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Marine Biology Phylum Echinodermata
I. Echinodermata (spiny skin) - the only major group of animals that are all confined to the
marine world. Contains over 5000 species
A. General characteristics 1. Radial symmetry - they actually have pentamerous radial symmetry (pentaradial/fivesided) in the adult stage and bilateral symmetry in the larval stage.
a. Dipleurula larva - a free living form
b. They undergo metamorphosis
2. Hydrovascular (water vascular system - a unique feature not found in any
other group of animals.
a. This system was once used to carry food to the mouth, but is now also used for
locomotion and to open the shells of bivalves for food.
b. The water vascular system is particularly important to echinoderms because it
enables locomotion.
3. Echinoderms have no brain.
a. Their nervous system follows the same pattern as their water vascular
system.
b. There is a circumoral ring with radial nerves extending into each arm.
4. Body plan - they contain a central disc with a true coelom (see-lum)(hollow body cavity)
and usually 5 or multiples of 5 arms extending freom the central disc.
a. Endoskeleton - in most cases these animals have an internal skeleton made
of calcium carbonate (a calcareous network called stereom).
b. Ossicles - these are the calcium carbonate plates which are fused together to form
the shell (internal skeleton)
5. In the adult stage they are all benthic organisms
6. Echinodermata encompasses a variety of lifestyles. Starfish and sea urchins are
abundant in shallow water while brittle stars and sea cucumbers crave the deep sea.
a. Sea urchins scrape the algae off rocks
b. Starfish are predatorial (predators)
c. Sea lilies are filter-feeders
d. Sea cucumbers, brittle stars, and sand dollars can burrow and feed on suspended
debris in the substrate.
7. Digestive system - well developed, with mouth, intestines, and anus
(Sub-class Ophiuroidea Brittle Stars and Basket stars have no anus)
8. Gas exchange - structures vary from class to class
9. No excretory system - organs are absent
10. Reproduction - all are dioecious, and carry out sexual external fertilization.
a. Eggs and sperm are released into the water.
b. A few are hermaphroditic (monoecious).
B. We will discuss the following classes of Echinoderms:
1. Class Stelleroidea - two subclasses: Asteroidea (starfish) and the Ophiuroidea
(brittle stars). Some authors have treated these as separate classes but, more recently,
they have been combined into the Stelleroidea.
a. Sub Class Asteroidea (sea stars)
b. Sub Class Ophiuroidea (brittle stars)
2. Class Echinoidea (sea urchins, sand dollars, and Heart urchins)
3. Class Holothuroidea (sea cucumbers)
4. Class Crinoidea (sea lilies and feather stars)
5. Class Concentriclycoidea
II. Class or Sub Class Asteroidea (as'ter-oi'de-a)(sea stars) gr. Aster, star + eidos,
form, +ea, characterized by)
These are the common sea stars. Their body consists of a central disc attached to five or more
arms. They are important predators in many marine systems. In some instances, rapid
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population increases can have detrimental effects on coral reefs, greatly reducing the
abundance and diversity of other important invertebrates. Sea stars are also unique in their
ability to regenerate an entire body from a single arm.
A. External structure - sea stars are typically pentamerous, with most containing 5 or
multiples of 5 arms.
1. Oral Surface – The ventral or underside of the sea star. The following
structures are located on the oral surface:
a. The mouth is located in the central disc on the oral surface
b. Ambulacral grooves - these grooves are located on the oral surface, and
house the tubed feet (podia).
c. The tubed feet (podia) are guarded by moveable spines around the margin of
the grooves.
d. There are also immoveable spines around the margin of the grooves.
e. The tubed feet are also used for locomotion
2. Aboral surface - the top or dorsal surface of the organism containing the
madreporite (sieve plate), anus and skin gills.
3. Light sensitive eye spots - at the tip of each arm are small tentacles that
contain a small pigment spot which is light sensitive
B. Body - the term spiny skin applies to these animals because of the presence of small subdermal spines and specialized pinchers called Pedicellariae, which capture very small
animals and carry them to the mouth.
1. The outer surface of the sea star is covered by a layer of epidermis, with many ciliated
cells.
2. This layer of epidermis is covered by a cuticle.
3. The epidermis has neurosensory cells connected to mucus glands.
4. When detritus falls on the body, it is coated with mucus, and the ciliated cells sweep the
particles away.
C. Endoskeleton - the calcareous plates called ossicles are different sizes and shapes, with
each species having its own definite pattern.
1. The ossicles are joined together by muscle fibers and connective tissue, so the
animal is very flexible.
D. Nutrition - asteroids are all carnivorous and feed on snails, bivalves,
crustaceans, polycheates, coral, and other echinoderms. (Mainly bivalves)
1. They will also scavenge and feed on the bodies of dead organisms
2. They have the ability to extrude (Evert) their stomachs out of their mouths and
externally digest.
a. They use their tubed feet to pry open the shells of bivalves
b. They secrete digestive enzymes which reduce meat to fluid (fluid).
3. They contain 2 stomachs
a. The lower cardiac stomach can be everted
b. The upper pyloric stomach is smaller and connects with digestive ceca in
the arms
4. Extracellular digestion occurs in digestive ceca
a. Because foods are almost completely digested before being absorbed by the sea star,
the intestine and anus are greatly reduced in size and function, (in other words,
very little material taken in by the seastar is made of solid undigestible particles
that might pass through the intestines and anus).
b. Some species lack an intestine and anus
5. Digested food is transported by the coelomic fluid, which fills the "hollow" spaces in the
arms and central Disc
E. The Hydrovascular system (4 important functions are movement with tube feet, gas
exchange, excretion of wastes and dispersal of sex cells)
1. Flow of water through a sea star - water enters through the seive plate (madreporite)
and then travels to the stone canal (a small tube). The stone canal leads to the ring
canal (around the mouth) and on out to the arms by way of the radial canals. Then
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into the balloon-like ampulla and finally to the tube feet.
a. A series of small lateral canals, each with a one-way valve, connects the radial canal
to the cylindrical podia or tubed feet, along the sides of the ambulacral groove in
each ray (arm).
b. Tubed feet are located along the radial canals and can be extended or
contracted by ampulla (ballon-like structures on the internal side).
c. Each podium is a hollow, muscular tube, and some species contains a sucker
others do not.
2. They can enlarge and suck water out of the tubed foot to contract or shorten it,
a. Or, water can be pushed into tubed foot to extend or elongate the foot.
b. Tubed foot (podia) used in movement - the expanded tubed foot can be
moved by muscles, so the sea star can travel along the bottom.
F. Gas exchange - oxygen enters by any one of the following:
1. Absorbed by diffusion through skin gills, (small finger-like projections of
ciliated tissue on the skin surface).
2. Through the tubed-feet
3. By water circulation through the animal
Part 2 Questions:
G. Waste Excretion – 3 ways wastes can be excreted from a sea star:
1. Occurs by the diffusion of waste products into the water flow as it is removed
from the animal.
2. Through thin membranes of the tubed feet
3. Through skin gills.
H. Nervous system - resembles the hydrovascular system
1. Nerve ring - a round ring in the area of the central disc, which contains
branches of nerves which run down each arm.
2. For regeneration, a portion (1/5) of the nerve ring must be present
I. Autotomy, regeneration and reproduction - seastars have strong powers of
regeneration.
1. If at least one-fifth of the nerve ring is removed with an arm, an entire new
seastar can be formed asexually.
2. Complete regeneration of a new sea star requires about 1 year
3. Most are dioecious (10 gonads-2 per arm)
4. Asexual reproduction is carried out by some species
5. External fertilization takes place in the water
6. There is usually only 1 breeding season per year, but reproduction can take
place more often in warm water.
7. One female may release as many as 2.5 million eggs at one time.
8. Larval stages - generally, larval stages remain near the bottom and do not
become part of plankton
III.
Class or Sub Class Ophiuroidea (brittle stars or serpent stars, and basket stars)
(Gr. Ophis meaning snake) largest in numbers of species and most abundant.
- Brittle stars and basket stars make up the bulk of the members in this class. Although
superficially resembling sea stars with five arms, their appendages are often much longer and
thinner. The arms lack ampullae, so locomotion is achieved by rapid "lashing" of the arms.
Most are detritivores, consuming small bits of organic debris from the ocean floor.
A. Body structure - these animals have extremely long slender arms with a well defined
central disc.
1. The ambulacral grooves are closed, they have no pedicellariae or papulae (skin gills).
2. They are relatively small and delicate, with a central disc of 1/"4 to 1 inch.
a. Basket stars - they are the largest members of this group, with a central disc of
about 4 inches.
b. These animals usually have 5 arms which sometimes branch at the ends.
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c. Some species have poisonous spines (basket and serpent stars)
d. There are no surface cilia, and some trap phosphorescent algae and are luminescent
B. Locomotion - Highly flexible arms allow these organisms to be the most highly mobile in
this phylum
1. Muscles in each arm contract to allow them to crawl rapidly along the bottom
(brittle stars).
2. They also use their arms to rake food into their mouths.
C. Water vascular system - same as sea stars. Most tubed feet do not have suckers,
but they do aid in food transfer.
D. Nutrition - feed on bottom detritus and small animals like worms and crustaceans.
1. They lift their arms to catch plankton on mucous secretions
2. Basket stars - they have a unique feeding method. They roll up into a ball,
travel in the surge, and then open up to capture prey.
3. Five movable plates surround mouth serving as jaws for feeding.
4. Visceral organs - all in central disc.
a. Stomach is saclike
b. No intestine
c. No anus
E. Circulation - the coelom, which is a hollow body cavity inside the animal is more
reduced than in sea stars.
1. Madreporite is located on oral surface
2. Some species can actively pump water through the body by internal cilia.
F. Gas exchange
1. 5 pairs of bursae open toward oral surface by genital slits were gas exchange occurs.
G. Autotomy, reproduction and regeneration
1. Gonads located on coelomic wall of each bursea, discharge sperm and eggs
directly into the water.
2. Sexes are mostly separate (some are hermaphroditic)
3. These animals can automize one or more arms to free themselves from
predators.
4. Autotomy and regeneration are even more pronounced in brittle stars than asteroids.
IV. Class Echinoidea - (Gr. Echin meaing spiny; prickly) sea urchins, sand dollars, heart
urchins, and sea biscuits
- Sea urchins and sand dollars are the most common members of this group. Unlike the
sea stars and brittle stars, they lack appendages. The body is either spherical, as in the sea
urchins, or flattened, like the sand dollars. Sea urchins are often covered with a series of long
spines that provide a formidable defense against would-be predators. The calcareous skeleton is
highly-developed, providing a box-like envelope for the soft tissues. The mouth, anus, and tube
feet extend through small openings in the skeleton. The spines of sea urchins can cause painful
swelling if broken off in the skin of humans.
A. Body structure - body is covered with spines (sea urchins have long spines, and
sand dollars have short spines)
1. The shell is called a “Test” - this endoskeleton is composed of sections(ossicles) fused
together to create a solid shell-like case to live in.
a. They lack arms but test reflects typical 5-part plan
b. Bilateral symmetry (especially accentuated in the heart urchin)
2. There are generally 10 double rows of these plates with 5 pair perforated for the
slender tubed feet to come through the test.
a. Ambulacral grooves are closed
b. Heart urchin - mouth at one end, and anus at the other end.
c. The spines are laid back pointing toward the anus.
d. The sea urchin has long moveable spines
e. Pedicellariae - some are poisonous for protection
3. Size - average 2 to 4 inches, however, some sea urchins reach a size of up to 1
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foot in diameter
a. Body structures are arranged in groups of 5
b. Longer spines are located around the equator, with shorter spines at the poles.
c. Some species have poisons at the tip of the spines
4. Locomotion - they move by using the spines and tube feet, which are
equipped with suckers
a. Sea urchins use their spines to wedge themselves tightly among rock crevices.
b. Sand dollars and heart urchins like to burrow in sandy substrate
B. Nutrition - some Sea urchins use their unusual five-toothed mouth, called: 'Aristotle's
Lantern', which they also use to chew into a stone surface, forming a cup-shaped hiding
place in the rocks.
1. Mouth - located on oral surface in center of animal
2. Anus - on aboral surface (top on animal)
3. Most sea urchins use their 'Aristotle's Lantern' to scrape and chew microscopic
algae clinging to rock surfaces
a. Unlike seastars, sea urchins possess a well-developed coiled intestine and anus
because a large proportion of their food is undigestible solids that must be
eliminated.
b. Sand dollars gather food on strings of mucous beneath their short spines.
c. They feed on small diatoms and organic debris found on the bottom.
C. Reproduction - they contain separate sexes (Dioecious).
1. The Eggs are said to be a delicacy in the sea urchin
2. Both eggs and sperm are shed into sea for external fertilization
V.
Class Holothuroidea (Gr. Holo meaning entire; whole)(sea cucumbers)
- Sea cucumbers and the next set of echinoderms, sea lilies, are the groups within the
phylum most observers would nominate as "Least likely to be an Echinoderm". At first
glance, sea cucumbers appear to be little more than a flabby, jelly-filled bag. The
calcareous skeleton has been reduced in this group to little more than a series of
microscopic plates spread throughout the surface of the body. Many have a series of
tentacles surrounding the oral opening. The tentacles are attached to the water vascular
system and allow the sea cucumber to capture prey items and bring them to the mouth for
ingestion.
A. Body structure - characterized by a soft, fleshy body resulting from a
tremendous reduction in the size of the Ossicles forming the endoskeleton
1. Very small Ossicles are embedded in the skin
a. Body can extend or contract
b. Small Ossicles in skin make animal harder when drawn up
c. Some crawl, other burrow, and some are found beneath rocks
2. Podia (tubed feet) - these are greatly enlarged around the mouth forming a
ring of feeding tentacles.
3. Like the sea urchin, sea cucumbers do not have arms
B. Survival - they are sluggish animals that have a unique methods of survival.
1. Evisceration - a tactic used to escape predators.
a. When threatened, some sea cucumbers may expel a mass of sticky tubes
from the mouth or anus, entangling an intruding crab, fish or lobster.
b. Lost organs that are spit out the mouth or anus can be regenerated
(digestive, respiratory, gonads).
C. Respiration - to obtain oxygen and remove carbon dioxide, most sea cucumbers
dialate the anal region (muscular cloaca) allowing sea water to enter.
1. Circular muscles around the anus contract, forcing water around special tubes.
2. Respiratory tree - Gas exchange occurs across the membranes of the respiratory
tree, which is a branched tube-like organ located in the cloaca.
3. Water is then pumped out by reversing the muscular contractions and opening the
cloaca.
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a. Cloaca - this is a common chamber into which the intestinal canal and respiratory
tree empty.
D. Movement - 5 areas run from mouth to anus where tubed feet are located.
1. 3 ventral areas where tubed feet are located are used for locomotion and contain
can contain suction cups.
E. "Trepang" - this is sea cucumber which is boiled several times and then dried.
Used in making soup.
1. Trepanging is the act of collection or harvesting of sea cucumbers, known in
Indonesia as "Trepang". The collector or fisher of Trepang is coined a trepanger.
F. Feeding –
1. most sea cucumbers feed by trapping bits of debris on their sticky feeding tentacles
either by sweeping these tentacles over the bottom or by waving them in the water.
2. Some swallow sand and mud while burrowing beneath the surface and expel a trail of
sand as waste.
G. Reproduction - sexes are separate, but holothuroidians are hermaphroditic fertilization is external in the water column.
VI. Class Crinoidea Crinoidea comes from the Greek word krinon, "a lily", and eidos, "form".
Though resembling a plant (sea lilies and feather stars)
- Sea lilies are the most primitive of the echinoderms. Their ancestors were an important
component of marine communities during the Paleozic era and their fossils are common in
sediments deposited in the ocean floor 300 to 500 million years ago. Most are sessile,
attached to the substrate by a long flexible stalk. Unlike all other echinoderms the oral
opening faces up and, like some of the Holothuroidea, is often surrounded by a series of
feathery tentacles used to capture food items. One group, the feather stars, have become
mobile.
A. Body - these organisms have comb-like feathery arms, which spread out into the
water currents to capture suspended food.
1. Sea lilies have a prominent stalk that serves as a permanent attachment to the
bottom.
2. The "crown" (body) is attached to the jointed stalk
3. Crinoids are the most ancient living echinoderms
4. The tubed feet on the feathery arms are used to capture particles of driftng food,
not for locomotion, and may contain cirri.
5. Madreporite, spines, and pedicellariae are absent
6. Both fossil and living Crinoids are also distinguished by being the only echinoderms
that are permanently attached to the substrate.
VII. Class Concentricycloidea (sea daisies)
A. Strange disc shaped animals
1. Described (1986) New Zealand at 1000 meters deep
2. So far there are only 2 species
3. They have no arms
4. Water vascular system contains 2 concentric ring canals
a. Podia arise from outer ring (radial canal)
b. Hydropore (madreporite) connects the inner ring canal to aboral surface.
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