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EXERCISE
ENCOUNTERS
WITH
LIFE
Kingdom Animalia:
Arthropoda, Mollusca,
and Echinodermata Phyla
BJ ECTIVES
.
After completing this exercise, the student
should be able to:
~
Identify the phylum and class of each of the animals in the jars on display.
~
Describe the characteristics contributing to the success of the arthropoda,
particularly on land.
~
List the three subphyla of the phylum Arthropoda, list the classes belonging
to each, and cite an example of each class.
el' Dissect a crayfish and identify the indicated parts.
~
Distinguish between a male and a female crayfish.
~
Identify each type of crayfish appendage.
@
List the six classes of Mollusca, and give an example of each.
• Identify the anatomical parts of a starfish.
@
Describe the water-vascular system in a starfish.
~
List the five classes of Echinodermata, and give an example of each.
~
List the features unique to each phylum and class in this exercise.
Phylum Arthropoda
About four-fifths of all living animal species are
arthropods-probably the most successful group of
animals ever to exist. Their success is attributed to
a basic body plan characterized by segmentation, a
hardened exoskeleton with jointed appendages, and a
high degree of specialization in the brain and central
nervous system, allowing for acutely instinctive be­
havior. The range of animals in this phylum is wide,
though they usually are small in size and are found in
almost every conceivable environment.
Members of several of the classes in this phylum­
such as the insects, centipedes, millipedes, and arach­
nids-are primarily terrestrial. They are better adapted
to a land environment than are any other invertebrates
largely because of the following characteristics:
1. a cuticle, which prevents water loss
2. efficient internal respiratory organs
3. jointed appendages with a hard, chitinous exo­
skeleton
Few arthropods are very large because of the
restrictions of their exoskeletons. Because the exo­
skeleton surrounds the body, it undergoes molting
periodically to allow for growth. Until the new exo­
skeleton hardens, the animal is helpless.
Below you will find a simplified scheme of classi­
fication for the phylum Arthropoda. The phylum is so
large that it is divided into three subphyla. Subphylum
Chelicerata include the arthropods that obtain their
food by way of hollow, fang-like structures called
chelicerae, using them to inject poisons/enzymes and
then suck up the liquid food through these hollow
structures. Members of the subphylum Mandibulata
use mandibles (jaws) to chew their food, although the
jaws move horizontally rather than vertically (how
odd!). Your textbook provides a more detailed de­
scription of each taxon. Table 24.1 lists the general
characteristics of the principal classes.
Furthermore, some (insects) have developed
wings, and their ability to fly has made possible their
distribution over the Earth. In addition, insects dis­
play wide variation in specialized mouthparts, allow­
ing for chewing, biting, piercing, lapping, or sucking.
This variety enables many insects to share the same
habitat without having to compete intensely for food.
TABLE 24.1
Phylum Arthropoda
Subphylum Trilobita: no living representatives
Subphylum Chelicerata
Class Merostomata: horseshoe crabs
Class Arachnida: spiders, mites, ticks,
scorpions, harvestmen, and daddy ~,
longlegs
Subphylum Mandibulata
Class Crustacea: lobsters, crabs, crayfish,
and shrimp
Phylum Arthropoda: General Characteristics of the Principal Classes
Cha racteristic
Crustacea
Insecta
Arachnida
Chilopoda
Diplopoda
Merostomata
body divisions
usually
cepha lothorax
and abdomen
head,
thorax,
abdomen
cephalo­
thoraxand
abdomen
head with
body of
similar
segments
head, short
thorax, long
abdomen
cephalothorax
and abdomen
2 pairs
1 pair
none
1 pair
1 pair
none
mouthparts
(pairs)
mandibles-1
maxillae-Z
maxillipeds-3
mandibles-1
maxillae-1
labia-1
chelicerae-1
pedipalps-1
mandibles-1
maxillae-2
maxillipeds
mandibles-1
maxillae-1
maxillipeds
chelicerae-1
pedipalps-1
legs
1 pair per
somite or
fewer
3 pairs on
thorax
4 pairs on
cepha lothorax
1 pair per
segment
2 pairs per
segment
4 pairs on
cephalothorax
gas exchange
gills or body
surface
tracheae
book lungs
and/ortracheae
tracheae
tracheae
book gills
principal
habitat
salt or
freshwater,
few on land
mainly
terrestriaI
mainly
terrestrial
all
terrestria I
all
terrestria I
marine
paired
appendages:
antennae
('
External Anatomy
Class Insecta: butterflies, bees, beetles,
mosquitos, etc.
Class Chilopoda: centipedes
Class Diplopoda: millipedes
Referring to Figure 24.1 for the external view,
note the exoskeleton. Anteriorly, it forms the carapace,
which covers the dorsal and lateral surfaces of the
fused head and thorax, the cephalothorax. The pos­
terior part of the body, or abdomen, is covered by
segmentally arranged chitinous plates. These plates
are named according to their positions: tergum =
dorsal plate; sternum = ventral plate; and pleuron =
lateral plate.
Examine the stalked compound eyes, a pair of
antennules, a pair of antennae, the six pairs of mouth
appendages, the large claws on the chelipeds, the
Examine the display jars containing arthropod
specimens. Attempt to determine the class to which
each belongs, using Table 24.1 as a guide.
CRAYFISH DISSECTION
Obtain a specimen of a crayfish, Cambarus (see
Figures 24.1, 24.2, and 24.3).
ABDOMEN
CEPHALOTHORAX
EVESTALK
ANTENNULE
TERGUM
..
";NTENNAi\====­
WALKING LEGS
(PERIOPODSI
A. LATERAL VIEW
SPERM DUCT OPENING
MANDIBLES
MOUTH
THIRD MAXILLIPOD
B. VENTRAL VIEW
Figure 24.1 Crayfish, Cambarus: External Features
......-- OSTIUM
HEART
__ -----fi'?,r-
~.::==-==-
GREEN GLAND / " MOUTH
OVIDUCT
DIGESTIVE GLAND
NERVECORD
Figure 24.2 Female Crayfish, Longitudinal Section
ANTENNULE (1st ANTENNA)
2nd MAXILLA (GILL BAILER)
1
2nd MAXILUPED
MANDIBLE
1st MAXILLA
Figure 24.3 Crayfish: Head Appendages
walking legs, the swimmerets on the abdomen, and
the broad uropods on the last abdominal segment.
These, together with the telson, form the fan-shaped
tail. All appendages are serially homologous. In the
early development and in basic adult structure, they
are all alike, even though they often differ in detailed
form and function. This basic structure may be exam­
ined in one of the swimmerets.
Remove the appendages on the left side of the
crayfish one by one, starting at the posterior end and
making sure that the whole appendage is taken off
exactly at the base.
Keeping track of both the numbers and the types
of appendages removed, lay them out in sequence on
a sheet of paper. From the abdominal segments you
should obtain, posterior to anterior, one telson, one
uropod, and five swimmerets. In the female, the an­
teriormost swimmeret is small or absent. In the male,
the swimmeret functions in sperm transfer and is
larger and anteriorly directed.
To obtain the thoracic appendages, remove the left
side of the carapace with scissors. This will expose a
gill chamber and the gills, which are attached to all of
the thoracic appendages except the first. Remove these
thoracic appendages with their gills attached. There
will be four similar walking legs and one large cheliped.
Next remove the mouthparts. From posterior to
anterior there are three maxilipeds, two maxillae, and
one mandible. The maxillipeds are more obviously
leglike than the other mouthparts and are used in
sensory functions and in handling and tearing pieces
of food. The maxillae are very thin and lie closely
pressed to the hard, clublike mandible (jaw).
Continue forward and remove in order the second
antenna and the first antenna. The first antenna is re­
ferred to as an antennule also. (Refer to Figure 24.3.)
Note the opening at the base of the second antenna.
This is the external opening of the green gland, the
excretory organ of the crustaceans.
INTERNAL ANATOMY
To locate the internal organs, refer to Figure 24.2.
Carefully loosen the remainder of the carapace and
the dorsal skeleton of the abdomen from the under­
lying membranous epidermis. Remove the exoskeleton
and cut the epidermis to expose the internal organs. If
muscles are in the way, do not tear them, but cut them
with scissors. Cover the animal with water, and study
the following: The small heart, showing several open­
ings, or ostia, is embedded in the pericardial cavity in
the middorsal region. Anterior to the heart is the stom­
ach, in the head region, a large sac containing a grind­
ing structure, the gastric mill, in its wall. Follow the
stomach posteriorly and trace the intestine to the
anus, located ventrally in the last abdominal segment.
Anteriorly, the stomach leads to a short esophagus,
which passes ventrally to the mouth. Locate the mouth
and probe through it to the stomach.
To each side of the stomach are the large, yellowish
digestive glands. Behind them, to each side of the heart,
are the gonads. The testes are difficult to distinguish
from the digestive glands, but the ovaries are coarser
in texture and darker in color (almost orange). Ducts
from the reproductive organs lead to the exterior
openings on the basal segments of the third pair of
walking legs in the female, and of the fifth pair of
walking legs in the male.
Starting in the abdomen and working forward,
carefully remove muscles and other structures to
expose the ventral nerve cords for their entire length.
Note the segmental thickenings of ganglional tissue.
Try to identify the ring of nerve tissue encircling the
esophagus and leading to the brain ganglia, which are
the anterior portions of the ring. Also, locate the pad­
like green glands (not green in preserved specimens),
which lie beside the esophagus.
Phylum Mollusca
The six classes of molluscs, containing an estimated
110,000 species, are represented by a tremendous
variety of body forms, all derived from the same
essential body organization. Molluscs are known as
the "soft-bodied animals."
Features unique to this phylum are:
1. A fleshy epithelial mantle that may secrete a
calcareous shell.
2. A muscular ventral foot.
3. A dorsal visceral body mass.
Some additional characteristics of the phylum are:
1. An unsegmented body.
2. An open circulatory system.
3. Respiration by a single or many ctenidia (gills), by
the mantle, or by the epidermis.
4. An extremely-reduced coelom.
S. Variety in feeding behavior and locomotion. The
chiton, for example, is designed for algal grazing
and adhering to wave-beaten rocks, the clam for
filtering fine food material, the snail for gliding and
protection, and the octopus for speed and predation.
Figure 24.4 gives examples from each of the mol­
luscan classes. Examine the specimens on display in
jars. Try to determine to which class each belongs.
(Monoplacophora)
FOOT
_--Jl:;;;'.
CHITON
(Polyplacophora)
TOOTH SHELL
(Scaphopoda)
SNAIL
(Gas1ropoda)
Figure 24.4 Representative Molluscs
CLASS POLYPLACOPHORA
The class Polyplacophora, which consists of algal­
grazing chitons, occurs mainly in marine intertidal
areas. Some preserved specimens are on display. Note
the foot, the gills, and the eight linked calcareous
plates that form a protective shell.
identifiable by their tubular shells, which are open at
both ends.
Dissection of a Clam
(a Representative Bivalve Mollusc)
EXTERNAL ANATOMY
CLASS MONOPLACOPHORA
Extremely primitive, somewhat segmented molluscs,
the class Monoplacophora is known from 300 million­
year-old fossils and from living specimens discovered
in 1952 off the coast of Central America.
Using Figure 24.5 as a guide, examine the external
shell of the clam. The more pointed end of the valves
is the posterior end. Find the posterior and anterior
ends. The valves or shells are secreted by the mantle
and are hinged together dorsally by the hinge ligament.
Locate the dorsal as well as the ventral regions of the
clam.
CLASS BIVALVIA
In general, bivalves are water-filtering organisms en­
closed in shells. Clams, oysters, mussels, scallops, and
similar organisms comprise this group, all of which
have shells consisting of two halves (bi = two) called
valves. The dorsally hinged valves are tightly closed
by well-developed adductor muscles.
CLASS GASTROPODA
The class Gastropoda includes snails, whelks, limpets,
slugs, and nudibranchs. Except for the slugs and nudi­
branchs, most gastropods possess a single spiral shell.
This class has a well-developed head with tentacles
and a rasping radula-a structure that enables them
to chew up vegetation.
CLASS CEPHALOPODA
The cephalopods are highly modified for motility and
active predation. Representatives of this class, such as
squid and octopus, possess a large head and eyes, a
highly developed brain and central nervous system,
8, 10, or more arms equipped with rows of sucking
discs, a mouth with horny beak and radula, and a
large siphon for controlled, rapid movement. The
arms, or tentacles, correspond to the foot in other
molluscs.
CLASS SCAPHOPODA
The elephant-tusk shells comprise a small class that
has adapted to life in mud or sand in marine waters.
They are seldom seen alive. Tusks, or tooth shells, are
INTERNAL ANATOMY
Referring to Figure 24.5, locate the internal organs.
Insert a scalpel between the lower edges of the valves,
and pry the valves apart enough to insert something
to act as a wedge. Cut the adductor muscles, which
hold the valves together, without damaging the internal
organs. Separate the valves to expose the mantle.
Once opened, and taking the valve without the
internal organs, locate the concentric growth lines on
the outside, and the remains of the adductor muscles.
Also note the creamy texture (mother of pearl) on the
inside of the valve.
On the other valve notice the adductor muscles
and the mantle, which lies over the visceral mass and
foot used for locomotion and digging. The visceral
mass contains the digestive and reproductive organs.
Locate the incurrent (more ventral) and excurrent
siphons at the posterior end. The siphons function to
draw water across the gills for respiration and feeding.
Food in the incurrent water is trapped in mucus on
the gills and is carried to the mouth by cilia on the
gills for ingestion.
Remove the mantle that covers the gills. Also
carefully remove the left gills and slit the bottom of the
foot to expose the labial palps, which aid in moving
the food trapped in mucus into the open mouth.
Usually you are able to see parts of the digestive
system. Locate the posterior portion of the intestine,
and note that the anus discharges into the excurrent
siphon. Trace the intestine forward. In the mid-dorsal
region of the body, it will pass into the pericardial
cavity or pericardial sac, which contains the heart.
Also locate the nephridium, or kidney, which is
beneath the pericardial cavity.
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DORSAL
FOOT
HINGE LIGAMENT
GROWTH LINES
External Features
PERICARDIAL CAVITY
ANTERIOR
ADDUCTOR
.. ~ MUSCLE
POSTERIOR ADDUCTOR MUSCLE
ANUS
---....L
'=71--
EXCURRENT SIPHON
MOUTH
FOOT
MANTLE
Internal Features
HEART
ESOPHAGUS
GONAD
Dissection of the Visceral Mass
Figure 24.5 Clam, Anodonta (a freshwater clam)
i ':;,;:-.
Phylum Echinodermata
CLASS ECHINOIDEA
Included in the phylum Echinodermata are the spiny­
skinned animals, such as the sea lilies, starfish, brittle
stars, sea urchins, and sea cucumbers. All of the species
are marine. Adult forms are sessile or slowly creeping
forms that are radially symmetrical around an oral­
aboral axis. The larvae are free-swimming, bilaterally
symmetrical forms. Echinoderms appear to have
evolved from an ancestral line having bilateral
symmetry, such as the flatworm or some similar
organism.
Features unique to this phylum include:
1. A water vascular system. This is a system of inter­
nal tubes communicating with the exterior by
way of a sieve plate or madreporite, which regu­
lates the amount of water in the system, and
ending in a paired series of tube feet running the
length of each ray, or along each section of the
fused endoskeleton, or test, in sea urchins. The
tube feet are extended by the contraction of mus­
cular bulbs, ampullae, at their inner ends, which
force water into the tube feet, making them turgid.
When the tube feet are brought into contact with
a surface, the ampullae relax, permitting the
echinoderm to adhere strongly to a surface with­
out further use of energy. This enables them to
withstand the crashing surf in the intertidal zone
as well as to open the shells of bivalve mollusks
for food.
2. Minute respiratory structures, skin gills, dermal
papillae, or dermal branchia.
3. A calcareous endoskeleton of movable or fixed
plates.
4. Numerous hard spines arising from the internal
skeleton.
S. Many minute pincers, or pedicellaria, which act
to keep the body surface free of debris, aid in
capturing food, and protecting the skin gills.
Spiny sea urchins and sand dollars are herbivorous
echinoderms constructed as if their arms were folded
back into a ball and fixed into a calcareous skeleton
or test, then covered with long, sharp, movable spines
and three-jawed pedicellaria. The test is globular in
sea urchins, and disc- or heart-shaped in sand dollars.
The tube feet are long, slender and equipped with
suckers. The mouth and anus are central or lateral.
The large gut fills much of the test cavity, except
during spawning periods.
CLASS HOLOTHLlROIDEA
Class Holothuroidea, the sea cucumbers, are sausage­
shaped garbage collectors that are among the chief
clean-up organisms of the ocean floor. With their
oblong shape and warty skin, sea cucumbers are well­
named. They vary in length from an inch to several
feet, with body wall consistency from leathery to
papery. Absent are arms, spines, pedicellaria, and
endoskeleton, except scattered tiny plates in the body
wall. Tube feet are present. The mouth with tentacles
is at one end of the body, and the anus is at the other.
The latter often bears a complex called the respiratory
tree, which functions in gas exchange.
CLASS CRINOIDEA
The stalked, flowerlike sea lilies and feather stars of
the class Crinoidea have five arms, which display up
to 10 or more branches, each bearing five branchlets
or pinnules to form a cuplike central disc. No spines,
pedicellaria, or suckers arise from the tube feet lining
the open ambulacral grooves. In a sea lily, the long,
jointed stalk with rootlike projections may attach the
animal to the substrate. In feather stars, the adult may
lack a stalk and be free-swimming, with motile, grip­
ping cirri and a mouth and anus on the upper, oral,
surface.
Some additional characteristics of the phylum are:
1. Ciliated organs.
2. Nervous system consisting of a circumoral ring
and radial nerves to the arms.
3. Lack of cephalization.
4. Complete digestive tract.
S. Lack of segmentation.
6. Open circulatory system.
The brittle stars of the class Ophiuroidea have a cen­
tral disc to which highly flexible, jointed limbs are at­
tached. Tube feet, confined to two rows and lacking
ampullae, have a sensory function. Pedicellaria and
anus are lacking, and the madreporite is aboral.
Living representatives are divided into five classes,
as illustrated on Figure 24.6. Also examine the many
specimens on display.
Class Asteroidea includes the predaceous star-shaped
or pentagonal sea stars. Other species in this class,
however, have as many as 50 arms. These ambulacral
CLASS OPHIUROIDEA
CLASS ASTEROIDEA
SEA URCHIN
SAND DOLLAR
ECHINOIDEA
SEA CUCUMBER
HOLOTHUROIDEA
STARFISH
ASTEROIDEA
SEA LILY
CRINOIDEA
grooves are separate, permitting movement. Short
spines and pedicellaria are present. The oral surface is
ventral, with two or four rows of tube feet lining the
open ambulacral grooves in each arm. The madre­
porite is aboral. Although sea stars are often called
starfish, this is a misnomer, as fish are vertebrates.
Dissection of Starfish.
Asterias forbesii
EXTERNAL ANATOMY
Use Figure 24.7 as a guide. Keep the specimen wet
by adding some water to the dissecting pan. Use
the stereomicroscope to aid you in the following
observations.
Note the central disk and five arms, or rays. On
the upper, aboral, surface, locate the madreporite, a
bright-colored area near the edge of the disc at the
junction of two rays. Can you find the red eyespot at
the tip of each arm? This structure is difficult to see in
preserved specimens. Observe the hard spines scat­
tered over the surface. Located among the spines are
the dermal branchiae and pincerlike pedicellaria.
On the oral or ventral surface, locate the mouth,
surrounded by large spines. Is any material protruding
from the mouth? If so, what might it be? Running
along the middle of each ray is an ambulacral groove,
with rows of tube feet. Identify the suction cups at the
ends of the tube feet.
LOCATION OF ANUS
MADREPORITE -.---,.,;,.
~~~rit:::.::::".
DERMAL BRANCHIAE
(SKIN GILLS)
Aboral Surface
AMBULACRAL GROOVES
RETRACTED}
EXTENDED
TUBE FEET
Oral Surface
Figure 24.7 Starfish, Asterias:External Features
INTERNAL ANATOMY
Cut off about one-half inch of the top of the ray
farthest from the madreporite and cut through the
aboral wall of the ray along each side toward the
central disc. Repeat this on an adjacent arm. Continue
cutting around the central disc such that you remove
a circular area from the disc, leaving only the madre­
porite and anus. Work carefully so the delicate organs
beneath are not macerated. Then, beginning near the
tip of the ray, carefully remove the aboral skeleton in
small sections, lifting and freeing it from the underlying
tissue before actually cutting off the skeleton. Carefully
examine Figure 24.8, which shows a partial dissection.
The mouth leads into a short esophagus (neither
of these is shown in Figure 24.8), which is connected
to a much-folded, saclike cardiac stomach, the portion
that can stick out through the mouth of the starfish
and start digesting the contents of an oyster. Above
the cardiac stomach is another portion of the stomach
known as the pyloric stomach. Connected to this are
five digestive glands, each of which is located in a ray.
Dorsal to the pyloric stomach are several rectal caeca
plus a short intestine attaching the stomach to the anus.
(Both of these are difficult to find.)
Each ray contains a pair of gonads, located under­
neath the digestive glands. They usually are a deep
brownish-red color and are smaller in size than the
digestive glands. After finding the gonads and digestive
glands, remove them from one of the rays. Notice the
skeletonous ambulacral ridge running down the center
of the ray. Along each side of the ridge are tiny, red­
dish, bulblike structures-the ampulla. Also, near
where the arms are attached to the central disc, find
the two bands of muscle per arm, which are responsi­
ble for movement of the arms.
To see the water-vascular system, again refer to
Figure 24.8. Remove the stomach and try to find the
stone canal leading orally from the madreporite to the
ring canal, a hard ring around the mouth, from which
branch five radial canals (one into each arm). Split the
ambulacral ridge lengthwise to find the radial canal.
It is connected to the ampulla and tube feet via short
lateral canals.
DIGESTIVE GLAND (HEPATIC CECA)
AMBULACRAL RIDGE
PYLORIC STOMACH
CARDIAC STOMACH
ANUS
:-'-o:--~
RECTAL CAECA
GONADS
(
MADREPORITE
STONE CANAL .
OSSICLES OF ENDOSKELETON
(BENEATH EPIDERMIS)
DIGESTIVE GLAND
SPINES
TRANSVERSE
CANAL
COELOM
GONAD. _ _---...
AMPULLA
DERMALBRANCHIAE
AMBULACRAL GROOVE
-.-«
Hj, - - -
TUBE FOOT
Water-Vascular System
Cross-Section of Arm
Figure 24.8 Starfish, Asterias: Internal Features
Review Questions
1. List the functions of the following appendages of arthropods:
Cheliped
Antenna
_
Wing
_
Maxilliped
_
Maxilla
_
Mandible
_
Swimmeret
_
Uropod
_
2. Use your text to help you find the function of the green glands in the crayfish.
_
3. What is the literal meaning of "arthropoda"?
_
4. In what respects do the Chelicerata and the Mandibulata differ?
_
5. What is the function of the water-vascular system in starfish?
_
6. Identify each of the following structures with respect to function, along with the phylum with which it is
associated:
Radula
-­
Ampulla
_
Madreporite
_
Siphon.
_
Compound eye
_
Respiratory tree
_
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7. What four characteristics have contributed to the success of Arthropods?
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a.
b.
c.
d.
8. What anatomical differences help the arthropods to survive away from water?
a.
b.
c.
d.
9. All members of the phylum Mollusca have the following three unique features:
a.
b.
c.
10.
MATCHING:
Match the class with the correct organism.
Common Name
---
chitons
Class
a. Gastropoda
_ _ _ squid, octopus
b. Bivalvia
_ _ _ oyster, clam
c. Cephalopoda
_ _ _ segmented and primitive
d. Polyplacophora
- - - snails
e. Monoplacophora
_ _ _ elephant tusk shells
f. Scaphopoda