Download A Squid: LoIigo

Marine Invertebrate Zoology
Phylum Mollusca: Gastropods and Cephalopods
Laboratory Procedures
Eastern Mud Snail: Class Gastropoda
Ilyanassa obsoleta
1. Obtain a gastropod provided by the instructor.
2. 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.
3. Posterior and ventral to the head is the large, muscular foot.
4. 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chitinous plate, the operculum, which covers the opening when the head and foot are withdrawn into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.
5. 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
The anatomy of most gastropods is difficult to work out
because the adult body is coiled 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.
RADULA REMOVAL
1. The radula is freed from the surrounding tissue by removing the snail’s body from the shell.
2. Once the animal is removed from its shell, 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.
3. 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!
4. Allow the solution to cool and locate the radula in a culture dish under a dissecting scope.
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5. Wash the radula in several changes of water and transfer to a clean slide.
6. Place a cover slip on the specimen and examine under the compound scope.
Food passes through the mouth and on to the stomach where it is digested. A digestive 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 cross-fertilization.
Later, each of the snails may deposits eggs in a gelatinous mass.
Class Collection
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
Loligo spp.
Squid are large, swift-moving, and highly specialized molluscs. They exhibit a well-developed 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 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.
EXTERNAL ANATOMY
1. 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.
2. 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.
3. 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 dorsal surface forward and its anterior surface up!
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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?
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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 structure by one sex (males
in this case) and not the other is an example of sexual
dimorphism.
4. Locate the muscular membrane at the base of the
arms and tentacles. In the center of the membrane
is the mouth opening.
5. 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.
6. 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.
7. Locate the funnel extending from the posterior side of the collar.
8. 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.
9. 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.
10. 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
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
1.
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4.
5.
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7.
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.
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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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Name __________________________________
Review Questions: Gastropods & Cephalopods
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. Squid usually employ a form of jet propulsion. What structure is used accomplish this method of
locomotion and how does it control direction?
5. How does fertilization occur in most cephalopods?
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