“Spiny skin”
Introduction to Phylum
Echinodermata
 Phylum Echinodermata is a group of invertebrates that
includes sea stars, sea urchins, sea cucumbers, and
sand dollars.
Sea star
Sea urchin
Sea cucumber
Sand dollar
Introduction to Phylum
Echinodermata
 The members of this phylum are called echinoderms
 They inhabit marine environments ranging from
shallow coastal waters to deep ocean trenches.
 They vary in diameter from 1cm to 1m and are often
brilliantly colored
Crown of thorns Starfish
Patrick Patiria miniata (Bat Star)
Why is this phylum placed
above the bilateral derived
characteristic when they are
obviously radially symmetrical?
Introduction to Phylum
Echinodermata
 They start off life as bilaterally symmetrical larvae and
develop into radially symmetrical adults. This feature
of development indicates that echinoderms almost
certainly evolved from bilaterally symmetrical
ancestors.
Step 1: Fertilization occurs
Step 2: Each fertilized egg divides into
bipinnaria
Step 3: Free swimming and eats
small plankton
Step 4: It has arms and a suction. Star
attaches to bottom of ocean and
metamorphosis begins developing
into pentaradially symmetrical adult
Introduction to Phylum
Echinodermata
 The fossil record of echinoderms dates back to the
cambrian period, more than 500 million years ago.
Early echinoderms from this period appear to have
been sessile, and biologists believe these animals
evolved radial symmetry as an adaptation to a sessile
existence. Echinoderms later evolved the ability to
move from place to place. Today the vast majority of
the 7,000 or so species can move by crawling slowly
along the ocean bottom.
Introduction to Phylum
Echinodermata
 Echinoderms are deuterostomes (radial cleavage; anus
first). This means that they are animals in which
embryonic development is controlled by genes and the 2nd
opening in the gastrula (embryo) becomes the mouth.
In protostomes (spiral cleavage; first mouth; mollusks,
annelids, arthropods), the 2nd opening becomes the anus.
Why do Echinoderms and
Chordates share a common
ancester?
Introduction to Phylum
Echinodermata
 Because both echinoderms and chordates are
deuterostomes, it is likely that they have a common
ancestor.
 Most echinoderms have a type of radial symmetry
called pentaradial symmetry, in which the body parts
extend from the center along five spokes.
Introduction to Phylum
Echinodermata
 In addition to their pentaradial symmetry,
echinoderms have three other major characteristics
that are not shared by any other phylum:
They have endoskeleton composed of calcium
carbonate plates know as ossicles. The ossicles may be
attached to spines or spicules that protrude through th
skin. In fact, the name echinoderm actually means
“spiny skin”.
2. They have a water vascular system, which is a network
of water-filled canals inside their body
1.
Introduction to Phylum
Echinodermata
3. They have many small, movable extensions of the
water-vascular system called tube feet, which aid in
movement, feeding, respiration and excretion.
Phylum Echinodermata
Taxonomists divide the 7,000 species of echinoderms
into six classes, five of which we will discuss
Class Crinoidea
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Called crinoids, include the sea lilies and feather stars.
The name crinoid means “lily-like.”
About 600 living species.
Sea lilies most closely resemble the fossils of ancestral
echinoderms from the Cambrian period. They are
sessile as adults, remaining attached to rocks or the sea
bottom.
Sessile
Freemoving
Class Crinoidea
 Feather stars, in contrast, can swim or crawl as adults,
although they may stay in one place for long periods.
 In both types of crinoids, five arms extend from the
body and branch to form many more arms – up to 200
in some feather star species.
Fossil specimen of
stalked sea lily
Class Crinoidea
 Sticky tube feet located at the end of each arm filter
small organisms from the water. The tube feet also
serve as a respiration surface across which crinoids
exchange oxygen and carbon dioxide with the water.
 Cilia on the arms transport trapped food to the crinoid’s
mouth at the base of the arms. The mouth faces up in
crinoids, while in most other echinoderms the mouth
faces toward the sea bottom.
Class Ophiuroidea
 Consists of basket stars and brittle stars.
 Distinguished by their long narrow arms, which allow
them to move more quickly than other echinoderms.
 About 2,000 species – the largest echinoderm class.
Basket stars
Brittle star
Class Ophiuroidea
 Brittle stars, so named because the coiled branches of
their flexible arms break off easily, can regenerate
missing parts
 Most basket stars and brittle stars are active at night
and are usually found beneath stones or seaweed or
buried under the sand.
Class Echinoidea
 Consists of about 900 species of sea urchins and sand
dollars.
 Echinoidea means “spinelike”
 Sea urchins are well adapted to live on hard sea
bottoms. They use their tube feet for locomotion and
feed by scraping algae from hard surfaces with the five
teeth that surround their mouth.
Underside of urchin
showing teeth
Class Echinoidea
 In some sea urchins, the spines are flexible, while in
others, they are hollow and contain a venom that is
dangerous to predators as well as swimmers.
Pencil Sea Urchin (flexible spines)
Purple Urchin
Class Echinoidea
 Sand dollars live along seacoasts. They are usually found in
coastal areas and have the flat, round shape of a silver
dollar. Their shape is an adaptation for shallow burrowing.
The short spines on a sand dollar are used in locomotion
and burrowing, and they help clean the surface of the body.
They use their tube feet to capture food that settles on or
passes over their body.
Class Holothuroidea
 Sea cucumbers belong to the class Holothuroidea
 Armless. Live on the sea bottom, where they crawl or
burrow into soft sediment
 About 1,500 species
 The ossicles that make up their endoskeleton are very
small and are not connected to each other, so their
bodies are soft.
Class Holothuroidea
 Modified tube feet form a fringe of tentacles around
the mouth. When these tentacles are extended, they
resemble the polyp form of some cnidarians. That
explains the name of this class, which means “water
polyp.”
 A sea cucumber uses it tentacles to sweep up sediment
and water. It then stuffs its tentacles into its mouth
and scrapes the food off them.
 Unique defensive behavior: Can forcefully eject part of
their internal organs when threatened.
Class Asteroidea
 The sea stars belong to the class Asteroidea, which
means “starlike”
 About 1,500 species
 They live in coastal waters all over the world
 They exist in a variety of colors and shapes and can
have dozens of arms
 They are economically important because they prey on
oysters.