Download A Squid: LoIigo

Marine Invertebrate Zoology
Phylum Mollusca
Gastropods and Cephalopods
Class Gastropoda
Obtain a gastropod provided by the
instructor. Identify its fleshy anterior head
with two pairs of tentacles, one pair of eyes
at the tip of the second pair of tentacles, and
a ventral mouth. Posterior and ventral to the
head is the large, muscular foot. Above the
foot is the visceral mass covered by the
calcareous, coiled shell. When the animal is
threatened or disturbed, the head and foot
can be retracted into the shell. Some snails
have a chitinous plate, the operculum, which
covers the opening when the head and foot
are withdrawn into the shell.
 Identify the following parts on the
gastropod, which you are assigned.
Shell
Whorl
Operculum
Body whorl
Anterior end Aperture
Posterior end Outer lip
Dorsal side Inner lip
Ventral side Apex
Spire
and has lost much of the bilateral symmetry
of its internal organs.
The digestive system consists of the ventral
mouth; a muscular pharynx with a dorsal
chitinous jaw and a ventral radula; a tubular
esophagus; a large, thin-walled crop for
storage; and a rounded stomach. The radula
is a chitinous structure used to shred the
food on which gastropods feed. The radula
consists of numerous teeth attached to a
muscular organ that can move the radula
back and forth against food materials. The
radula is a constantly renewing ribbon of
transversely oriented teeth. Both herbivores
and carnivores use their radula to rasp their
food. Some snails even use their radula to
drill holes in other molluscs. The shredded
particles are then ingested.
The anatomy of most gastropods is difficult
to work out because the adult body is coiled
Food passes through the mouth and on to the
stomach where it is digested. A digestive
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
Radula Removal
The radula is freed from the surrounding
tissue by removing the snail’s body from
the shell. Then treat the snail with a
10% solution of NAOH or KOH. Treat
only the head region on large specimens.
On small mud snails treat the entire
body.
Treat the snail for 15 minutes (or until
the tissue dissolves) in a water bath
allowing the water to almost boil. Be
careful because the KOH or NAOH is
extremely caustic! Allow the solution
to cool and locate the radula in a culture
dish under a dissecting scope.
Wash the radula in several changes of
water and transfer to a clean slide. Place
a cover slip on the specimen and
examine under the compound scope.
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gland located in the apex of the shell
releases enzymes into the stomach.
Connecting to the stomach is a long, coiled
intestine terminating in the anus, which
empties through the fleshy mantle on the
right side of the shell.
Gastropods have no real brain, but the
nervous system consists of four pairs of
ganglia connected by nerves and with major
organs of the body. The principal sense
organs are the two eyes, an olfactory organ,
and two statocysts, which serve as balancing
organs.
Most gastropods are hermaphroditic. A
single, unpaired gonad, located near the
apex of the shell, produces both eggs and
sperm. Mating between two snails’ results in
the exchange of sperm and crossfertilization. Later, each of the snails may
deposits eggs in a gelatinous mass.
ocean depths.
Several species of the genus Loligo are
found in the coastal waters of the Atlantic
and Pacific Oceans and in the Gulf of
Mexico. Loligo pealeii is a common small
Atlantic squid; Loligo opalescens is a small
squid abundant in Pacific coastal waters.
Architeuthis spp., the giant squid found in
deeper waters of the Atlantic Ocean, is the
largest known invertebrate animal, with
some individuals reaching a length of 15—
20 meters.
Class Collection
Using a preserved specimen observe the
head, tentacles, and two large eyes. The eyes
are equipped with a pupil, iris, cornea, lens,
and retina and are capable of forming clear
images; they are remarkably similar to
vertebrate eyes in many respects. This is an
interesting and important example of
convergent evolution, in which two
organisms with quite different evolutionary
histories (squid and humans) have evolved
quite similar structures.
Note that the body of the squid is slender
and tapered, which facilitates its swift
movement through the water. The
orientation of the squid is rather unusual,
since morphologically the head and tentacles
represent the ventral surface of the body and
the pointed end opposite the head represents
the dorsal surface. The funnel or siphon is
located on the posterior surface. While
swimming, a squid actually travels with its
1. Examples of dextral and sinistral shells.
2. Examples of gastropods with reduced,
internal, or no shell (slugs, nudibranchs,
etc.).
3. Preserved specimens of other
gastropods.
Squid
Class Cephalopoda
Squid are large, swift-moving, and highly
specialized molluscs. They exhibit a welldeveloped nervous system, complex sense
organs, and sophisticated patterns of
behavior. They are good representatives of
the Class Cephalopoda, the most highly
evolved class of molluscs. The name
cephalopoda means “head-foot” and is quite
appropriate because the foot, one of the
basic anatomical elements of molluscs, is
highly modified in this class and actually
serves as a head. The “head-foot” contains
several large nerve ganglia, which
coordinate nerve impulses and several sense
organs that provide important sensory
information.
Squid are often abundant in the surface
waters of the oceans, where they may form
large schools. Dozens or hundreds of squid
may be captured together in trawl nets cast
overboard in Atlantic and Pacific coastal
waters. Other species of squid inhabit the
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External Anatomy
• Examine a preserved squid to identify its
major external features. Living squid in
an aquarium are interesting to watch and
to observe for swimming, feeding, and
other types of behavior, but are difficult
to obtain and keep alive.
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dorsal surface forward and its anterior
surface up!
Squid swim by a type of jet propulsion in
which water is squirted out of the funnel.
Since the funnel is under muscular control,
the direction of water ejection and thus the
direction of movement can be swiftly
changed. Movements of the arms and the
two lateral fins also aid in steering the
animal.
Locate the four pairs of arms on the head of
your specimen and two pairs of elongated
tentacles. The tentacles are retractile and
play an important role in feeding. Study one
arm and observe that it bears two rows of
suckers. Are the suckers all the same size?
Which suckers are largest? Which are the
smallest?
Each sucker is made up of a rounded cup
attached to the arm by a stalk or pedicle.
A special modification of the suckers on one
of the arms of mature males results in a
reduction in the size of the suckers and an
increase in the length of the pedicle
supporting the terminal suckers. This
modified arm is called a hectocotylus and
serves in the transfer of sperm bundles
(spermatophores) to the female in mating.
Such a modification of an anatomical
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structure by one sex (males in this case) and
not the other is an example of sexual
dimorphism.
Locate the muscular membrane at the base
of the arms and tentacles. In the center of the
membrane is the mouth opening. Inside the
mouth find the two chitinous beaks, which
are used to seize and tear the prey in
feeding. Prey organisms are captured by
swift movements of tentacles, which seize
the prey, hold it fast with the suckers, and
draw it toward the mouth with the aid of the
other arms. Poison glands provide
secretions, which aid in subduing active
prey.
The fleshy mantle surrounds the remainder
of the body and encloses the internal organs.
The ventral edge of the mantle adjacent to
the eyes is called the collar. Locate the
funnel extending from the posterior side of
the collar. On the anterior side of the collar,
you should find a projection of the mantle
that represents the ventral tip of the pen. In
the squid, the skeleton is reduced to a
translucent, internal, chitinous structure
called the pen plus several cartilages, which
protect the cephalic ganglia or “brain”
within the head-foot, and other cartilages
embedded in the collar region of the mantle
and funnel.
Locate also the two large lateral fins
extending from the mantle. As noted above,
the fins are movable and serve as steering
aids during swimming. On the surface of the
mantle are numerous chromatophores,
specialized pigment containing cells that
allow the squid to change the color of its
integument. A chromatophore consists of a
small sac of pigment surrounded by an
elastic membrane attached to many small
muscles. Contraction of these tiny muscles
causes the pigment to spread out and add to
the general coloration of the squid. A
chromatophore whose pigment is not spread
out contributes little to the apparent color of
the squid.
Since squid have several types of
chromatophores, each with a different
colored pigment, many shades of color can
be achieved. This aids greatly in the
camouflage and protective coloration of the
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squid. Live squid show almost constant
changes of color because of the continuous
activity of the chromatophores, which are
under nervous (and muscular) control.
Internal Anatomy
Study a demonstration of a dissected
specimen to observe the principal organs.
Posterior to the head is the funnel connected
with two large funnel retractor muscles.
Dorsal to the funnel, locate the rectum and
ink sac. A median kidney surrounds the
intestine. Two branchial hearts pump blood
to the two gills. Anterior to the paired
branchial hearts is the intestinal caecum. A
chitinous pen is attached to the anterior wall
of the mantle cavity and provides support
for the body.
Octopus
Class Cephalopoda
The body of an octopus is globular and
saclike with well-developed eyes and a
complex brain. Octopi usually crawl about
the sea bottom using their eight flexible
arms equipped with numerous suction cups
for locomotion and to grasp prey. They can
also swim by jets of water forced through
the funnel from the mantle cavity when
startled.
Octopi are carnivores and feed on molluscs
by drilling holes in the shells with their
radula and injecting a poison to kill their
prey. They also have a powerful chitinous
beak by means of which they can shred
tissue or attack enemies.
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Review Questions
1. Briefly, describe the concept of convergent evolution and how it applies to humans and some
cephalopods.
2. In squid what is the function of the chromatophores?
3. In gastropods, what is the function of the radula?
4. What is the sexuality of most gastropods (dioecious or hermaphroditic).
5. Squid usually employ a form of jet propulsion. What structure is used accomplish this
method of locomotion and how does it control direction?
6. How does fertilization occur in most cephalopods?
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