliated tracts become larval arms
Larva grows three adhesive arms and a sucker at
the anterior
 Now called a brachiolaria
Brachiolaria attaches to substrate and undergoes
metamorphosis into a radial juvenile
Arms and tube feet appear, animal detaches from
stalk and becomes a young sea star
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22-38
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Class Asteroidea
Sea Daisies
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Diversity and Characteristics
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Small, disc-shaped animals discovered in deep
water off New Zealand
Described in 1986, only two species are known
Phylogenetic analysis of rDNA places them within
Asteroidea
Pentaradial but have no arms
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22-40
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Class Asteroidea
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Tube feet are located around periphery of the disc
rather than in ambulacral areas
Water-vascular system has an outer ring and a
hydropore homologous to madreporite that
connects inner ring canal to aboral surface
One species has a shallow, sac-like stomach
The other species has no digestive tract
 Velum covers oral surface and absorbs
nutrients
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Class Ophiuroidea
Form and Function
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Largest in number of species
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Over 2000 extant species
Arms of brittle stars are slender and distinct
from the central disc
Lack pedicellariae or papulae
Ambulacral groove is closed and coated with
ossicles
Madreporite is on the oral surface
Tube feet lack suckers and ampullae
Protrusion generated by proximal muscles
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Class Ophiuroidea
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Each jointed arm has a column of articulated
ossicles called vertebrae
Arms are moved in pairs for locomotion
Five movable plates act as jaws and
surround mouth
No anus
Skin is leathery and surface cilia are mostly
lacking
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Class Ophiuroidea
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Visceral organs are in the central disc
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Arms are too slender to accommodate them
Stomach is saclike
No intestine
Water-vascular, nervous, and hemal systems
resemble those of sea stars
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22-47
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Class Ophiuroidea
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Reproduction
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5 invaginations called bursae open to oral surface
by genital slits at bases of the arms
Gonads on wall of each bursa discharge ripe sex
cells into water for external fertilization
Sexes usually separate
 Few are hermaphroditic
Larva is an ophiopluteus
Larva has ciliated bands that extend onto delicate
and beautiful larval arms
Lack any attached phases during metamorphosis
Regeneration and autonomy are more pronounced
than in sea stars
 Very fragile
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22-49
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Class Ophiuroidea
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Biology
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Brittle stars are secretive and live on hard or
sandy bottoms where little light penetrates
 Often under rocks or in kelp holdfasts
Browse on food or suspension feed
Basket stars perch on corals and extend
branched arms to capture plankton
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Class Echinoidea
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Diversity
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Approximately 950 species of living echinoids
Sea urchins lack arms but their tests show fivepart symmetry
Up-folding brings the ambulacral areas up to the
area of the anus
Most sea urchins have a hemispherical shape with
radial symmetry and long spines
Sand dollars and heart urchins (irregular
echinoids) have become bilateral with short spines
Regular urchins move by tube feet
Irregular urchins move by their spines
Echinoids occur from intertidal regions to deep
ocean
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22-52
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22-54
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22-55
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Class Echinoidea
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Form and Function
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Echinoid test has ten double rows of plates with
movable, stiff spines
Tube feet extend along the 5 ambulacral rows
Spines articulate on “ball-and-socket” joints moved
by small muscles at the bases
Among the several kinds of pedicellaria, the threejawed variety on long stalks is most common
Some species have pedicellariae with poison glands
that secrete a toxin that paralyzes small prey
5 converging teeth and sometimes branched gills
encircle the peristome
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22-57
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Class Echinoidea
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Anus, genital pores, and madreporite are aboral
and in the periproct region
Sand dollars and heart urchins have shifted the
anus to the posterior and can be defined bilaterally
Inside the test is Aristotle’s lantern
 Complex set of chewing structures
Ciliated siphon connects esophagus to intestine
 Food can be concentrated in the intestine
Sea urchins are largely omnivorous
 But primary diet consists mainly of algae
Sand dollars filter particles through spines
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22-59
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Class Echinoidea
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Hemal and nervous systems resemble those in
asteroids
Ambulacral grooves are closed and radial canals
run just beneath the test in each radii
In irregular urchins, respiratory podia are
arranged in fields called petaloids on the aboral
surface
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Class Echinoidea
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Reproduction
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Sexes separate
Gametes are shed into sea for external fertilization
Some, including pencil urchins, brood young in
depressions between the spines
Echinopluteus larvae of nonbrooding echinoids live
a planktonic existence before becoming urchins
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Class Holothuroidea
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Diversity
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Approximately 1150 species of holothuroids
As common name suggests, these animals
resemble cucumbers
Greatly elongated in the oral-aboral axis
Ossicles are greatly reduced and body is soft
Some species crawl on the ocean bottom, others
are found under rocks or burrow
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22-63
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Class Holothuroidea
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Form and Function
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Body wall is leathery with tiny ossicles buried in it
 A few have dermal armor
In some, locomotor tube feet are distributed to all
five ambulacral areas
Most have them only on the ambulacra that faces
the substratum
The side that faces the substratum (the sole) has
three ambulacra, adding a secondary bilaterality
All tube feet, except oral tentacles, are absent in
burrowing forms
Oral tentacles are 10–30 tube feet surrounding the
mouth
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22-65
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Class Holothuroidea
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Coelomic cavity has many coelomocytes
Digestive system opens into a cloaca
Respiratory tree also empties into cloaca
Madreporite lies free in the coelom
Hemal system is more developed than in other
echinoderms
Respiratory tree also serves for excretion
Gas exchange also occurs through the skin and
tube feet
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Class Holothuroidea
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Reproduction
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Sexes are separate in most
Some are hermaphroditic
Sea cucumbers have a single gonad
 Considered a primitive character
Fertilization is external and produces freeswimming auricularia
A few brood their young inside the body or on the
body surface
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Class Holothuroidea
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Biology
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Sea cucumbers use ventral tube feet and muscular
body waves to move
Some trap particles on the mucus of tentacles,
ingesting food particles in pharynx
Others graze sea bottom with tentacles
Cast out part of viscera when irritated
 Must regenerate these tissues
 Organs of Cuvier are expelled in direction of an
enemy
 Sticky and have toxins
One small fish, Carapus, uses the cloaca and
respiratory tree of a sea cucumber for shelter
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Class Crinoidea
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Diversity
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Crinoids include both sea lilies and feather stars
Have primitive characters
Far more numerous in fossil record
Unique in being attached for most of their life
Sea lilies have a flower-shaped body at tip of a stalk
Feather stars have long, many-branched arms
 Adults are free-moving but may be sessile
Many crinoids are deep-water species
Feather stars are found in more shallow water
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Class Crinoidea
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Form and Function
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Body disc or calyx covered with a leathery skin or
tegument of calcareous plates
The 5 arms branch to form more arms, each with
lateral pinnules as in a feather
Calyx and arms form a crown
Sessile forms have a stalk formed of plates
 Appears jointed and may bear cirri
Madreporite, spines, and pedicellariae are absent
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Class Crinoidea
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Upper surface has a mouth that opens into an
esophagus and intestine
Tube feet and mucous nets allow it to feed on
small organisms in the ambulacral grooves
Has a water-vascular system, an oral ring, and a
radial nerve to each arm
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Class Crinoidea
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Reproduction
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Sexes are separate
Gonads are masses of cells in the genital cavity of
arms and pinnules
Gametes escape through ruptures in the pinnule
wall
Some brood eggs
Doliolaria larvae are free-swimming before they
become attached and metamorphose
Most living crinoids are 15–30 centimeters long
Some fossil species had stalks 20 meters long
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Phylogeny and Adaptive Diversification
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Phylogeny
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Fossil record is extensive but there are many
theories about their evolution
From the larvae, we know the ancestor was
bilateral and the coelom had three pairs of spaces
One theory states sessile groups derived
independently from free-moving adults with radial
symmetry
Traditional views consider the first echinoderms
sessile and radial, giving rise to free-swimming
forms
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Phylogeny and Adaptive Diversification
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Early forms may have had endoskeletal plates
with stereom structure and external ciliary
grooves
Carpoids may be an extinct variation, or a
separate subphylum
Echinoids and holothuroids are related
The relationship of ophiuroids and asteroids is
controversial
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Phylogeny and Adaptive Diversification
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Adaptive Diversification
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If ancestors had a brain and sense organs, these
were lost in adoption of radial symmetry
Current evidence suggests that the oral surface is
anterior and the aboral surface is posterior
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Indicates the arms represent lateral growth zones
Basic body plan has limited evolutionary
opportunities to become parasites
Only the most mobile ophiuroids have any
commensal species
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Phylogeny and Adaptive Diversification
Classification
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Subphylum Pelmatozoa
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Class Crinoidea
Subphylum Eleutherozoa
Class Asteroidea
 Class Ophiuroidea
 Class Echinoidea
 Class Holothuroidea
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Phylum Hemichordata
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Diversity and Characteristics
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Formerly considered a subphylum of chordates
based on presence of gill slits and a rudimentary
notochord
The “notochord” is really a buccal diverticulum
 A stomochord, and not homologous to
chordate notochord
Vermiform bottom dwellers, living in shallow
waters
 Most are sedentary or sessile
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Phylum Hemichordata
Class Enteropneusta
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Form and Function
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Wormlike acorn worms
Mucus-covered body
Active proboscis
 Collects food in mucous strands
Cilia carry particles to groove at the edge of the
collar, then to mouth
Thrust proboscis into mud and ingest mud to
extract the organic matter
Buccal diverticulum connects protocoel with a
proboscis pore to the outside
Contraction of body musculature
 Forces excess water out through gill slits
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22-83
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Phylum Hemichordata
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Roll of gill pores is part of branchial system that
connects with series of gill slits in sides of
pharynx
Primarily ciliary-mucus feeders using U-shaped
gill slits
Middorsal vessel expands into a sinus and heart
vesicle above the buccal diverticulum
Blood enters network of blood sinuses called
glomeruli and then through an extensive system
of sinuses to the gut and body wall
Nervous system consists mostly of a subepithelial plexus
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22-85
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Phylum Hemichordata
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Dorsal nerve cord (neurochord) formed by an
invagination of ectoderm
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Hollow in some species
Sexes are separate
Fertilization is external
In some species a ciliated tornaria larva develops
similar to that of echinoderm larva
At least one species undergoes asexual
reproduction
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22-87
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Phylum Hemichordata
Class Pterobranchia
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Form and Function
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Basic plan similar to that of Enteropneusta
Small animals, usually 1 to 7 mm in length
Many individuals may live together in collagenous
tubes
 Zooids are not connected
Body divided into three regions
 Proboscis, collar, and trunk
Ciliated grooves on tentacles and arms collect food
Both dioecious and monoecious species
Asexual reproduction is by budding
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22-89
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22-90
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Phylum Hemichordata
Phylogeny and Adaptive Diversification
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Phylogeny
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Phylogeny far from being completely understood
Share characters with both echinoderms and
chordates
 Pharnygeal slits (chordates)
 Diffuse epidermal nervous system
(echinoderms)
 Tripartite coelom (echinoderms)
Buccal diverticulum is now believed to be a
synapomorphy of hemichordates only
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Phylum Hemichordata
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Early embryogenesis of hemichordates
remarkably like echinoderms
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Suggests that echinoderms are a sister group of
hemichordates
Tornaria larva almost identical to bipinnaria
larva of asteroids
Sequence analysis of the gene encoding the
small subunit of rRNA
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Supports a deuterostome clade
 Echinodermata, Hemichordata, and Chordata
Supports placement of Chaetognaths among
protostomes
 Possible that Chaetognaths are neither
protostomes nor deuterostomes but originated
independently from an early coelomate lineage
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Phylum Hemichordata
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Adaptive Diversification
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Undergone little adaptive divergence and have
retained a tentacular type of ciliary feeding
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Enteropneusts
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Possibly because of sedentary lifestyles
Lost tentaculated arms
Use a proboscis to trap small organisms in mucus
Have diversified only slightly
Recent molecular evidence suggests that
pterobranchs are derived from within the
enteropneust lineage