Chapter 38: Arthropods

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Chapter 38 Arthropods
38-1 Phylum Arthropoda
38-2 Subphylum Crustacea
38-3 Subphyla Chelicerata and Uniramia
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38-1 Phylum Arthropoda
I. Characteristics of Arthropods (75 of ALL
Animals)
(1) Bilateral symmetry AND true coelom (2)
Segmented body WITH jointed appendages (3)
Exoskeleton AND Cephalization (eyes, feelers,
sensory hairs) (4) Ventral nerve cord AND
Open-Circulatory System
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NOTE The EXOSKELETON has THREE layers.
(1) OUTER WAXY layer repels WATER and prevents
dehydration.
(2) HARD MIDDLE layer ? MAIN protection (CaCO3).
(3) INNER layer ? FLEXIBILITY at joints greater
MOVEMENT.
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Critical Thinking
(1) Aquatic arthropods, such as crabs and
crayfish, typically have thicker, stronger
exoskeletons than do terrestrial arthropods, such
as spiders and insects. What advantage does a
thick, strong exoskeleton provide in an aquatic
environment?
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(1) Appendages

• JOINTED extensions of BODY, including LEGS and
  ANTENNAE.

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(2) Compound Eyes (in MOST arthropods)

• Eyes with MANY individual light detectors, EACH
  with its own LENS.

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II. Molting (hormone INDUCES molting, triggered
by GROWTH)

• NO growth without periodically SHEDDING its
  exoskeleton (many times over ones life).

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III. Evolution and Classification ( 600 m.y.a.)

• CLASSIFIED into 4 subphyla using DEVELOPMENT and
  MORPHOLOGY.

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(1) Tagmata (pl. tagma)

• Several segments FUSED to perform SPECIFIC
  functions. (i.e.,
  specialized for feeding, locomotion, and
  reproduction)

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(2) Subphylum Trilobita (e.g., extinct TRILOBITES)

• ANCESTRAL arthropods with ONE pair of similar
  appendages on EACH body segment (i.e., LITTLE to
  NO specialized tagmata).

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(3) Subphylum Crustacea

• Branched ANTENNAE and a PAIR of chewing
  MOUTHPARTS (mandibles) (e.g., shrimps,
  lobsters, crabs, crayfish, barnacles, daphnia)

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(4) Mandibles

• CHEWING mouthparts (primitive jaws).

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(5) Subphylum Chelicerata

• Arthropods WITHOUT antennae BUT have PINCER-like
  mouthparts (chelicerae).

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(6) Suphylum Uniramia (one-branch, includes
Myriapods and Insects)

• Antennae AND mandibles, BUT appendages are
  UNBRANCHED. (NOTE Uniramians are
  ONLY group evolving from TERRESTRIAL habitat)

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38-2 Subphylum Crustacea
I. Characteristics and Diversity of Crustaceans
( 40,000 species)

• 2 pairs of appendages on HEAD (ONLY arthropods
  w/2),
• 1 PAIR on BODY segments
• Most have GILLS, NAUPLIUS larvae, CaCO3 in
  exoskeleton (aquatic).

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Critical Thinking
(2) The freshwater flea Daphnia consumes algae
for nutrition. It also contains a prominent
eyespot. How might the eyespot be connected with
the ability of Daphnia to find food?
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(1) Nauplius (FREE-swimming larvae)

• 3 PAIRS of appendages and a SINGLE EYE in middle
  of head (following a series of MOLTS, it
  takes on ADULT body form).

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(2) Plankton (i.e., Zooplankton)

• MIXTURE of COPEPODS or DAPHNIA, and a LARGE
  percentage of BIOMASS in an ecosystem.

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(3) Cirrus (cirri, pl.)

• 6 pairs of ciliated legs REPLACE swimming parts
  of barnacle LARVAE and pull FOOD into mouth.

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Critical Thinking
(3) Barnacles are sessile crustaceans. What
structural adaptation do barnacles have that
enables them to compete with motile organisms for
food? What structural adaptation do barnacles
have that might protect them from predators?
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(4) Isopods (equal-legs)

• Terrestrial AND aquatic with 7 pairs of
  identical legs (e.g., sow bugs and pill bugs).

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II. The Crayfish

• FRESHWATER crustacean ( lobsters, marine
  relatives) classified as DECAPODS (with shrimps
  and crabs)

(1) Decapods

• TEN feet crustaceans with FIVE pairs of legs.

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(A) External Structure

• BODY is DIVIDED into 2 major sections
• (1) Cephalothorax
• (2) Abdomen

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(1) Cephalothorax

• Consists of 2 tagma HEAD (5 segments) and
  THORAX.

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Critical Thinking
(4) The cephalothorax of a crayfish is covered by
the carapace, a single, fused plate of
exoskeleton. What are some possible advantages
and disadvantages of this fused structure?
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(2) Thorax

• 8 segments found POSTERIOR to head.

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(3) Carapace

• Tough covering of fused DORSAL segments of
  cephalothorax.

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(4) Abdomen

• 7-segmented tagma lies POSTERIOR to
  cephalothorax.

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(5) Telson (NO appendages attached HERE)

• 7th segment forms a flat PADDLE at posterior end.

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Critical Thinking
(5) Like other arthropods, crayfish are
cephalized, with a variety of specialized sensory
structures on their head. However, crayfish also
have a high concentration of sensory hairs on the
telson. What might be the evolutionary advantage
of having so many sensory structures at the
posterior end of the animal?
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(6) Antennae Antennules

• Serve as FEELERS sensitive to touch, taste, and
  equilibrium.

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1. Maxillae (2 pairs) Maxillipeds (3 pairs)

• Chew food and help with respiration.

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(8) Chelipeds

• Large pincers used for capturing food and for
  defense.

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(9) Swimmerets (create WATER currents)

• Attached to ABDOMINAL segments, used for
  locomotion reproduction.

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(10) Uropods

• On TELSON, help to propel the crayfish during
  TAILFLIPS.

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(B) Digestion, Respiration, Open-Circulation, and
Excretion

• Gills are attached to walking legs ?DURING
  walking, legs circulate water across its GILLS
  (NOTE TEETH are in the STOMACH).

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(1) Digestive Gland (e.g., associated with
STOMACH)

• Food is ground up into a PASTE and mixed with
  ENZYMES further digestion AND absorption occurs
  in INTESTINE.

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(2) Green Glands (e.g., excretory organs,
crayfish live in HYPOTONIC)

• Eliminate EXCESS water from tissues through a
  PORE at base of ANTENNAE.

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Critical Thinking
(6) The American lobster, Homarus americanus, is
a NOCTURNAL organism. Marine biologists have
discovered that the lobsters senses of taste and
smell are over 1,000 times MORE powerful than
those senses in humans. The lobster uses taste
and smell both to search for food and to detect
mates. What adaptive advantage would these
highly developed senses provide for the lobster?
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(C) Neural Control

• Brain with VENTRAL nerve cord connect GANGLIA to
  muscles.

NOTE Crayfish sense VIBRATIONS and CHEMICALS
with THOUSANDS of small SENSORY HAIRS that
project from the exoskeleton.
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38-3 Subphyla Chelicerata and Uniramia
I. Class Arachnida (Subphylum Chelicerata
70,000 species)

• Body divided into CEPHALOTHORAX, an ABDOMEN,
  CHELICERAE, PEDIPALPS, and FOUR pairs of WALKING
  legs.

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(1) Pedipalps (on the cephalothorax)

• Hold FOOD during chewing, and (in spiders),
  transfer SPERM to the female during MATING.

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(A) Anatomy of a Spider

• HOLLOW fangs (modified chelicerae) and 6 to 8
  SIMPLE eyes.

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(1) Spinnerets (3 pairs of ORGANS on the TIP of
the abdomen)

• Made of hundreds of TUBES that connect SILK
  glands to ABDOMEN.

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(2) Book Lungs (i.e., spiders are terrestrial)

• SACS in abdomen with many PARALLEL FOLDS
  (resemble a book) and carry out GAS EXCHANGE.
  (NOTE MOST spiders have book lungs)

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(3) Tracheae (found in place of OR in addition to
book lungs)

• System of TUBES carry AIR directly to TISSUES
  from OPENINGS in exoskeleton (i.e., called
  spiracles).

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(4) Spiracles (in exoskeleton)

• OPENINGS that bring AIR into the tracheae during
  GAS EXCHANGE.

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(5) Malpighian Tubules (EXCRETORY system
terrestrial adaptation)

• HOLLOW TUBES that COLLECT body fluids and WASTES
  and carry them to INTESTINES, reabsorbing WATER.

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(6) Coxal Glands (EXCRETORY organs found in some
spider species)

• Organs remove WASTES through openings at BASE of
  legs.

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(B) Life of a Spider

• STEALTH hunters, most are VENOMOUS, males are
  smaller ? mate (and quickly leave) result in a
  female laying a COCOON of young spider eggs.

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(C) Scorpions (nocturnal chelicerates)

• Evolved large, pincer-like PEDIPALPS and a
  STINGER on LAST segment of abdomen (i.e., the
  telson).

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(D) Mites (CHIGGERS-larvae) and Ticks (MOST
diverse 30,000 species)

• COMPLETELY FUSED cephalothorax and abdomen
  (i.e., NO separation between them).

NOTE Although venom is NOT a characteristic,
species are PATHOGENIC or PARASITIC and inhabit
all types of environments.
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II. Myriapods (Subphylum Uniramia e.g.,
millipedes and centipedes)

• HIGHLY segmented bodies, a single pair of
  ANTENNAE, no waxy exoskeleton, and ALL are
  terrestrial.

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(A) Class Diplopoda (e.g., millipedes are
HERBIVORES)

• SLOW moving, ROUND-bodied with TWO pairs of legs
  on each body segment adapted for BURROWING
  through vegetation.

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(B) Class Chilopoda (e.g., centipede are
CARNIVORES)

• Flattened body, ONE pair of legs per segment
  appendages on first segment modified into a
  pair of VENOMOUS CLAWS.

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Extra Slides AND Answers for Critical Thinking
Questions
(1) The lobsters senses of taste and smell would
enable the animal to find food and mates at
night, when it is active and its vision is
limited by darkness.
(2) Most algae live in lighted areas. The
eyespot of Daphnia senses where light is
brightest and therefore where most algae are
likely to be.
(3) Barnacles have long appendages called cirri
that sweep small organisms and food particles
from the water into the barnacles mouth.
Barnacles have a very hard shell that completely
encloses the body in most species.
(4) Fusion of the exoskeleton strengthens a
vulnerable joint where the head and thorax join.
However, it also eliminates movement at this
joint, limiting the flexibility of the animal.
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(5) Sensory hairs on the telson can pick up
vibrations in the water that may signal the
approach of a predator from the rear. Thus, they
provide a defensive warning system for the
posterior end of the animal.
(6) A thick, strong exoskeleton is better able to
prevent an arthropods body from collapsing under
the pressure exerted by water.
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