Psolus fabricii Skeletal structures In all echinoderms the epidermis ...

Skeletal structures
In all echinoderms the epidermis covers the body and contains ossicles
In sea urchins and sand dollars, ossicles are attached to each other and form a rigid test.
Muscles are weakly developed.
In sea cucumbers, ossicles are scattered in the fleshy epidermis. Layered muscles are
developed.
In sea stars and ophiurods,
skeletal plates articulate and
allow arm motion.
Psolus fabricii
B-Spines of the sand dollar
Echinarachnius parma
C-Skeletal ossicles from central discs
of 4 ophiurid spp.
D-Ossicles from holothurian Psolus
chintinoides

Skeletal structures
Skeletal plates remain single
or fuse. They give rise to
spines, tubercles, and
pincer-like pedicellariae
(starfish, urchins)
Pedicellariae respond
external stimuli.
Independent from nervous
system. Function: defense,
cleaning, predation
Stylasterias forreri
E-Kinds of echinoid pedicellariae surrounding a spine
F,G-Pedicellariae used for prey capture in starfish Stylasterias forreri
H-Generalized pedicellariae
I-2 types of muscle systems in pedicellariae
J-A movable spine

Support and Locomotion
General body shape maintained by skeletal elements except in sea cucumbers (hydrostatic skeleton)
Crinoids can be sessile (sea lilies) or crawl and swim. They move with oral side away from
substrate and use cirri for temporary attachment or balance
Ophiurids use flexible articulated arms for crawling and clinging. Tube feet no suckers and ampullae
Asteroids use podia. No metachronal waves. Some excrete mucus for defense
Luidia ciliaris
Urchins use both podia and spines.
Holothurians crawl by podia or by action of body wall muscles
Crinoid Antenon
A,B- Resting, moving
C-Starfish with tube feet in
motion
D-Changes in position of
individual podia
Amphiura

Feeding And Digestion
Digestion is both extracellular and
intracellular
ASTEROIDS
1. Ecology: Mostly carnivores,
many feed on bivalves, barnacles,
other starfish.
Tube feet are used to open the
valves
2. Anatomy
Short gut extends from oral mouth
to aboral anus. Mouth in the
center of the peristomial
membrane.
Pyloric ceca (digestive glands) of the pyloric
stomach. The gut lined with ciliated epithelium
3. Digestion: Cardiac stomach is everted to engulf prey
outside the mouth. It can be slipped through the gape
between clam valves. Digestion begins externally
Extracellular digestion occurs in the stomach. Absorption, phagocytosis, and intracellular digestion
occur in the pyloric ceca
4. DOM
30% or food needs met by DOM absorption

1. Ecology: Most are carnivores, deposit feeders, or suspension feeders
Some capture (lasso) small invertebrates with their arms
Many use tube feet to move suspended or deposited particles (food bolus) to the mouth
2. Anatomy: Gut is a blind
pouch with mouth on oral
surface, with no rectum
or anus
Digestion and absorption
occur in the stomach
Tentacular scale near
basis of each podium.
Trap plankton
Amphiura brachiata
OPHIUROIDS

1. Ecology
Regular urchins are grazers
Irregular urchins are deposit
feeders
Aristotle's lantern used to
scrape algae, etc from rocks
It’s a complex masticatory
apparatus, just inside the mouth
with 5 protractible teeth
Made of 5 triangular plates
(pyramids), moved by
comminator muscles
Tube feet are used to move
organic particles to the mouth
2. Anatomy
Mouth at center of oral surface
A long intestine extends to anus
Siphon bypasses the stomach
ECHINOIDS

HOLOTHUROIDEA
1. Ecology
Most are suspension or deposit feeders
Direct deposit feeders ingest sediment
Sediment is pushed into mouth by tentacles
Suspension feeders and selective deposit feeders capture individual
organic particles with adhesive papillae on tentacles
Tentacles are stuffed into the mouth one by one.
Evisceration: Partial or complete expulsion of digestive tract, gonads
Chemical or physical stress, defense mechanism?
Discharge of Cuverian tubules: defensive action, sea cucumber
points the anus at predator, and discharges the tubules by rupturing
the hindgut. All tissues can be regenerated.
2. Anatomy
Body is elongate on oral-aboral
axis
Mouth at oral end, anus at aboral
Mouth surrounded by 10-30
dendritic tentacles
Tentacles are modified tube feet
Respiratory trees-internal
structures where water is
pumped via anus for gas
exchange

Cuvierian tubules released
Neothyonidium magnum
C-Parastichopus feeding-deposit feeder
D-Cucumaria showing feeding tentacles
E-Mucus-secreting papillae on tentacles of Aslia lefevrei
F-Cucumaria minuta
G-Psolidium suspension feeder

1. Ecology: Suspensions feeders similar to ancestral echinoderms
2. Anatomy: Feed with oral surface up, unlike most
Echinoderms. Arms surround the mouth
Arms and ambulacra are branched
Mouth at center of oral surface, anus is also oral,
but displaced to periphery, gut is U-shaped.
3. Feeding
Food particles are
captured by podia
Particles are transferred
to the mouth by ciliary
currents
The original function
of tube feet was for
feeding, not locomotion
CRINOIDS
A-Central disc and base of one arm
B-An arm with open ambulacral (food) groove
C-Oral surface of Antedon. Note alphabetical
position of ambulacral radii

Asexual and sexual reproduction (by far the most common)
A. ASEXUAL
Most echinoderms are capable of regeneration of lost parts. Cut-off arms soon die,
however Linckia can regenerate from single arms
Brittle stars and crinoids cast off whole arms
(autotomy) when disturbed
Reproduction
In asexual reproduction, disk divides (fissiparity)
Best developed in brittle stars and starfish
Arm with 20% of disk can produce
a new star
B. SEXUAL
Gonochoric, few are hermaphrodites
External fertilization. Spawning usually nocturnal.
Both sexes release gametes into the sea
A few species brood; female retains eggs and embryos
Some brittle stars brood in the genital bursae
Linckia guildingi

Sexual Reproduction
Gonads are housed in genital sinuses. Sea Cucumbers with 1 gonad.
Crinoids with no distinct gonads. Gametes arise from the peritoneum of genital canals.
No gonoducts. Gametes are released by rupturing the pinnule walls.
Ophiurids with 1 to many gonads attached next to bursal slits. Gametes are released into
bursae and released though slits
Starfish, urchins with
multiple gonads
Gonoducts lead to
interambulacral
gonopores
Regular sea urchins
with 5 gonads (located
on 5 genital plates
Brooding in all groups
in polar and boreal
species.

Development
A. GENERAL
Classical deuterostome development
Radial, regulative (indeterminate) cleavage
Coeloblastula
Gastrulation by invagination
Enterocoelous coelom and mesoderm formation
Tricoelomic
Blastopore becomes the anus
B. LARVAE
Dipleurula is the first larva (a gastrula)
Becomes a specialized larva for each class
Larvae are plesiomorphically bilaterally
symmetrical, have a recurved gut and transparent
ectoderm, and feed by upstream particle capture
using the ciliated band (planktotrophic).
Metamorphosis is typically radical and occurs
during settlement onto the benthos.
Bilateral larvaradial adult
A-Vitellaria (crinoid)
B,C-Bipinnaria and later brachiolaria of starfish
D-Ophiopluteus of an ophiurid
E-Auricularia of sea cucumber

Phylogeny
Current hypothesis: lineage leading to the Crinoidea branched most basally,
Echinoidea and Holothuroidea are sister taxa.
Current debate: Are Ophiuroidea and Asteroidea sister taxa? The two best-supported
hypotheses are:
"Asterozoan" hypothesis proposed by Bather (1900), and supported by Mooi and David
(1997). Mitochondrial gene order supports this hypothesis (Smith et al. 1993), but crinoid
info is needed to clarify the polarity of the character transformations.
“Cryptosyringid” hypothesis proposed by MacBride (1906). Smith 1988 advocated the
same topology.
Both hypotheses are supported equally, by 18S and 26S rRNA analyses (summarized in
Littlewood et al. 1997).