سفالوپدا - Cephalopoda
Cephalopoda
A class in the phylum Mollusca that is
exclusively marine and contains the extant members—squids, cuttlefishes,
octopuses, and the chambered nautiluses—and the fossil taxa—ammonoids,
nautiloids, and belemnoids. The earliest known cephalopod fossils date to
roughly 500 million years ago from the Upper Cambrian of northeast
General
characteristics
Living cephalopods are bilaterally symmetric
predatory mollusks that typically have conspicuous, laterally oriented eyes.
Most have 8–10 appendages (8 arms and 2 tentacles) around the mouth; species of
nautilus have over 90 arms. The appendages are lined with one to several rows of
suckers or hooks, although the arms of nautilus lack conspicuous armature.
Paired chitinous jaws, similar to an upside-down parrot's beak, with large
muscles form the spherical buccal mass that contains the mouth and a tonguelike,
toothed radula (a character that, in part, defines the Mollusca). The highly
developed brain is positioned between the eyes where it is the center of the
extensive, proliferated nervous system. The shell, which was external in most
fossil cephalopods, as it is in modern species of nautilus, has become in all
other living forms internal, highly modified, reduced, or absent. Paired fins
that emerge from the side of the body in most coleoid cephalopods aid
locomotion. The main thrust of primary movement is generated by jet propulsion
in which water that is drawn into the sides of the mantle cavity is forcibly
expelled through the nozzlelike funnel.
Classification
Among mollusks, cephalopods are thought to
be most closely related to the class Gastropoda, although each group has had a
very long and distinct evolutionary history. The earliest fossils are external
shells recognized as those of cephalopods by internal septa thought to relate to
buoyancy control. The illustration shows the taxonomic hierarchy to the level of
suborder in the extant coleoid cephalopods, with a representative figure of each
taxon. See also: Gastropoda
Classification and morphology (body form) of
major groups of the class Cephalopoda.
Morphological characters that form the basis
of fossil cephalopod classification are those associated with shells. Characters
that form the basis of coleoid cephalopod classification, taxa that lack
external shells, include the presence or absence of a cornealike membrane over
the eye that separates nearshore or myopsid squids from oceanic or oegopsid
squids. Although squids share general characters, the more than 20 families are
diagnosed by a combination of the kind and arrangement of hooks or suckers on
the arms and tentacles, shape of the cartilaginous connections between the
mantle and funnel, and the presence or absence of tentacles at maturity.
Cuttlefishes are recognized by their characteristic cuttlebone.
Among octopods, the presence or absence of
fins separates the two suborders. Among finned or cirrate octopods, fin size and
the shape of the supporting structure that underlies the fins provide taxonomic
identification. Within-group distinctions among the finless or incirrate
octopods include the arrangement of the suckers, relative arm lengths and web
sector depths, skin color and texture, and copulatory organ features. Most
modern cephalopod diversity lies within the three nearshore genera Loligo,
Sepia, and Octopus.
Phylogenetic relationships among cephalopods
are poorly resolved. The octopods are recognized as the sister group to the
vampyromorphs, as both groups share characters that are otherwise unique in the
cephalopods. Relationships within the squids and between them and the cuttlefish
remain poorly understood, with several enigmatic groups complicating the issues.
Habitat
Cephalopods live in most marine habitats,
from the intertidal zone to depths of over 16,500 ft (5000 m). Each extant
cephalopod group tends to share ecological distribution. Chambered nautiluses
are restricted to tropical and subtropical waters of the Indo-West Pacific,
where they live associated with vertical reef faces from near-surface to 1600 ft
(480 m) deep. Squids typically spend their lives swimming; nearshore squids
typically occur on continental shelves and oceanic squids in open water.
Cuttlefishes tend to occur in shallow water where they rest on the ocean floor,
but they actively swim while hunting. Most octopod species are bottom-dwelling
animals that occur in virtually all benthic habitats. In shallow water,
octopuses tend to associate with shelters, which they either find or excavate if
in heavily sedimented areas. Lesser-known groups of octopods are pelagic at
various ocean depths. Cirrate or finned octopus that live at 660 ft (200 m) and
deeper were once thought to spend most of their time in the water column;
sea-floor observations show that they may also occur on the bottom at depths of
660 to 16,500 ft (200 to 5000 m). The only species in the order Vampyromorpha
occurs at midwater depths of 1650–3960 ft (500–1200 m) worldwide. Most
cephalopods are thought not to tolerate brackish water, although the myopsid
squid Lolliguncula brevis is known to enter water with salinities as low as 8.5
parts per thousand, much more dilute than the normal ocean salinity of 35 parts
per thousand.
Brain and
nervous system
Cephalopods are characterized by their
ability to change their behavior based on past experience, which requires memory
and is consistent with learning. Their eyes are extraordinarily well developed
as are their multilobed brains, which by some measures (ratio of brain to body
weight) exceed those of most fishes and reptiles. Diffuse ganglionated nerve
cords in the arms of octopuses and mantles of squids further indicate that their
advanced nervous system approaches that of some birds and mammals. Since
cephalopods are direct competitors with and frequent prey of marine fishes, some
experts argue that fishes have generated selective pressure resulting in the
rapid evolution of skin color and texture, predator detection abilities, and
eyes of modern cephalopods. See
also: Nervous system (invertebrate)
Eye
structure
Cephalopods—with the exception of species of
nautilus that have pinhole eyes and one species of finned octopod that has
reduced eyes—have large eyes that are very similar to those of vertebrates. Two
features demonstrate that the structures evolved in convergence. First, to focus
the eye on nearby subjects, cephalopods change the shape of the eyeball rather
than the thickness of the lens, as do vertebrates. Second, in cephalopods the
photosensitive cells point toward the lens, allowing the nerves that connect
them to the brain to exit directly out of the back of the eye. In vertebrates,
the photosensitive cells point backward, toward the retina, so they detect light
that bounces off the retina; their nerves must extend into and their common
optic nerve must exit out of the eyeball, creating a blind spot. Because
cephalopod eyes are better engineered, they have uninterrupted vision. See also: Eye (invertebrate); Eye
(vertebrate)
Respiration,
circulation, and excretion
Cephalopods extract oxygen from the water
using gills that are fully contained within the muscular mantle. The gills lie
on the side of the mantle cavity between the mantle wall and the viscera, where
they are immediately exposed to water as it is drawn inside the mantle. The skin
may act as an additional surface for gas exchange. The gills extend most of the
length of the mantle in squids. In some cirrate octopods, however, the gills are
reduced—rather than two attachments, a single one creates a semicircular
gill. See also: Arecidae
Gas exchange, oxygen uptake and release of
carbon dioxide from the bloodstream, is somewhat limited in cephalopods because
their copper-based oxygen-carrying blood pigment, hemocyanin, is not highly
effective in binding oxygen. The nearly closed circulatory system is pressurized
by right and left branchial hearts that force deoxygenated blood through their
respective gills and a medially situated systematic heart that pumps oxygenated
blood through the rest of the body. The elastic nature of the arteries and veins
is considered by some to contribute significantly to blood circulation. The same
ability to contract also allows the blood vessels to stop blood loss in the
event of an injury.
Nitrogenous waste is excreted in the form of
ammonia, with several organs involved. The renal sacs, branchial heart
appendages, digestive duct appendages, and even the gills contribute, with the
branchial heart appendages being the site of ultrafiltration. The intestine and
rectum prepare and expel indigestible material eaten with food from the body.
The renal sacs are unusual in harboring minute wormlike members of the
Dicyemida, which are only known from these organs of cephalopods. Ingested
material that resists breakdown is passed through the intestine to be released
from the viscera by the rectum, at the base of the nozzlelike funnel. A vigorous
exhalation flushes it from the mantle.
Food, feeding,
and digestion
Cephalopods are predators that feed on a
variety of invertebrates, fishes, and even each other. The relatively sluggish
nautiluses feed primarily on slow-moving prey such as reed shrimps, and even
scavenge castoff shells of molted spiny lobsters. Cuttlefishes prey on shrimps,
crabs, and small fishes, while squids eat fishes, pelagic crustaceans, and other
cephalopods. Benthic octopuses prey mostly on clams, snails, and crabs. Prey
capture is treated in Coleoidea.
Salivary glands secrete enzymes that begin
digestion, and in octopuses contain toxins that subdue their prey. The digestive
tract begins with the buccal mass that forms a mobile muscular sphere containing
the chitinous beak that holds or bites off chunks of prey. The toothed radula
working as a rasp moves these bits into the esophagus. Because the esophagus
passes through the brain, prey morsels need to be fairly small and lack hard
points. Digestion occurs primarily in the stomach where prey bits mix with
digestive juices from the salivary glands, the digestive gland (the analog of
the vertebrate liver and a lot more), and its appendages. The cecum is where
digestion is completed; absorption of food also occurs in this organ and in the
digestive gland and its appendages. Indigestible waste is moved through the
system to the rectum, where it is released from the anus at the base of the
funnel. It leaves the mantle cavity with water expelled during respiratory or in
jetting contractions of the mantle (body).
Size, age, and
growth
Cephalopods efficiently incorporate their
food into their bodies, allowing very rapid growth rates. Many species of small
cuttlefishes, bobtail squids, and octopuses reach reproductive maturity in less
than a year, while larger forms hatch in the summer, overwinter, and spawn at
12–14 months of age. The life spans of giant species of octopuses and squids are
not known but are estimated to be 5 years or less. The smallest adult
cephalopods are the Indian Ocean sepioid Idiosepius pygamaea at 0.2–0.8 in.
(6–22 mm) body length, the Caribbean myopsid Pickfordiateuthis pulchella at 0.8
in. (22 mm), and the octopuses Octopus micropyrsus from California at 0.4–1.4
in. (10–35 mm) and O. nanus from the Red Sea at 0.5–1 in. (12–26 mm). The true
giant squids of the family Architeuthidae are the largest invertebrates in the
world, with a total length (tip of tail to tip of outstretched tentacles) of
nearly 64 ft (20 m), a body length of 13–16 ft (4–5 m), and a weight of up to 1
ton. Most Architeuthis specimens are 32–38 ft (10–12 m) total length, 6–10 ft
(2–3 m) body length, and about 450–660 lb (200–300 kg). The largest octopus,
Enteroctopus dofleini, can have an arm span of 32 ft (10 m).
Defense
Because most cephalopods are unprotected by
hard coverings, they are the prey of many other animals. Cephalopods have a
myriad of defenses, most of which rely on escaping detection. To this end, they
have become masters of camouflage and escape.
Chromatophore organs allow the skin of
cephalopods to change color to match the background or mimic less desirable
targets for predation. In addition, musculature in the skin can erect papillae,
flaps, and knobs to match the background and break up the outline of the body.
Many midwater oceanic squids have photophores on their ventral surface that
match the intensity of light from above, thus reducing the conspicuousness of
their silhouettes. Most cephalopods have an ink sac, from which blobs of dark
ink are ejected to form a “decoy” for a pursuing predator. The two components of
the ink, mucus and pigment, protect the cephalopod because when the predator
bites the ink decoy, the ink temporarily blinds it and the mucus clouds its
chemoreceptors, allowing the cephalopod to escape undetected. Jet propulsion,
which despite its energy-intensive nature allows both rapid movement and
extraordinarily high maneuverability, also contributes to cephalopod defenses.
Benthic octopuses typically take refuge in shelters or dens that provide
protection from predators. Some octopuses excavate holes in areas of soft
sediment in which to hide. Squids occur in schools that may provide safety in
numbers. Cuttlefish rest on the bottom and often cover their dorsal surface with
a coat of sediment that effectively camouflages them. See also: Chromatophore; Coleoidea;
Photophore gland; Protective coloration
Locomotion
Cephalopods have perfected jet propulsion
for many modes of locomotion, from hovering motionless, to normal cruising, to
extremely rapid escape swimming. Water is drawn into the mantle cavity through
openings on the sides and ventral mantle when the muscular mantle (body)
expands. In squids and cuttlefish, the mantle opening can be closed by
cartilaginous thickenings to ensure that contraction drives all water out of the
mantle through the narrow funnel, propelling the cephalopod tail-first through
the water. A complex of nerves controls this action, the most notable of which
is the giant axon. Because nerve impulses travel faster in larger nerves, the
giant axon allows the impulse that starts the contraction of the mantle to reach
the dead-end part of the mantle first. As the wave of contraction moves forward
toward the funnel, it moves the largest possible volume of water out to generate
the most possible force, moving the squid as efficiently as possible. The giant
axon of cephalopods, the largest single nerve fiber in the animal kingdom, was
used as a model for neurobiology to increase our understanding of how nerves
function. Although the fins of sepioids, teuthoids, and cirrate octopods are
conspicuous, they primarily serve as rudders and stabilizers, providing only
limited locomotory power.
Benthic octopuses normally move by walking
or scuttling on their arms; when hunting they use each arm to explore the
substrate. Most species, however, will use jet propulsion when threatened or if
they are anxious to get somewhere. The use of buoyancy devices—such as the
cuttlebone of cuttlefish (Sepia), gas chambers in other shells (species of
nautilus, Spirula), low-density ions, especially of ammonium in special body
reservoirs (Cranchiidae) or even in the body tissues (Histioteuthidae,
Architeuthidae), and large amounts of oils in the liver (Bathyteuthidae)—reduces
the energy required when these cephalopods are swimming.
Reproduction
Cephalopods have separate sexes and internal
fertilization. Copulation in some species (such as nearshore squids) occurs at
spawning, that is, when the eggs are released; in other taxa (octopods), sperm
can be stored for periods of at least several months. Females produce eggs
inside their (single) ovary. When the eggs mature, they are released through the
paired oviducts to be inseminated by sperm from the male. Eggs may be encased in
individual capsules (most octopods, sepioids, some oceanic squids), or may be
enveloped in a gelatinous matrix and shaped into fingerlike clusters (nearshore
squids) or formed into sausage- or ball-shaped masses that drift in the open
ocean (some oegopsids). The fragile “shell” of the epipelagic argonauts
(Argonauta) is actually secreted by the female's arms to hold her eggs during
incubation. In one midwater pelagic octopod, the eggs develop entirely within
the female's long, convoluted oviducts.
Males produce sperm in their single testis;
it moves through a series of organs where it is packaged into cylinders
(spermatophores) that the males store. The fluid inside the spermatophore is of
comparatively high salinity. When the spermatophore is exposed to normal
seawater, osmosis drives water inside it. The resultant increase in internal
pressure forces the spermatophore to open and eject its sperm. A male transfers
spermatophores to females either with a specially modified arm, the
hectocotylus, or in those squids that lack a modified arm by a penis, which is
normally contained inside the mantle. Sperm in octopods is stored in glands in
the oviduct, but in cephalopods without sperm storage organs, spermatophores may
be injected into the female's mantle muscles, around the neck, under the eyes,
or around the mouth, depending on the species. In these species, sperm is mixed
with eggs as the female prepares the egg cluster.
Although most cephalopods leave their eggs
to develop on their own, females of incirrate (finless) octopods care for their
eggs during development, as does at least one species of oceanic squid. Egg
development proceeds over a few weeks to a few months depending on the egg size
and the temperature at which they develop. In deep-sea octopuses that produce
very large (over 50 mm or 2 in. long) eggs, development may take years. Most
hatchlings emerge from the eggs as planktonic “larvae” that, as they grow,
slowly take on the adult form of their species; in some species hatchlings look
like miniature adults.
Ecology
Cephalopods are important marine predators
that kill and eat most marine animal taxa. They, in turn, are important prey to
animals able to capture them. Because cephalopod beaks resist digestion,
analysis of gut contents can identify cephalopods eaten by large predators. They
are important components in the diets of toothed whales (sperm whales,
dolphins), pinnipeds (seals, sea lions), pelagic birds (petrels, albatrosses),
and predatory fishes (tunas, billfishes, groupers). In one case, North Atlantic
pilot whales were found to prey almost exclusively on one species of squid,
Illex illecebrosus, that aggregates for spawning in the summer. As cephalopods
grow, they become less vulnerable to smaller predators, but they also become
more attractive to larger predators.
Squids tend to form schools of individuals
of the same size. The similar size of squids within a given school has been
suggested to be due to the elimination of any smaller-than-average individuals
by cannibalism. Octopuses tend to be solitary, with individuals coming together
for copulation or cannibalism (the two are not exclusive, as females can
cannibalize their suitors). Observational studies of a limited number of octopus
species suggest that in part their high growth rates relate to the amount of
time they spend resting, which is typically interrupted only by bouts of
feeding.
Janet Voight
Clyde F. E. Roper
Fossils
Modern representatives of the Cephalopoda
are dominated in numbers by forms without a calcareous skeleton (such as
Octopus) or by those taxa with only a reduced or rudimentary internal skeleton
(most squids). Only a small minority of forms, such as the externally shelled
Nautilus and Argonauta, the internally shelled cuttlefish (Sepia), and the squid
Spirula, possess significant skeletal hard parts.
Fossil
record
The fossil record, however, suggests that
the typical morphologies of the world's cephalopodan fauna were quite different
in the past. During the Paleozoic and Mesozoic eras (from about 560 to 65
million years ago), the majority of cephalopod fossils came from species with
external shells, similar to that produced today by Nautilus. Although fossils of
the subclass Coleoidea (to which squids and octopus are today assigned) are
known, and in some deposits can even be extremely common, it appears that they
represented only a small fraction of Paleozoic and Mesozoic diversity. Even
accounting for their lesser chance of incorporation into the fossil record
because of their reduced internal skeletons, it appears that the coleoids were
never as abundant as the groups bearing external shells during the Paleozoic and
Mesozoic eras. Over 15,000 Paleozoic- and Mesozoic-aged species belonging to the
subclasses Nautiloidea and Ammonoidea are now known to science. In contrast, the
modern-day cephalopod fauna, dominated by the squids and octopuses, has only
several hundred species described to date, and appears to be very different from
the typical cephalopod faunas characteristic of the Paleozoic and Mesozoic
oceans. Most evidence suggests that the modern fauna is composed of taxa which
have evolved only after the end-Mesozoic extinction of most externally shelled
forms. See also: Mesozoic;
Paleozoic
Chambered
shell
Much of what is known and inferred about the
habits of the ancient cephalopods comes from study of the small number of extant
forms still possessing the external shells characteristic of the Paleozoic and
Mesozoic cephalopod faunas. For this reason the modern-day Nautilus has been the
subject of much research, for it represents the last living, externally shelled
cephalopod. (Although Argonaut, a very specialized pelagic octopus, also has an
external shell, this shell is produced by females only and serves as a
specialized brooding organ for developing eggs.) The Nautilus shell has the dual
purpose of lending protection to the vulnerable soft parts and serving as a
buoyancy device. The rear of the shell contains numerous compartments
partitioned by calcareous walls or septa; each chamber contains a central tube
within which is a strand of living flesh communicating with the rear of the soft
parts. This strand, called the siphuncle, is instrumental in achieving overall
neutral buoyancy of the entire animal by removing a seawater-like liquid from
each chamber. Although the modern Nautilus shows soft-part specializations which
may be adaptations to its typical deep-water existence in front of coral reefs
(and hence atypical of most extinct species, which appear to have lived in water
depths and environments different from those typical of Nautilus), the nautilus
shell shows close similarity with the shells of all fossil nautiloids and their
closely allied descendants, the ammonoids.
In both nautiloids and ammonoids, the soft
parts rest in a large space at the front of the shell. The shell enlarges
through new calcification along its aperture. New chambers are produced at the
back of the soft parts and are originally filled with the seawater-like liquid.
This liquid is removed via the siphuncle (a tabular extension of the mantle
passing through all chambers to the apex of a shelled cephalopod) by an osmotic
process at a rate that balances the density increases brought about by the
production of new shell and tissue growth. After completion of a new chamber,
the soft parts of the nautiloid or ammonoid move forward in the shell, creating
a space behind the body for another chamber. All fossil cephalopods reached a
final mature size and hence ceased shell growth.
With this cessation of shell growth, new
chamber formation no longer became necessary. The buoyancy function of the
chambered shell portion changed from one primarily dedicated to offsetting
density increase due to shell and tissue growth, to one concerned with making
small buoyancy compensations in response to slight, nongrowth weight changes,
such as weight increases brought about by windfall feeding, or concerned with
producing weight reduction, which could occur if shell material was broken off
during predatory attacks on the nautiloid or ammonoid. There is reason to
believe that buoyancy change was not used to power any sort of movement
vertically through the water column. At least in the modern Nautilus (and by
inference in the fossil nautiloids and ammonoids as well), the process of
buoyancy change appears too slow to aid in any type of active locomotion brought
about by rapid weight gain or loss.
Origins
The evolution of the chambered cephalopod
shell, with its neutral buoyancy capability, dates back to the Cambrian Period;
the shell was the central adaptation leading to the origin of the cephalopods
from their snail-like molluscan ancestors. The earliest cephalopods are assigned
to the subclass Nautiloidea. The first of these, known from Late Cambrian rocks
in China, were small (less than 1 ft or 0.3 m long) and bore straight to
slightly curved shells. By Ordovician time, however, adaptive radiations among
the ancestral nautiloid stocks gave rise to a wide variety of shell shapes and
sizes, including giants at least 10 ft (3 m) long. The subclass Ammonoidea arose
from nautiloid ancestors in the Devonian Period, and can be differentiated from
the nautiloids in details of shell morphology, especially relating to the
complexity and position of the septa and siphuncle. It was also during the
Devonian Period that the earliest internally shelled forms assignable to the
Coleoidea appeared. See also:
Cambrian; Devonian; Ordovician
Evolutionary
radiations and extinctions
The early Paleozoic nautiloids predated the
first-known fishes and hence may have been among the earliest large predators in
the sea. The subsequent evolutionary history of the nautiloids and their
ammonoid descendants was one of successive waves of rapid evolutionary
radiations. These externally shelled forms, however, were also very susceptible
to extinction, especially during the rare intervals of mass extinction which
occasionally occurred during the Paleozoic and Mesozoic eras. Following a period
of extinction, a new fauna of nautiloids and ammonoids would rapidly evolve.
This characteristic pattern of rapid speciation rates and high extinction rates
tends to make most species chronologically short-ranging and hence useful in
subdividing strata through biostratigraphy. Ammonoids are among the most useful
known fossils for subdividing the stratigraphic record into small, chronological
units.
The modern-day cephalopod fauna originated
only after one of the most spectacular of the mass extinctions, occurring at the
end of the Cretaceous Period. At this time, about 65 million years ago, a
still-flourishing ammonoid fauna was totally destroyed; most nautiloids also
died out as well. Although the record of Cenozoic cephalopod fossils is spotty,
it appears that much of the cephalopod fauna as it is known today evolved only
after the demise of the externally shelled forms. Thus, much as in the case of
mammals and dinosaurs, the so-called modern fauna (the coleoid cephalopods)
inherited the seas not because they were inherently superior to the ancient
shelled nautiloids and ammonoids, but because they found themselves in an ocean
emptied by mass extinction. See
also: Cenozoic; Extinction (biology)
Janet Voight
Peter D. Ward
Bibliography
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W. Arkell et al., Treatise on Invertebrate Paleontology, pt. L, 1996
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M. R. Clarke and E. R. Trueman (eds.), The Mollusca, vol. 11: Form and Function, 1988
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M. R. Clarke and E. R. Trueman (eds.), The Mollusca, vol. 12: Paleontology and Neontology of Cephalopods, 1988
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U. Lehmann, The Ammonites: Their Life and Their World, 1981
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R. T. Hanlon and J. B. Messenger, Cephalopod Behaviour, Cambridge University Press, 1996
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K. N. Nesis, Cephalopods of the World, T. F. H. Publications, Neptune City, NJ, 1987
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M. Norman, Cephalopods: A World Guide, Conch Books, Hackenheim, Germany, 2000
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P. Ward, The Natural History of Nautilus, Allen and Unwin.
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alifazeli=egeology.blogfa.com
Additional
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P. R. Boyle (ed.), Cephalopod Life Cycles, vols. I and II, Academic Press
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M. Nixon and J. Z. Young, The Brains and Lives of Cephalopods,
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Tree of Life: Cephalopoda
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Fossil Coleoidea Page
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CephBase
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Loligo pealei: The long-finned squid
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Cephalopod Research
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Cephalopods at the
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alifazeli=egeology.blogfa.com
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