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Phylum Arthropoda and Echonodermata
Chapter 28
The word Arthropoda means “joint foot’. That means that the wonderful
world of insects all have feet with joints, otherwise they would be walking on
pegs.
1. Introduction:
If we consider both named and as yet unnamed members of the phylum
Arthropoda, there are most likely more of these animals than all other living
organisms combined. The arthropods are, by any criteria, THE MOST
SUCCESSFUL GROUP OF ANIMALS ALIVE. They have adapted successfully
to life in water, on land and in the air. About 80% of all known animal species
belong to the Arthropoda - about 800,000 species have been described, and
recent estimates put the total number of species in the phylum at about 6
million.
As their name indicates, they are animals with `jointed appendages".
Included in the phylum are spiders, scorpions, ticks, crabs, barnacles, ants,
bees etc.
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Arthropods are found in all types of environments. They live in the ocean
depths, high mountain peaks, deserts, tropical rain forests and in or on the
bodies of other animals and plants. They obtain their food as carnivores,
herbivores, omnivores and parasites. The largest arthropod is the Japanese
crab (3.7m) and the smallest is a parasitic mite, (0.1mm). There is no group
of animal that does more harm to humans and few that do as much good.
2. Subphyla of Arthropoda:
a. Trilobites: extinct multishelled sea
animals
b. Chelicerata: spiders, horseshoe crabs,
mites, scorpions
c. Crustaceans: crab, shrimp, lobster
d. Uniramians: insects, centipedes,
millipedes.
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3.
Unifying characteristics of Arthropods:
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Coelomate and bilaterally symmetrical
Jointed appendages
Exoskeleton of chitin secreted by the epidermis
3 body regions (head thorax and abdomen)
Respiration by body surface, gills, trachea, or book lungs
Open circulatory system
Sensory organs are well-developed: compound eyes, tympanum (drumlike ear) antenna (touch, smell, chemical reception)
Complete digestive systems with mouth, specialized organs and anus
Dioecious: fertilization is usually internal
Developed nervous system with dorsal brain connected by a ring and
double ventral nerve cords
Found in marine, freshwater, terrestrial and aerial environments
4. Form and Function in arthropods:
a. Feeding
Every mode of feeding is seen in arthropods: herbivores,
carnivores, parasites, detritus feeders and filter feeders
b. Respiration and circulation
 Three types of respiratory structures: gills, book gills and book
lungs, and tracheal tubes.
 Crabs and shrimp have gills
 Book gills and book lungs are sheets of gills and spiracles are
tubes which connects the outside air to them.
 tracheal tubes travel throughout the insect and are connected
 by spiracles to the outside.
c. Internal Transport
Uses a heart and an open circulatory system.
d. Excretion
Solid wastes leave by the anus. Nitrogenous wastes are collected
by Malpighian tubules. They are organs bathed in blood that
remove nitrogenous wastes and add them the undigested waste
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that leaves by the anus. Some arthropods use their gill to get rid
of the waste.
e. Response
 Arthropods have a brain and nerves that run the length of
the animal. The nerves coordinate leg and wing movement.
 They have simple sense organs like statocysts, chemical
receptors (taste) and compound eyes and eardrums.
f. Movement
The exoskeleton has muscles attached that move the arthropod
g. Reproduction
Involves males depositing sperm into the female or dropping off a
sperm packet for the female to pick up. Ovipositors are used to
deposit the fertilized eggs
5. Class Insecta: Bugs and the Creepy Crawlies
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Exoskeleton made of chitin
Have a head segmented into 6 with sensory antennae and compound
eyes
Have a thorax with attached segmented legs and possibly wings
Have an abdomen with 11 segments which houses most of the
digestive, respiratory, excretory and reproductive systems
Insects respire through a system of internal tubes and sacs that lie
directly next to the circulatory system, therefore the circulatory
system does not need closed vessels
• Most insects hatch from eggs
which are formed by sexual
reproduction
• Insects molt as they grow in
size (undergo metamorphosis)
• Metamorphosis includes the
larval, pupa, nymph, and adult
phases to life
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Echinoderms: Starfish
Echinoderms are characterized by spiny skin an internal skeleton, a water
vascular system, and suction cuplike structures called tube feet. The
phylum Echinodermata represents a relatively small group of unique,
marine animals. The phylum contains such interesting animals as the
starfish, sea cucumbers, sand dollars, sea urchins and brittle stars. They
appear embryologically most closely related to the chordates but they
have several features that set them apart.
The echinoderms are all marine and have no ability to osmoregulate. They
are all benthic organisms that are located in every ocean and at almost all
depths. Some are active predators, others are scavengers and still others
filter their food from the water.
 most adults exhibit five part radial symmetry
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Coelomate and radially
symmetrical
Body is usually 5 rayed
(pentaradial)
Have a calcareous skeleton
Possess a water vascular
system used for gas exchange,
feeding and locomotion
Complete digestive system
Open/reduced circulatory system (no heart) with a central ring and
vessels
Simple radial nervous system (no brain)
Reproductive system consists of gonads which take up a significant
amount of space in the body cavity
Sexual reproduction with fertilization outside body (most)
Have ciliated, free-swimming larvae
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Abundant on most sea coasts especially on rocky shores
1. Form and Function in Echinoderms:
Echinoderms have internal water vascular systems which is filled with
fluid.
 They carry out respiration, circulation, and movement.
 It opens to the outside through the madreporite.
A tube foot is a structure that operates much like a suction cup.
Each tube foot has a sucker on the end.
a. Feeding:
Echinoderms have several methods of feeding.
 Sea Urchings use a five part jawlike structure to scrape algae from
rocks
 Sea cucumbers move like bulldozers across the ocean floor, taking in
sand and detritus.
 Starfish usually feed on molluscs such as clams and mussels.
Digestion
• Sea stars have 2 stomachs that perform different functions
1. Cardiac stomach can be pushed outside of the mouth to engulf and
digest food. Once digested the food is brought inside to the pyloric
stomach
2. The pyloric stomach further digests food and passes it on to the
intestine and the anus which is located on the aboral surface (top)
This allows a sea star to consume food that would normally not fit into its
mouth!!!
b. Respiration and Circulation
The water vascular system consists of a central ring and a series of canals
that eventually end in tube feet which you can see protruding from the
underside of this sea star. These tube feet are used for movement,
feeding and bringing in water for gas exchange.
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c. Excretion:
Most wastes are released through the anus. Nitrogen based cellular waste
are excreted in the form of ammonia.
d. Response:
They do not have highly developed nervous systems. They have a nerve ring
which surrounds the mouth and radial nerves that run up the arms.
 They have sensory cells that detect light, gravity, and chemical
released by potential prey at the end of each arm
e. Movement:
Most echinoderms move using their tube feed. It’s mobility is determined
by the structure of it’s endoskeleton.
 Sand dollars and sea urchins have movable spines which help them.
 Seat starts and brittle stars have flexible joints.
 in Sea Cucumbers don’t have the hard spiny skin. The use tube feet
and muscles to move along the sea floor.
f. Reproduction:
Reproduce by external fertilization. Sperm and eggs are produced in
gonads. Both gametes (Sperm and egg) are released into the water where
fertilization takes place.
g. Defence
i. Spines are observed on many echinoderms and can be
soft or hard and sharp. Their presence deters predators
ii.
Pedicellariae are pincer-like structures that
protrude from the body surface between the spines
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Pedicellariae keep the body surface clear of
encrusting organisms
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Echinoderms also camouflage with their natural color
into their environment
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2. Groups of Echinoderms:
a. Sea Urchins and Sand Dollars.
b. Brittle Stars
c. Sea Cucumbers
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d. Sea Stars
3. Ecological Role of Ecinoderms:
 Starfish and brittle stars prevent the growth of algal mats
on coral reefs
 Ossified skeletons of echinoderms are major contributors to
many limestone formations
 Part of the food chain, larvae produce food for other
creatures
 The death of echinoderms often is followed by an increase in
seaweed growth and destruction of reefs
 Sea urchin grazing reduces the rate of colonization of bare
rock
 Burrowing of sand dollars and sea cucumbers depletes the
sea floor of nutrients and encourages deeper penetration
which increases levels of oxygen and ecological tiering
 Sea urchins bore into rocks releasing nutrients
 Sea cucumbers provide habitats for parasites
 Echinoderms feed on dead and decaying organisms
4. Cool things about Echinoderms:
• Sea stars are capable of regeneration of body parts as long as
part of the central disk remains
• The gonads of Sea urchins and cucumbers are considered a
delicacy in SE Asia
• Sea cucumbers will eviscerate (shoot out parts of their
internal organs) when they sense extreme danger (the organs
then re-grow)
• Sea urchins can replace spines that are damaged or lost
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