Transcript Lecture #13

Lecture #13
Phylum Echinodermata
Features common to Phylum
1. triploblastic
2. pentaradial symmetry in adults; bilateral symmetry in larvae
3. coelomate
4. endoskeleton made of calcerous plates called ossicles
5. water vascular system for locomotion
6. complete digestive tract
7. hemal system derived from the coelom – fluid filled canals,
function unknown
• 8. nervous system consisting of a nerve net, a nerve ring and
radial nerves
Phylum Echinodermata
• pentaradial symmetry
– body parts are arranged in fives or multiples of fives around an oral-aboral
central axis
– adult form is pentaradial
– larvae have bilateral symmetry
• internal/endoskeleton made of calcium carbonate
– consisting of calcium carbonate plates called ossicles
– skeleton can be modified to form articulating or fixed spines that project
from the body surface
Water vascular system
• water-filled canals
• extensions are called tube feet
• includes a ring canal that surrounds the
– ring canal opens to the outside through a
stone canal and a sieve-like pore called the
• five (or a multiple of five) radial canals
branch from the ring canal and run down
each arm or arch down the body wall
toward the aboral end
• multiple lateral canals branch off of the
radial canals and end in tube feet
-tube feet
• extensions off the lateral canals
• emerge to the outside through
openings between the ossicles
• inside the body they end as a bulblike,
muscular ampulla
• when the ampulla contracts- forces water
into the tube which then extends
• outside the body - tube feet often
have suction cups at its distal end
• function of tube feet: locomotion
tube feet:
Phylum Echinodermata
• approximately 7,000 species
• all are marine
• living echinoderms are classified into 6 classes
– 1. Asteroidea – sea stars
– 2. Ophiuroidea – brittle stars and basket stars
– 3. Echinodea – sea urchins and sand dollars
– 4. Holothuroidea – sea cucumbers
– 5. Crinoidea – sea lilies and feather stars
– 6. Cocnentricycloidea – sea daises
Class Asteroidea
• sea stars
• about 1,500 species
• live on hard substrates in marine
• five arms radiating from a central disc
• mouth & tube feet are found on the
ventral side (oral surface) of this disc
• endoskeleton comprised of ossicles
• moveable spines project from the
ossicles – on the dorsal or aboral
• dermal branchiae – thin folds of the
body wall found between ossicles
– function in gas exchange and
excretion of metabolic wastes
Class Asteroidea
• in some species – the aboral surface has pincer-like
appendages called pedicelleriae
• series of ossicles on the oral surface of each arm form an
ambulacral groove
– each groove houses a radial canal
– paired rows of tube feet protrude through the body wall
on either side of the groove
ambulacral groove
Class Asteroidea
• locomotion: tube feet move in a stepping motion
– contraction of ampulla moves fluid into the tube foot
– alternate extension, attachment, and contraction of these feet move the sea star
across its substrate
– suction discs on the tube feet allow effective adhesion in strong currents
Class Asteroidea
• nervous system:
– consists of a nerve ring
that encircles the mouth
– plus radial nerves down
each arm or running
down the body wall
toward the aboral end
• responsible for
coordination of the tube
– body wall also houses a
nerve net
– sensory receptors are
distributed over the
surface of the body and
the tube feet
• digestion and feeding:
– mouth surrounded by moveable oral
– mouth opens to a short esophagus
– esophagus leads into a larger cardiac
stomach (oral stomach) for the receipt
of food
– then into a smaller pyloric stomach
(aboral stomach)
– pyloric stomach leads into digestive
glands - called pyloric cecae – 2 per arm
• secretory and absorptive functions
– large, undigested materials are expelled
out through the mouth NOT the anus
Class Asteroidea
sea stars feed on bivalves by prying open the shell
tube feet attach to the outside of the shell and forces the valves apart
once open slightly - everts the cardiac portion of the stomach into the mollusc
releases digestive enzymes and partial digestion weakens the bivalves adductor
muscles - the sea star then pulls the bivalve open completely
• takes in the partially digested mollusc for continued digestion in the cardiac
• reproduction:
– dioecious
– two gonads present in each arm – open via gonopores at the base of the
– external fertilization
– gamete release is timed with pheromone release that induces other sea
stars to spawn – increases chances of fertilization
– embryology has been studied extensively – easy to spawn and fertilize in
the lab
• form a bilateral embryo = bipinnaria larva
• float around in the plankton
• development of radial arms results in a branchiolaria larva – develops into a
juvenile sea star
– regenerative powers are remarkable – can regenerate entire radial arms
Class Echinoidea
sea urchins and sand dollars
about 1,000 species
nearly all marine
sea urchins specialized for living on hard
substrates – wedging themselves into cracks and
• sand dollars – burrow into sand or mud
– use their tube feet to catch organic matter settling on
them or passing over them
• sea urchin:
– skeleton is called a test
– 10 closely fitted plates that arch between
oral and aboral ends
– five rows of ambulacral plates have
openings for tube feet
– alternate with 5 rows of interambulacral
plates with spines
– pedicelleriae & moveable spines - for prey
capture and cleaning
– water vascular system is similar to sea stars
– radial canals run along the inner wall –
between the oral and aboral ends
some spines are hollow and
contain venom to kill prey
• feeding and digestion:
– feed on algae, bryozoans, corals and other
– oral end with a mouth and oral tube feet –
faces toward the substrate
– possess a chewing apparatus called
Aristotle’s lantern
• can be projected from the mouth
• cuts food into small pieces for ingestion
• gas exchange: diffusion across the tube feet
• similar nervous system to the sea stars
• reproduction and development:
– dioecious
– five gonads along the body wall of
interambulacral plates
– the gonads open at ossicles found at the aboral
end - called genital plates
– external fertilization
genital plate with
Class Holothuroidea
sea cucumbers
about 1,500 species
found at all depths of the ocean
have no radial arms
elongated along the oral-aboral axis
lie on one side which is flattened – called the ventral side
thick and muscular body wall
– lacks spines or pedicelleriae
• outer epidermis covering a dermis with embedded ossicles
• 10 larger ossicles form a ring canal that surrounds the oral end
• below the dermis – layer of circular muscle overlying longitudinal
• water-vascular system is filled with coelomic fluid
• ring canal – surrounds the oral end of the digestive
– 5 radial canals arise from the ring canal – give rise to
tube feet that run from oral to aboral poles
• ventral side has 3 rows of tube feet used for
• dorsal surface has two rows of tube feet – may be
absent in some
• tube feet surrounding the mouth – can elongate and
are often referred to as tentacles
• internal madreporite
– connected to the ring canal via a stone canal
tube feet
• digestive system – ingest matter using their tentacles
– mouth leads to a stomach plus a long, looped
– then leads to a rectum and an anus
– tentacles are thrust into their mouths to “wipe off”
the food trapped in mucus
– extracellular digestion in the intestine
– hemal system for food distribution??
• nervous system – similar to other echinoderms
• Polian vesicles
– maintain fluid pressure and volume in the water vascular
• respiratory trees for gas exchange
– attach at the rectum and branch throughout the
– pumping action of the rectum circulates water
through these trees
– water is drawn into these trees – gases diffuse into
the colemic fluid surrounding the trees
– fluid is circulated through the coelom for
distribution of respiratory gases, wastes and