Download Arthropods

Coelomates have a body design that:
1. Repositions the body’s fluid
2. Allows complex tissues/organs to develop
3. Allows for a larger body size
Coeloms evolved multiple times during animal
evolution
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Mollusks are second in diversity only to arthropods
-Exhibit a wide variety of sizes and body forms
-Live in many different environments
-Include snails, slugs, clams, octopuses and others
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Mollusks evolved in the oceans and most groups have
remained there
They are an important source of human food
They are economically significant in other ways
-Pearls are produced in oysters
-Mother-of-pearl is produced in the shells of abalone
Mollusks can also be pests
-Zebra mussel (Dreissena polymorpha)
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Mollusks are bilaterally symmetrical
-Characterized by a reduced coelom surrounding the
heart and excretory organs
The digestive, excretory and reproductive organs are
concentrated in a visceral mass
Mollusks may have a differentiated head at the anterior
end of the body
Mantle = A thick epidermis that
covers the dorsal side of the
body
-Forms a cavity which houses
the respiratory organs
(ctenidia, or gills) and the
openings of excretory,
reproductive & digestive
organs
The muscular foot of a mollusk
is adapted for locomotion,
attachment, food capture
-Or a combination of the
above
Most mollusks produce an
external calcium carbonaterich shell
-Used for protection
-Some species have
internalized or reduced
shells
Most mollusks have a rasping
tongue-like organ called the
radula
-Used for feeding
Nephridia = Special excretory structures that remove
nitrogenous wastes
-Consist of cilia-lined openings called nephrostomes
Except for cephalopods, all mollusks have an open
circulatory system
Cephalopods have a closed circulatory system
Most mollusks have distinct male
and female individuals
Most engage in external
fertilization
In marine mollusks, embryos
develop through spiral cleavage
-Trochophores: Freeswimming larval stage
-Veliger: Second freeswimming larval stage
-Only in bivalves and snails
There are eight recognized classes
-Four are representatives of the phylum
1. Polyplacophora
2. Gastropoda
3. Bivalvia
4. Cephalopoda
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Class Polyplacophora (Chitons)
-Marine mollusks that have oval bodies with eight
overlapping dorsal calcareous plates
-Body is not
segmented under
the plates
-Most chitons are
grazing herbivores
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Class Gastropoda (Snails and slugs)
-A primarily marine group,
But only group to have terrestrial
organism
-Heads typically have pairs
of tentacles with eyes at the ends
-During embryological development, gastropods
undergo
1. Torsion – Mantle cavity and anus are
moved from the posterior to the front
2. Coiling – Spiral winding of the shell
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Class Bivalvia (Bivalves)
-Includes clams,
scallops, mussels,
oysters and others
-Have two lateral
(right and left) shells (valves) hinged
together dorsally
-Most are sessile filter-feeders
-Water circulation is mediated by
siphons and rhythmic beating of
cilia on
gills
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Class Cephalopoda
-Active marine predators
-Foot has evolved into a series of arms equipped
with suction cups
-Squids have 10; octopuses, 8; and nautiluses,
80 to 90
-Have highly developed nervous systems
-Exhibit complex patterns of behavior and a high
level of intelligence
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Annelid worms exhibit segmentation (building of body from repeated
units): Allows for specialization An annelid consists of a series of ringlike elements running the length of the body
-Divided internally by septa
The anterior (front) segments contain specialized sensory organs
A ventral nerve cord connects the ganglia in each segment with each
other and the brain
1. Class Polychaeta
2. Class Oligochaeta
3. Class Hirudinea
Annelids move using their hydrostatic skeleton -Each
segment contains chitin bristles (chaetae) that help
anchor the worms
Most have a closed circulatory system
They exchange O2 and CO2 through their body
surfaces
Excretory system consists of a pair of ciliated, funnelshaped nephridia per segment
Class Polychaeta (Polychaetes)
-Mostly marine worms, such as tubeworms
-Unusual forms and iridescent colors
-Have a differentiated head
-Have paired parapodia on most segments
-Used in swimming, burrowing, crawling
-Sexes are usually separate
-Typically lack permanent gonads
Class Oligochaeta (Earthworms and leeches)
-Mostly terrestrial
Earthworms
-Consist of 100-175 segments, with a mouth on the
first and an anus on the last
-Lack eyes, parapodia and head
-Have fewer setae than polychaetes
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Include two phyla of mostly marine animals: Bryozoa
and Brachiopoda
Both convergently evolved a lophophore
-A circular or U-shaped ridge around the mouth with
1-2 rows of ciliated tentacles
-Functions in gas exchange and feeding
Most members undergo radial cleavage
Bryozoans are small and live in colonies
-Their anus opens near their mouth
-Phylum also called Ectoprocta
-Individuals secrete a tiny chitinous chamber called a
zoecium
-Used for attachment
-Asexual reproduction occurs frequently by budding
Brachiopods have two
calcified shells
-Valves are dorsal and
ventral (not lateral as in
bivalves)
-Solitary lophophorates
Phoronids were once a
separate phylum
-Each individual secretes
a chitinous tube and lives
out its life within it
-Develop as protostomes
(unlike brachiopods)
Arthropods are the most successful animals
-1,000,000 species (2/3rd of all species)
-About 80% are insects
-For each human, 200 million insects are alive at
any one time
Arthropods affect all aspects of human life
Are divided into four major classes:
-Arachnids, myriapods, crustaceans and insects
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All arthropods have jointed
appendages
-Modifications: antennae,
mouthparts, legs
Arthropods also have an
exoskeleton made of
secreted chitin and protein
-Functions: protection &
muscle attachment
-Its thickness limits arthropod
body size
Arthropod bodies are
segmented
-Some segments are
specialized into functional
groups, or tagmata
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Arthropods have an open
circulatory system
Compound eyes are found in
many arthropods
-Composed of independent
visual units called
ommatidia
Other arthropods have
simple eyes, or ocelli
-Have single lenses
-Distinguish light from
darkness
Nervous system consists of
a double chain of
segmented ganglia on
ventral surface
-Brain seems to be an
inhibitor, rather than as a
stimulator, as it is in
vertebrates
Respiratory system
consists of tracheae and
tracheoles
-Connected to the exterior
by spiracles
Arthropods have a unique
excretory system consisting
of Malpighian tubules
-Eliminates nitrogenous
wastes as concentrated uric
acid or guanine
Arthropods periodically
undergo ecdysis or molting
-Shedding the outer cuticular
layer
Arachnids are largely terrestrial organisms
-Spiders, ticks, mites, scorpions, and others
The most anterior appendages, chelicerae, often function as fangs
or pincers
Body is divided into two main tagmata
-Prosoma (anterior): Bears all appendages
-Opisthosoma (posterior): Contains the reproductive organs
- Pedipalps (palps) are posterior to chelicerae
-Resemble legs, but have one less segment
-Used as copulatory organs, pincers, or sensors
Most arachnids are carnivorous
-Mites are largely herbivorous
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Order Araneae (spiders)
-About 35,000 species
-Many spiders catch their
prey in silk webs
-Silk protein forced out
of spinnerets found on the
posterior of the abdomen
-Other spiders actively hunt
their prey
-All spiders have poison
glands leading through
their chelicerae
Order Acari (mites and ticks)
-Largest and most diverse arachnid order
-Most mites are small
-Cephalothorax and abdomen are fused into an
unsegmented ovoid body
-Ticks are larger than mites
-Are blood-eating parasites
-Can carry many diseases (spotted fevers, Lyme
disease)
Centipedes (class Chilopoda) and Millipedes (class
Diplopoda) have bodies with a head followed by
numerous segments
Centipedes are all carnivores (eat insects)
Millipedes are largely herbivores
In both fertilization is internal
-The sexes are separate
Centipedes have fewer legs than millipedes
-Centipedes: one leg pair on each segment
-Millipedes: two on some or all segments
Crustaceans are primarily aquatic organisms
-Crabs, shrimps, lobsters, barnacles, others
Have three tagmata
-The two most anterior fuse to form a cephalothorax
Have two pairs of antennae, three pairs of
appendages, and various pairs of legs
Most appendages are biramous
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Decapod crustaceans include shrimps, lobsters,
crabs and crayfish
-Have ten feet
-Exoskeleton usually enforced with CaCO3
-Most body segments are fused into a cephalothorax
-Lobsters and crayfish have appendages that aid in
swimming
-Swimmerets and uropods
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Order Cirripedia
-Barnacles are crustaceans that are sessile as
adults
-Free-swimming
larvae
-Are hermaphroditic
-Some have stalks
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Insects are by far the largest group of animals
-More than half of all named animal species
Insects are primarily a terrestrial group
Approximately one billion (1018) insects are alive at
any one time
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External features
-Three body regions
1. Head = Has pair of antennae and modified mouthparts
2. Thorax = Has three segments, each with a pair of legs
-May have one or two pairs of wings
3. Abdomen
Most insects have compound eyes
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Echinoderms are an ancient group of
marine animals, with about 6000 living
species
-Characterized by deuterostome
development and an endoskeleton
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The echinoderm body plan undergoes a fundamental
shift during development
-Larvae = bilateral symmetry
-Adults = pentaradial symmetry
Body structure is discussed in reference to their
mouths, which define the oral surface
They are important marine predators
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Water-vascular system
-A hydraulic system
that aids in movement and
feeding
-Composed of a central
ring canal from which five
radial canals extend into
each of the body’s five parts
-Madreporite = Opening for water entry
-Ampulla = Muscular sac for tube feet control
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