Download Invertebrates: Marine Animals Without a Backbone

Invertebrates: Marine Animals
Without a Backbone
Chapter 7
Invertebrate Introduction
Most species of multicellular organisms on Earth are animals and cannot
manufacture food on their own. As result, we need to get it from other
places.
The need to eat has contributed to the evolution of different means of
obtaining and processing food, as well as diverse ways to avoid being
eaten.
Many marine organisms are traditionally classified into two major groups:
1.
2.
Vertebrates (animals WITH a backbone)
Invertebrates (animals WITHOUT a backbone)
At least 97% of all species of animals are invertebrates, many of which reside
within the marine environment.
Sponges
Sponges are best described as aggregations of specialized cells. Sponges have
cellular organization; meaning, they are largely independent of one another and
do not form true tissues and organs. Sponges are, as result, among the most
simplistic multicellular animals.
Characteristics:
 Nearly all of the 6,000 species of sponges are marine and can live from the poles
to the tropics, with the shallow tropics being the habitat of choice
 All are sessile organisms, living permanently attached to some sort of surface
 Simplistic body plan
 Numerous pores (ostia) allowing water to enter and circulate
 Sponge cells are plastic, changing from one type to another, and can “regroup” if
cells are separated
 Sponges may grow into branching, tubular, round, or volcano-like looking
masses reaching very large sizes.
Architecture of Sponges
 The outer surface is covered with
flat cells (pinacocytes)
 Tube-like pore cells (porocytes)
where water can enter the sponge
 Water is then pumped into a
larger feeding chamber lined with
collar cells (choanocytes) that
traps food particles to be ingested
into the body of the cell.
 Water then leaves through a large
opening on top of the sponge
(osculum)
As Sponges Grow…
Sponges need structural support as they grow larger.
 Most have spicules to support structures of different shapes and
sizes and many have a “skeleton” of tough, elastic fibers made of a
protein called spongin. When present, they are found mostly
between the outer and inner layers of cells.
 Wandering cells (amebocytes) secrete the spicules and spongin
and sometimes transport and store food particles. Some can even
repair damage to the sponge.
Sponge Species
Glass Sponge
Encrusting Sponge
Boring Sponge
Coralline Sponge
Sponges – Commercial Use
Some marine sponges are of
commercial importance:
 Bath sponges (Spongia) are still
harvested in a few locations in the
Gulf of Mexico and the eastern
Mediterranean in what was once
flourishing industry
 Synthetic sponges consist of
spongin fibers remaining after cells
and debris are washed away
 Some sponges produce potentially
useful chemicals, such as those
used in anti-inflamatory or
painkilling drugs
Cnidarians
After the sponges, the next step of organizational complexity of
animals involves quite a big step: the evolution of tissues that
perform specific functions.
Tissue development makes it possible for organisms to swim, respond
to external stimuli, and engulf prey.
In addition to tissue development, cnidarians have radial symmetry –
which means that parts of their body are arranged and repeated
around a central axis.
Radial Symmetry
 Animals with radial symmetry look the same
from all sides, and have no head, front or
back. As result, if a radial symmetrical animal
were cut like a pizza, all “slices” would be
similar
 Although they do not have “traditional” body
parts, they do have an oral surface (where
the mouth is) and an aboral surface (on the
opposite side)
 Cnidarians have a centrally located mouth,
surrounded by tentacles, which opens into
the gut where food is digested.
 Within the tentacles are nematocysts,
which are stinging structures used to capture
small prey
Two Basic Forms
Polyp
Sac-like attached stage with the
mouth and tentacles oriented
upward
Medusa
An upside down polyp adapted
for swimming
The life history of some cnidarians includes both polyp and medusa stages;
however, some spend their entire lives as either a polyp or a medusa.
Types of Cnidarians
They basic cnidarian body plan, though structurally simple, has been
very successful with known species totaling 10,000. Almost all of
the known species are marine.
Types:
1. Hydrozoans
2. Scyphozoans
3. Anthozoans
Hydrozoans
Characteristics:
 Wide range of forms and life histories
 Feathery or busy colonies of tiny polyps
that attach to shells, seaweeds, and other
surfaces
 Polyps are specialized for feeding, defense,
or reproduction
 Can lack either polyp or medusa stage
Siphonophores:

Form drifting colonies of polyps, where
some develop a specialized float
(Portuguese Man-of-War) that is either
gas-filled or contains oil droplets.

Some develop long tentacles to capture
prey – these develop a harmful toxin
that causes a painful reaction to
swimmers / divers
Portuguese Man-of-War
http://www.youtube.com/watch?v=qI7plX8to5o
Scyphozoans
Characteristics:
 Larger jellyfishes common in all
oceans
 Polyps are very small and release
juvenile medusae (few species lack
polyp stage completely)
 Rounded body (bell) of some may
reach up to 10ft in diameter in
deep waters
 Swim with rhythmic contractions
of the bell, mainly carried by
currents
 Some are among the most
dangerous marine species known,
giving fatal stings
Anthozoans
Characteristics:
 Solitary or colonial polyp lacking
medusa stage
 Largest number of cnidarian species
 Most complex type of cnidarian, with
additional surface area for digestion of
large prey
Sea Fans are gorgonians with branches that
grow in only one plane and have many cross
connections.
Types:
 Sea Anemones
 Gorgonians
 Black Corals
 Soft corals
 Sea Pens
 Sea Pansies
Biology of Cnidarians
Feeding and Digestion
 Practically all cnidarians are
carnivores and prey on other
animals.
 Many are capable of catching large
prey by using their nematocysts,
which consists of toxins and fluids
that track their prey.
 After ingestion, food passes through
the gut to be digested.
 The initial phase of digestion occurs
outside of the cells (extracellular)
but the complete breakdown of the
food takes place inside of the cells
lining the gut (intracellular)
Behavior
 Though lacking a brain or true
nerves, cnidarians do have specialized
nerve cells that interconnect to form
a nerve net to transport impulses.
 Although simple, the nervous system
can be quite complex.
 In some anemones, they can tell if
other members of their species are
from the same clone and have been
known to attack and kill those that are
not!
 Some medusae have primitive eyes
surrounded by fluid-filled chambers
(statocysts) and sensitive hairs,
which provide a sense of balance.
Comb Jellies
Comb jellies are an exclusively marine
group of about 100 species.
Radial symmetry and gelatinous body
resembles a medusa, but there are
unique traits.
Comb jellies swim with 8 rows of long
cilia (ciliary combs) fused at the
base that beat in waves, which
creates a prism like multi-color
effect.
Bilaterally Symmetrical Worms
Radial symmetry works fairly well in animals that attach to surfaces or
drift in currents, but animals that crawl or swim in one direction
have different needs.
Most animals show bilateral symmetry, which is an arrangement of
body parts in such a way that there is only one way to cut the body
in order to get two identical parts.
Characteristics of
Bilaterally Symmetrical Organisms
 Have a front (anterior) end and a
back (posterior) end
 Have a head with a brain, or an
accumulation of nerve cells, and
sensory organs at the anterior end
 Have a back (dorsal) surface that is
different from the belly (ventral)
surface
 Animals that are bilaterally
symmetrical are able to be more
active in the pursuit of prey and
develop more sophisticated
behaviors than those with radial
symmetry
Flatworms
Characteristics
 The simplest bilateral symmetry
plan because they are flattened
(with a definite back and belly)
 The most simple tissue
organization into real organs and
organ systems
Biology of Flatworms
 Central Nervous System:
information is stored and
processed here and consists of a
brain (which is an aggregation of
nerve cells in the head)
 Several nerve cords running
through the length of the worm
that coordinates movement of a
well-developed muscular system
 Only one opening to outside
(mouth)
There are 20,000 known species of flatworms, with the most common marine flatworms being the
turbellarians (free-living carnivores). Most turbellarians live inside or on the surface of oysters, crabs,
and other vertebrates.
Parasitic worms also live within marine vertebrates and feed on their tissues, blood, or intestinal
contents. Parasitic worms are common in fish, seabirds, and whales.
Ribbon Worms
Characteristics
 Longest invertebrate on Earth
 More complex organization than
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flatworms
All are predators, feeding on worms
and crustaceans
Found in all oceans, although most
common in shallow temperate waters
Some are nocturnal and not easily
seen, others are bright and can be
found under rocks at low tide
Incredibly elastic, proboscis can
extend a meter or more beyond its
body
Biology of Ribbon Worms
 Complete digestive tract, with
a gut including a mouth and
anus
 Circulatory system for blood
to transport nutrients and
oxygen to tissues
 Long, fleshy tube (proboscis)
to entangle prey
There are approximately 900 species of Ribbon Worms, most being marine.
Nematodes
Characteristics
 Often known as roundworms
 Hardly ever seen, but numbers
are extreme
 Many are parasitic, most marine
species have nematode parasites
 Adapted to live in either
sediment or in tissues of other
organisms
Biology of Nematodes
 Mostly small, cylindrical
bodies with points at each end
 Feed mostly on bacteria or
organic matter
 Gut lies within a body cavity
filled with fluid that transports
nutrients
 Contains hydrostatic skeleton
to provide support and aid in
locomotion
The actual number of species of nematodes is debatable, but estimates are
between 10,000 and 25,000.
The adult of nematodes inhabit the intestines of seals and dolphins, but their
larvae reside in fish and can ultimately harm humans who eat raw or poorly
cooked fish.
Segmented Worms
Characteristics
 Large group (contains
earthworms) that is further
classified into Polychaetes,
which are marine worms.
Biology of Segmented Worms
 Body contains a series of “segments” that
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act as a hydrostatic skeleton for
movement and flexibility
Gut goes through all segments and lies
in a cavity (coelom) that is completely
surrounded by different tissue
Flattened extensions (parapodia) on
body segments with stiff, sharp bristles
Circulatory system transports nutrients,
oxygen, and carbon dioxide
Closed circulatory system to transport
blood within distinct vessels
Contains gills on parapodia to assist in
obtaining oxygen from water
A: Fireworms
B: Fanworms
C:Tubes (habitat)
There are more than 10,000 species of polychaetes, which are almost entirely marine. Many
of these polychaetes crawl on the bottom, hiding under rocks or coral, but many burrow into
the mud or sand. Some polychaetes live in tubes made of mucus, protein, seaweed bits, or
mud and sand.
Regardless of their chosen habitat, polychaetes have a successful lifestyle.
Peanut Worms
Characteristics
Biology of Peanut Worms
 Soft, unsegmented bodies
 Long anterior portion
 Burrow in muddy bottoms,
containing a mouth and set of
small lobes or branching
tentacles
 Tentacles can be pulled into
the body, which makes the
worm look like a bundle
(peanut)
rocks, corals, or hide in empty
shells
 All are marine
 Live in mostly shallow water
 Peanut worms are 0.4-14 inches
long
There are appoximately 320 known species of peanut worms, all
of which are deposit feeders.
Euchiurans
Characteristics
 Burrow into the mud or corals
 Similar to peanut worms in size
and shape
Biology of Euchiurans
 Have a non-retractable, spoon-like
proboscis
 Deposit feeders, use the proboscis
to gather organic matter
All of the 135 known species are marine.
Molluscs:
The Successful Soft Body
Snails, clams, octopuses, and
other familiar forms are
members of the Mollusc
species.
There are more species of
molluscs in the ocean than
any other animal group.
Body Characteristics
“Most” Molluscs
 Soft body, enclosed in a calcium

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
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carbonate shell
Body is protected by a tissue
(mantle) that secretes the shell
Unsegmented bodies are bilaterally
symmetrical
Contains a foot, typically involved
in locomotion
Most have a head that typically
includes an eye and other sensory
organs
Contains radula (ribbon of small
teeth to feed)
Exceptions
 In a squid, the shell is internal
and in an octopus, there is no
shell at all!
 Portions of a snails body are
coiled and asymmetrical
 Some molluscs have either
modified radula… or none at
all!
Types of Molluscs
Molluscs exhibit an immense diversity of structure and habit,
occupying all marine environments – from the rocky shores to
hydrothermal vents in the deep sea. Molluscs thrive on practically
all types of diet.
Despite the diversity, most molluscs belong to one of three major
groups:
1.
2.
3.
Gastropods
Bivalves
Cephalopods
Gastropods
 Gastropods are the largest, most common, group of molluscs with 75,000
species (Snails being the most familiar.)
 Can be best described as a coiled mass of vital organs enclosed by a dorsal
shell (usually coiled) that rests on a ventral creeping foot.
 Most use their radula to scrape algae from rocks although some are
bottom feeders on soft bottoms.
 Gastropods prey on clams, oysters, worms, or small fish.
Exceptions:
Sea slugs are gastropods that have lost their shell completely. They have
colorful branches along their gut, making them appear vibrant in the sea.
Sea slugs prey on sponges or other invertebrates. To protect themselves,
since they have no shell, they are able to release a toxic chemical or retain
undischarged nematocysts taken from their prey.
Marine Snail
Sea Slug
Bivalves
 Contains clams, mussels, oysters, and other similar molluscs.
 The body is laterally compressed (flattened sideways) and enclosed in a

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shell with two parts (valves).
No head or radula are present, gills are expanded and used to obtain
oxygen and filter food particles from the water.
Contains a mantle cavity, which is a large space between two halves of the
mantle.
Strong muscle system, used to close off valves as needed.
Use foot to burrow into the sand or mud.
Exceptions:
 Not all bivalves are burrowers.
 Mussels attach themselves to rocks or other surfaces and oysters cement
themselves to hard surfaces.
Clam
Zebra Mussel
Cephalopods
 Contains 650 species, all marine, including octopuses, squid and
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cuttlefish.
Predators that are specialized for locomotion, nearly all are agile
swimmers with a complex nervous system
Either a reduced shell, or none at all!
Head is pushed down toward foot, like gastropods, but foot is modified
into arms and tentacles equipped with suckers used to capture prey
Large eyes, set on sides of head, and rounded body protected by thick and
muscular mantle that forms a cavity behind the head to enclose two or
four gills
Swim by forcing water out of the mantle cavity through a flexible siphon
that allows them to move in all directions
Atlantic Giant Squid
Cuttlefish
Octopuses
Characteristics:
 Have 8 elongated arms and lack a shell,
with each arm having suction cups on the
underside
 Common bottom dwellers
 Size varies from 2in (dwarf) to 30ft
(Pacific Giants)
 Efficient hunters, with crabs, lobsters,
and shrimp among their favorite prey
 Bite prey with jaws (and paralyzing
secreted substance) and use radula to rip
away flesh. Despite this, most are
harmless
 Contain an ink sac to distract predators
 Different stages of gonad development
with maturity, males reaching this stage
before females… at around 4 months
The Blue Ringed Octopus is the most deadly in
the world, found in tide pools of the Pacific
Ocean, particularly in Japan and Australia
http://www.youtube.com/watch?v=SNxJwCGJtsA
Other Types of Molluscs
Chitons
 About 800 species are known,
all being marine
 Identified by: eight overlapping
shell plates that cover arched
dorsal surface with uncoiled
internal organs
 Most live on shallow, hard
bottoms where they feed on
algae
 Many return to a “home base”
after feeding
Tusk Shells
 About 350 species
 Elongated shells, open at the top
and tapered like elephant tusks,
that are narrow at the top
 Has a foot at the bottom of the
shell, projecting from a wide end
 Many species have thin tentacles
with adhesive tips that are used
to capture young bivalves
 Live in sandy or muddy bottoms
Chiton
Tusk Shell
Arthropods: The Armored Achievers
Arthropods make up the largest
group of animals, with more
than a million known species.
(Of all animals on Earth, 3 out
of 4 are arthropods, with the
most dominant being insects.)
In the marine environment,
crustaceans are the dominant
arthropodic life form.
Crustaceans include the familiar
barnacles, shrimp, lobsters, and
crabs.
Body Characteristics
 Flexible, segmented body with bilateral symmetry
 Jointed appendages, such as legs and mouthparts, that are moved by
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attached muscles
Tough, non-living, exoskeleton composed of chitin and secreted by
underlying layer of tissue
Gets protection, support, flexibility, and increased surface area for
muscle attachment from appendages and exoskeleton
Grow through shedding (molting) exoskeleton, which is quickly
replaced by a new shell that has developed underneath the old one
Most are small due to rigid exoskeleton, but their legs can reach
great lengths.
Crustaceans
Contains approximately 68,000
species, most being marine.
Specialized for life in water, most
possess gills to obtain oxygen
Chitinous skeleton is hardened by
calcium carbonate
Appendages are specialized for
swimming, crawling, attaching to
other animals, mating, and feeding
Possess two pairs of antennae, used
for sensing surroundings
Body plan is repeated
Small Crustaceans
Small crustaceans are everywhere: in plankton, on the bottom within
sediments, on and in other animals, and crawling among the
seaweed.
Types:
1. Copepods
2. Barnacles
3. Amphipods
4. Isopods
5. Krill
6. Shrimps, Lobsters, Crabs
Copepods
 Extremely abudant and
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important in the plankton
Use mouthparts to filter out or
capture food
Some species are so common
they’re among the most
abundant animals on Earth
Many keep from sinking by
using enlarged antennae to
swim
Many are parasitic
Barnacles
 Filter feeders that live attached
to surfaces, including living
surfaces (whales and crabs)
 Common barnacles look like
molluscs due to their body shape
and appearance
 Some are highly parasitic,
although they all have typical
crustacean larvae that swim and
attach to surfaces before
becoming adults
Copepods
Barnacles
Amphipods
Isopods
 Small crustaceans with a curved
 Found in similar environments
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body that is flattened sideways
Most are under 2cm in length,
some deep water species are much
larger
Head and tail curve downward,
appendages are specialized for
function
Some are strong jumpers (beach
hoppers) that stretch out their
curved bodies, while others crawl
among the seaweed
They often burrow in whales
(whale lice)
to amphipods
 About same in size as
amphipods, but have legs on
their “main” body and have flat
backs
 Fish lice are parasites of fish
and other crustaceans
Amphipods
Isopods
Krill
 Planktonic, shrimp-like
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crustaceans that grow up to 6cm
long
Head is fused with body segments
to form a carapace that covers the
anterior half of the body like
armor
Most are filter feeders
Extremely common in polar
waters, aggregating in giant
schools
Almost exclusive food source to
many Antarctic whales, penguins,
and fishes
Can also live in the deep sea
Shrimp, Lobster, Crab
 Around 10,000 species, making the
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decapods the largest group of
crustaceans
Largest crustaceans in size
5 pairs of legs, the first set being
heavier and with claws for feeding and
defense
3 pairs of maxillipeds (closer to
mouth, turned forward, and
specialized to sort out food and push
it toward the mouth
Shrimp and lobster have laterally
compressed bodies with distinct and
elongated abdomen
Shrimp are typically scavengers who
specialize in bottom feeding
Shrimp
Lobster
Crayfish Dissection
Other Marine Arthropods
Very few arthropods other than crustaceans are common in the ocean. Most
belong to two small and entirely marine groups. A third group, huge and
mostly terrestrial, includes a few shy invaders of the sea.
Groups:
1. Horseshoe Crabs – widely represented in the fossil record, only
surviving members left within their group. Not “true” crabs, considered
to be living fossils
2. Sea Spiders – four or more pairs of jointed legs stretch from a small
body. Found commonly in cold waters
3. Insects – distinguished from other arthropods by having only 3 pairs of
legs as adults. Rare in the sea, most live at the water’s edge and scavenge
among seaweeds, barnacles, and rocks
Horseshoe Crab
Sea Spider
Marine Water Strider
Lophophorates
There are three groups of marine invertebrates that have a unique
feeding structure, the lophophore, that consists of a set of ciliated
tentacles arranged in a horseshoe-shaped, circular, or coiled fashion.
Lophophorates are suspension feeders, using their cilia to create
feeding currents.
Groups:
1. Bryozoans
2. Phoronids
3. Lamp Shells
Bryozoans
 “Moss animals” that form colonies
on seaweed, rocks, and other
surfaces
 Approximately 4500 species,
almost all marine, are grouped
together.
 Bryozoan colonies consist of
minute interconnected individuals
(zooids) that secrete skeletons of
a variety of shapes. The colonies
may be encrusting or take an
upright form .
Phoronids
 Easily confused with
polychaetes at first glance
 Worm-like and build tubes
made of sand grains
 They have a horeshoe-shaped
or circular lophophore, their
gut is u-shaped in contrast to a
straight gut in polychaetes
 All known species (20) are
marine, burrowing in sand and
rocks of shallow water
Bryozoan
Phoronid
Lamp Shells
 350 living species, also called
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brachiopods
Thousands of other species are
known only as fossils
Consists of shell with two valves –
dorsal and ventral to the body
When valves are open, you see a
lophophore, consisting of at least
two coiled and ciliated arms
occupying most of the space
between the valves
Most are found attached to rocks
or burrowing in soft sediments
Echinoderms: Five-Way Symmetry
Characteristics:
 Radially symmetrical, like cnidarians and comb jellies, but it is a secondary
development
 Planktonic larvae are bilaterally symmetrical – only adults develop radial
symmetry
 Pentamerous radial symmetry (based on five parts)
 Lack a head, have no anterior or posterior end, and no dorsal or ventral side
 Does have oral (where mouth is) and aboral sides
 Complete digestive tract, well-developed coelom, and internal skeleton
 Water vascular system (network of water-filled canals) with muscular tube feet
as extensions of these canals and ampullae (muscular sacs)
 Tube feet end in a sucker and are used for attachment, locomotion, and
receiving stimuli
 Sea stars and sea urchins have a madreporite (a porous plate on the aboral
surface) that connects the system to the outside
Types of Echinoderms
 Large group of about 7,000 known species, all marine
 Important members of the bottom community, from the poles to
the tropics
Specific Types:
1. Sea Cucumbers
2. Crinoids
3. Sea Urchins
4. Sea Stars
5. Brittle Stars
Sea Cucumbers
Crinoids
 Worm-like with tough skin
 Do not have spines and lack obvious
 Suspension feeders that use
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radial symmetry
Body plan appears to be elongated
along the oral-aboral axis
Animal lies on side, where 5 rows of
tube feet are concentrated
Deposit feeders that use tentacles
branching from mouth to pick up
organic matter
Burrow or hide and release toxic
substances as defense mechanism.
Can also expel internal organs
(evisceration) to distract
prey…and later grow back organs
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outstretched, feathery arms to
obtain food from the water
Approximately 600 species of
feather stars (shallow water) and sea
lilies (deep water)
Body appears as upside-down brittle
star with ambulacral grooves and
mouth directed upward
Some have 5 arms, some have up to
200 that branch off of initial 5
Has tiny tube feet that secrete
mucus, which aids in catching food
particles
Sea Cucumbers
Crinoids
Sea Urchins
 Exoskeleton forms a round, rigid, shell-
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like with movable spines used in
locomotion
Flat and radiating body that can transform
into a sphere by dropping the arms and
pulling the oral and aboral surfaces into
shape.
Mouth is on the bottom, anus is on top
Graze on attached or drifting seaweed and
seagrasses, where they also ingest sponges
and bryozoans in the process of feeding
Contains Aristotle’s Lantern, which is a
system of jaws and muscles used to bite
off algae
Commonly found on rocky shores
 Have 5 arms that radiate from a central disk,
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though some have more than 5… up to 50!
Hundreds of tube feet protrude from the oral
surface along channels on each arm
(ambulacral grooves)
Can move in any direction, slowly, by reaching
out their tube feet and pulling themselves along
Endoskeleton has connected calcium carbonate
plates that form a relatively flexible framework
allowing their arms to be flexible
Aboral surface is often covered with spines to
help keep the surface clean
Predators to bivalves, snails, barnacles, and
other slow-moving animals
Feed by extending part of their stomach
through their mouth to envelop the food. The
stomach then secretes digestive enzymes that
extend into its arms and carry the food back
into the glands for absorption. The stomach is
then pulled back inside of the body.
Sea Stars
 Also have star shaped bodies
 5 long, flexible arms protruding
from a central disk
 Arms move quickly and are used
for locomotion whereas the tube
feet, which lack suckers, are used
for feeding
 Prey on small animals picked up
from the bottom of the water,
where they pass the particles
from foot to foot – and to the
mouth
 More species (2,000) than any
other echinoderm and are widely
distributed
Brittle Stars
Biology of Echinoderms
Radial Symmetry is associated with sedentary lifestyles. With the exception of limited
swimming in some feather stars and deep-water sea cucumbers, adult echinoderms are
slow bottom crawlers.
Feeding and Digestion:
 Sea Stars (expels stomach)
 Sea Urchin / Sea Cucumber (long coiled gut to allow for lengthier digestions of plants and
sediment)
Circulatory System:
 Lacking in most echinoderm, use coelomic fluid filling body cavity to transport oxygen
and gas exchange
Nervous System / Behavior:
 Nerve net to coordinate movement of tube feet and spine in absence of brain
http://shapeoflife.org/video/phyla/echinoderms-ultimate-animal
Hemichordates: A “Missing Link?”
 The search for evolutionary links between chordates and other
groups of animals has been a challenge, with several proposed
explanations.
 As strange as it seems, chordates and echinoderms share several
features related to the development of our embryos.
 The wide evolutionary gap can be filled by a small, infrequently seen
group of worms – the hemichordates.
Characteristics of Hemichordates
Acorn worms are the most
common form of hemichordates.
Characteristics:
 Some have a larva similar to that of
some echinoderms
 Dorsal, hollow nerve cord and
openings along anterior part of gut
 85 known species, most worm-like
deposit feeders that live free or in
U-Shaped tubes
 Range in length from 3-18in, some
reach more than 8ft
 Ingest sediment using a thick
mucus-secreting proboscis to
collect materials swept toward the
mouth
Chordates without a Backbone
 Divided into three major groups, two of which lack a backbone.
(The third group is the largest and is has a backbone.)
 Collectively called “protochordates”
 During at least part of their development, they all share the
following characteristics:
1.
2.
3.
4.
5.
A single, hollow nerve cord that runs along the dorsal length of the
animal
Gill slits, small openings along the anterior part of the gut
Notochord, a flexible rod for support that lies between the nerve
cord and the gut
Post-anal tail extending beyond the anus
Ventral heart
Tunicates
Lancelets
 Largest group of protochordates with
 23 known species

 Body grows up to 3in long and




3000 known species, all marine
Often attach their sac-like bodies to
hard surfaces or they anchor
themselves within soft sediments
Only sessile chordates
Body is protected by a tunic (leathery
outer covering)
Filter feeders, using their u-shaped
gut to pass water and expel it through
a second siphon opening
Examples: Sea Squirt, Salps, and
Larvaceans
is laterally compressed and
elongated like a fish
 Inhabitants of soft bottoms
 Filter feeders, using their gill
slits to capture and
concentrate organic particles
Tunicates – Sea Squirt
Lancelets