Download Invertebrates III

Biology 4B Laboratory
Invertebrates III: Arthropoda and Echinodermata
Objectives
 To understand the basic differences among the invertebrate animal phyla
 To investigate and learn the obvious external and internal characteristics of annelids, nematodes, arthropods
and echinoderms
 To investigate at the microscopic level the organization and function of selected tissues and cells within these
groups
Figure 1. Cladogram of the Major Animal Phyla based upon SSU-rRNA
INTRODUCTION
In this laboratory, we will continue to survey the remaining two invertebrate phyla: Annelida, Nematoda, Arthropoda and
Echinodermata (figure 1). We have already studied four of the five major protostome phyla. The only remaining
protostome phylum remaining, arthropoda, will study in this laboratory. Arthropods, like annelids exhibit metamerism,
the division of the body into segments. Segmentation is advantageous during development, where greater efficiency is
obtained by constructing a whole organism out of identical somites or segments. In the adult, locomotor activity is
enhanced because of the independent nature of each segment and the flexibility afforded by a series of segmented parts.
Segmentation also gives these phyla a survival advantage. Since many segments are similar to other segments in form and
function, damage to one or several segments does not necessarily compromise body functions.
PHYLUM ARTHROPODA
The phylum Arthropoda is the largest in the animal kingdom. More than 75% of all living organisms are arthropods with
insects contributing the greatest numbers. Like annelids they are characterized by metamerism, i.e. the body is
segmented. In addition, they have a chitonous exoskeleton. The segmented body is divisible into functional units called
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tagmata. In some arthropods three tagmata are present - a head (involved in feeding and sensory functions), a thorax
(involved mostly in locomotion), and an abdomen (which performs the visceral functions). In many arthropods the head
and thorax are fused, forming a cepahalothorax.
The phylum contains three extant subphyla - Chelicerata, Crustacea, and Uniramia. The subphylum Chelicerata contains
arthropods in which the first appendages are modified into chelicerae (pincer-like feeding structures). Well-known
representatives of this subphylum include the class Arachnida (scorpions, spiders, ticks, etc.) and the class Merostomata
(horseshoe crabs).
Major Arthropoda subdivisions (you are only responsible the taxonomy down to the subphylum level)





Subphylum Trilobita
Subphylum Chelicerata
o Class Merostomata - horsecrab
o Class Pycnogonida – sea spiders
o Class Arachnida
 Order Araneae - spiders
 Order Scorpionida - scorpions
 Order Opiliones - harvestmen
 Order Acari – ticks & mites
Subphylum Crustacea
o Class Branchiopoda
 Order Cladocera – water fleas
o Class Maxillopoda
 Subclass Copepoda - copepods
 Subclass Cirripedia - barnacles
o Class Malacostraca
 Order Isopoda – isopods, pill bugs
 Order Euphausiacea - krill
 Order Decapoda – crabs, shrimp, lobster, crayfish, etc.
Subphylum Myriapoda
o Class Chilopoda - centipedes
o Class Diplopoda – millipeds
Subphylum Hexapoda
o Class Insecta - insects
OBSERVATION OF ARTHROPODA
SUBPHYLUM TRILOBITA
This extinct group has members dating back to the Carboniferous to the Cambrian. The body has
two longitudinal furrows that run down the entire length. There is three distinct body sections:
head, thorax, and abdomen.
o Examine the fossil of a trilobite (Figure 2). Do not get it confuse with chitons.
Figure 2. Fossil trilobite
SUBPHYLUM CHELICERATA
Organisms that you will examine in this group includes: horseshoe crabs, spiders, ticks and scorpions. These organisms
are grouped here because the first pair of appendages is modified into chelicerae for feeding. They also have a pair of
pedipalps for capturing prey and four pairs of legs. There are two body segments: the cephalothorax and abdomen.
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 CLASS MEROSTOMATA
This group contains the aquatic chelicerates such as the horseshoe
crab and the extinct
eurypterids.
o
o

Examine the horseshoe crab (Limulus).
On the
horseshoe
shaped
carapace
comprises
the
cephalothorax, the simple eye and pair of compound
eyes can be found on the dorsal surface. Behind the
hinge is the abdomen. The telson is the tail.
Examine the ventral surface of Limulus (Figure 3). Find
the mouth. The chelicerae are the first pair of
appendage used to manipulate food. The pedipalps are
the second pair of appendages, used to capture prey.
The remaining four pairs of appendages are the walking
legs. On the abdomen, find the six leaf-like structures.
The first is the genital opercula and the remaining five are
the book gills (used for respiration).
Figure 3: Dorsal and ventral view of Limulus
CLASS ARACHNIDA
This class consists of members that are rather familiar
to most. They include spiders, scorpions, ticks and
mites. All members possess a pair of cherlicera,
pedipalps and four pairs of walking legs.
o
Examine the cephalothorax of the garden
spider Argiope (Figure 4). Note the number of
eyes. Identify the chelicerae. They have been
modified into fangs for the injection of poison.
Find the pedipalp. What’s its general function?
In males, the pedipalps are modified as an
intromittent organ to deliver sperm to the
female. How many walking do spiders have and
what body segment (tagmata) are they located?
o
Figure 4. Ventral view of a garden spider, Argiope.
Obtain a dissecting scope and examine the
ventral abdominal region of Argiope. Look for the
lung slit at the anterior portion of the abdomen. Towards the posterior end of the abdomen, you will notice
three pairs of spinnerets on a raised surface responsible for silk production.
o
Examine a scorpion. The pincers are the pedipalps. Note the stinger with venom sac at the distal portion of
the abdomen.
o
Examine the slide of a tick. These are ectoparasites on various vertebrates. Many can transmit diseases such
as Lyme disease and Rocky Mountain spotted fever.
o
Examine the slide of a mite. Mites are some of the smallest archnids.
SUBPHYLUM CRUSTACEA
In the subphylum Crustacea, mandibles are the primary feeding appendages. All crustacean appendages are biramous i.e.
they have two processes extending from the base. Gills are used in respiration. Shrimp, crabs, lobsters, and many
microscopic species are included in this subphylum.
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
CLASS MAXILLOPODA
You will examine one group of organisms within this class, the barnacles. The body of
the barnacle is sessile as an adult and is housed within a calcareous shell. We will have
the opportunity to see living barnacles at the tidepools. When the tide is out (when
we’ll be there); you will not be able to see the cirri (feeding legs).
o Examine the shell of the acorn barnacle (Balanus). These are attached directly
to the substrate.
o Examine the gooseneck barnacle (Figure 5). The main body is attached to the
substrate via a stalk.

CLASS MALACOSTRACA
We examine members in the order Decapoda only in this class. Decapods, as the name implies, have ten walking legs
on the Cephalothorax which is covered by a hard carapace. Many have the first walking leg modified into a cheliped
that is used in capturing prey and defense.
o Examine the dorsal and ventral surface of a preserved crayfish (Figure 6). Locate the following paired
structures:
Head
 Antennules – two, short filamentous structures at the tip of the rostrum for touch, taste and equilibrium
 Antennae – the long filamentous structure lateral to the antennules for touch and taste
 Mandible – bears teeth for crushing food
Thorax
 First maxilla – for handling food
 Second maxilla – food handling and bailing water from gill chamber
 Maxilliped (1 – 3) – touch taste and food handling
 2nd & 3rd maxilliped – have gills for respiration
 Cheliped (1st walking leg) – grasping food, defense and respiration
 Walking legs (2 – 4) – walking and respiration
Abdomen
 Swimmerets – circulates water
 Males – 1st is modified to transfer sperm to female seminal receptacle
 Females – assists in carrying eggs and young (2 – 5)
 Uropod & Telson – locomotion & protecting eggs (female)
Figure 5. Gooseneck barnacle, Lepas.
Figure 6. Dorsal (left) and ventral (right) external crayfish structure.
o
Examine other representative malacostracans (crabs, shrimp, etc.).
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SUBPHYLUM Myriapoda
Organisms in the subphylum Uniramia also have mandibles as the primary feeding appendages, however their appendages
are uniramous (having only one process extending from the base), and a tracheal system is used for respiration. This large
subphylum includes the following classes: lnsecta (Insects), Diplopoda (millipedes), and Chilopoda (centipedes).

CLASS CHILOPODA
o Examine preserved centipedes. They do not have a
hundred legs. They do have one pair of legs per body
segment (Figure 7) The maxilliped is modified into a fang
for the delivery of venom. Centipedes are active
predators living in moist places. If live ones are available,
observe their locomotion.

CLASS DIPLOPODA
o Examine preserved millipedes. These do not have a
thousand of legs. They do have two pairs of legs per
body segment (Figure 7). Like centipedes, millipedes can
be found living in moist habitats. However, they are
herbivores or scavengers feeding on decaying wood or
leaves. Some millipedes produce cyanide as a chemical
defense mechanism. Observe the locomotion of live
millipedes if available.
Figure 7. Centipede and millipede
SUBPHYLUM HEXAPODA

CLASS INSECTA
This is by far the largest group of animals with estimates of over one million named species. The major characteristics of
insects are: three walking legs on the thorax, one pair of antennae, three body segments (head, thorax and abdomen).
Many insects have either one or two pairs of wings.

Examine a preserved grasshopper and observe its external features (Figure 8). The exoskeleton is divided by
sutures into plates called sclerites.
o HEAD: The head consists of fused sclerites forming a cranium and mouth parts. A pair of antennae arise in
front of the compound eyes. Three ocelli (simple eyes) can be seen - one in the center, between the antennae,
and two located above the base of the antennae.
o THORAX: The thorax consists of three segments. The anterior prothorax bears the first pair of legs. The
mesothorax (middle segment) bears a pair of legs and a pair of leathery wings. The metathorax (third segment)
bears a pair of highly modified jumping legs and a pair of membranous wings which are extension of the
respiratory system. The legs are jointed.
 Examine the wings of a beetle (Orthoptera). The forewing is called the elytra which functions to
protect the membranous hindwing that’s used for flight.
 Examine the wings of a cranefly (Diptera). The forewing is for flight and the hindwing is reduced
and modified for balance.
o ABDOMEN: The abdomen is simple, devoid of appendages, and made up of 10 to 11 segments. Note the
terminal structures and use them to determine the sex of the specimen. Be sure to compare your grasshopper
to one of the opposite sex. In the female, the ovipositor is for laying the eggs inside the earth. At the tip look
for a pair of sensory structures known as cerci.
 Observe the female cricket and notice the long ovipositor for depositing eggs.
 On either side of the first abdominal segment you might see a thin membrane, called the
tympanum - a hearing organ. Spiracles are present on either side of most of the segments. The
spiracles are most prominent in the thorax region. They are the breathing pores of the elaborate
network of the tracheal system.
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Insect have mouth parts that are adapted for
the type of feeding they specialize in. There are
four basic mouth parts: sucking mouthparts,
sponging/lapping mouthparts, siphoning and
chewing/biting mouthparts. You want to be
able to differentiate these mouthparts for they
type of feeding (Figure 9).
o Examine the slide of the mosquito
head (sucking mouthpart)
o Examine the slide of the butterfly head
(siphoning mouthpart)
o Examine the slide of the fly head
(lapping mouthpart)
o Examine the slide of the honeybee
(chewing)
a.
Figure 8. External features of a female grasshopper.
c.
b.
d.
Figure 9. Insect mouthparts: a. Chewing, b, Sponging/Lapping, c. Siphoning and d. Sucking.
PHLYUM ONYCHOPHORA
Members in this group are often referred to as velvet or walking worms. They are an unusual group in that they have
characteristics of annelids and arthropods. Onychophorans have changed very little in the past 500 million years.
Aysheaia is a fossil onychophoran from the Burgess shale deposit that dates back to the mid-Cambrian. It looks very much
like the modern day onychophorans. Some have called this the “missing link” between these two phyla.
Onychophorans are a terrestrial species. They are active predators at night or when there is very high humidity.
o Examine the preserved velvet worm (Peripatus).
PHYLUM ECHINODERMATA
Echinoderms are a group of animals that arose from bilaterally symmetrical ancestors even though the animals show
pentaradial symmetry. Many of them have a bilateral larval stage and hence the radial feature may be secondarily
acquired. As you have already studied, most radially symmetrical animals are sessile, however echinoderms are free
moving. They are triploblastic and eucoelomate. Echinoderms are marine animals and that include: sea stars, sea urchins,
sea cucumbers, and sea lilies. The body parts are arranged in "fives" around the oral/aboral axis.
The most noticeable characteristics for echinoderms are the calcareous ossicles for the endoskeleton, the water vascular
system with tube feet, pedicellariae, dermal branchiae and pentaradial symmetry.
Major classes of Echinodermata include:
Asteroidea – sea star
Ophiuroidea - brittle stars, basket stars
Echinoidea - sea urchins, sand dollars
Holothuroidea - sea cucumbers
Crinoidea - sea lilies, feather stars
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OBSERVATION OF ECHINODERMATA
CLASS ASTEROIDEA
Sea stars are found in relatively shallow waters, and range in size from less than an inch to nearly three feet in diameter.
They feed primarily on bivalves, prying the shell to open with their tube feet, everting their stomach into the victim's body
cavity, and digesting it. The larvae are known as bipinnaria and have bilateral symmetry, whereas the adult form is star
shaped with arms not sharply marked off from the central disk. Sea stars can perform autotomy (self-amputation) of their
arms. However, if a small portion of the central disc remains attached to it, the amputated arm can then regenerate and
form a new individual (a clone).
o
Examine live, preserved or dehydrated sea stars (Figure 10). Identify the oral and aboral surfaces. Radiating from the
central disk are the five arms, noting their spiny texture (from which they get the name echinoderm - spiny skin). At
the tip of each arm is the eyespot. Note the calcareous spines, dermal branchiae (skin gills - little sac-like structures
on the skin) and pedicellariae (claws - tiny pincer-like structures on some living sea stars that can aid in food capture
or keep the sea star clean of debris).
o
The madreporite (a light colored, circular, slightly raised structure located on the aboral surface near the base of two
arms) is the opening, or intake, of the water vascular system. The anus is seen as a minute opening at the center of
the aboral surface. Ambulacral grooves are the deep grooves that extend from the oral surface along the midline of
each arm. The tube feet are seen as double rows of soft tubular "feet" on each arm, lying along and just inside the
ambulacral groove. On the
dehydrated specimen, the tube
feet may or may not be present.
o
Examine the living sea star, if
available,
observe
the
madreporite plate, eyespot,
sensory tentacles (located at
the tip of each arm/ray),
ambulacral groove, tube feet
and pedicellariae (if present).
CLASS OPHIUROIDEA
Figure 10: Dorsal and ventral sea star surfaces
Brittle
stars
are
secretive
echinoderms found from tidepools to great depths in the ocean. Although
they are one of the more agile and abundant echinoderm, they are not
frequently seen. On our tidepool trip, you will need to carefully turn over
rocks to find brittle stars. If caught, brittle stars will often detach an arm
(autotomize) in hopes that the predator is attracted to the wiggling arm as
the brittle star escapes. Brittle stars differ from sea stars in that brittle star
in that their ambulacral grooves are closed. Their tube feet are reduced
and to do not have suckers. As a result, the tube feet are not used for
locomotion. Instead, they “walk” with their arms.
o
o
Examine a preserved brittle (Figure 11). Do not handle them
Figure 11: Oral view of the central disk of a brittle star.
roughly. On the oral surface find: mouth, five triangular jaws,
oral shield located between the arms. Find the oral shield that’s modified into the madreporite plate. Note the spines
on each arm.
Examine the preserved basket star.
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CLASS ECHINOIDEA
This class includes sea urchins, heart urchins and sand dollars. This group is distinct in that they do not have arms and are
more or less globular. They have tube feet with suckers and movable spines. Their ambulacral grooves are closed and
covered by ossicles.
o Examine the test of sea urchin (Figure 12). On the aboral surface, fine the madreporite plate, anus, ambulacral groove
with tube feet pores, spine tubercle with or without the spine. On the oral surface, find the mouth and tooth.
o Examine the Aristotle’s lantern from the chewing
complex of a sea urchin.
o Examine a living sea urchin. Note the moveable spines
and the tube feet. Do the tube feet have suckers?
o Examine the sand dollar and heart urchin or sea biscuit
on display.
Figure 12. External structure of the sea urchin
CLASS HOLOTHUROIDEA
Sea cucumbers are placed into this class and have cucumber shape. There are no arms or spines. The ambulacral grooves
are closed. Sea cucumbers have soft bodies because their ossicles are microscopic and embedded in the thick muscular
wall.
o Examine the sea cucumber (Cucumaria) and find the mouth at one end with the anus at the other. The mouth is
surrounded by modified tube feet called tentacles.
CLASS CRINOIDEA
Most crinoids (seas lilies and feather star) are an ancient group of enchinoderms with few living members. The oral end
is “up” where as the aboral end has the attachment stalk or cirri.
o
Examine fossil or preserved specimens as available. Note the 10 arms with pinnules. The arms radiate from the calyx
where the digestive and other organs are located. The calyx and arms are collectively called the crown.
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