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Contents

A Broadclub Cuttlefish (Ascarosepion latimanus) camouflaged against the sandy ocean floor. Photo: Nick Hobgood (Source).

Introduction

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	Class Name Number of Species Percent   Gastropoda ca. 65.000 - 80.000 ~ 76% Diagram: Enlarge! Bivalvia ca. 20.000 ~ 21% Cephalopoda ca. 900 ~ 1% Scaphopoda ca. 900 ~ 1% Monoplacophora ca. 25 < 1% Polyplacophora ca. 1.000 ~ 1% Solenogastres ca. 300 < 1% Caudofoveata ca. 150 < 1% Mollusca ca. 85.000 - 100.000
	Number of recent species in Mollusca, displayed by classes, including percentage. Sources: WoRMS: MolluscaBase eds. (2025): Mollusca Linnaeus, 1758.

The cephalopods (Cephalopoda) are, by several criteria, considered the most highly developed group of molluscs (Mollusca). Like most molluscs, they share a basic body plan consisting of a head, a foot and a visceral mass enclosed by the mantle. In cephalopods, however, the foot has evolved into several arms or tentacles, as well as into the so-called funnel, a tube-like extension of the mantle cavity opening. By pressing water out of the mantle cavity through the funnel, cephalopods can cause a jet propulsion effect and thus are able to move through the water very quickly. Some squids are even capable of gliding for short distances through the air.

Another common name sometimes used for cephalopods is "inkfish" (e.g. in German or Dutch). Of course, cephalopods are not fish, and some species do not even possess an ink sac. Those that do, however, are able to release a dark cloud of ink when threatened and retreat under its cover.

Most cephalopods lack an external shell, with the exception of the Chambered Nautilus (Nautilus pompilius), which, because of this and many other primitive characteristics, is often regarded as a living fossil. The fossil record, however, contains a great diversity of shell-bearing cephalopods from many periods of Earth's history, including ammonites and belemnites. Living cephalopods instead are either equipped with an internal shell, such as the cuttlebone of cuttlefish or the gladius of squids, or like the octopuses have almost completely lost their shell.

Although some fossil cephalopods have reached enormous sizes, most living species are comparatively small, with a few remarkable exceptions. Among the ten-armed squids, the Giant Squid and the Colossal Squid may reach lengths of several metres and weights of several hundred kilograms. Yet despite the prominence of giant octopuses and the Kraken in myths and legends throughout human history, the actual giant octopuses living today are comparatively modest in size.

With almost 1,000 known recent species, cephalopods account for only around 1% of all molluscs present on Earth today. Unlike snails (Gastropoda) and clams (Bivalvia), they never left the marine environment during their evolutionary history. Octopuses in particular, however, are capable of brief excursions out of water and may also move between tidal pools.

Particularly remarkable is the highly developed nervous system of cephalopods. In addition to a central brain arranged in a ring around the oesophagus, octopuses, for example, maintain additional peripheral nerve centres (ganglia) within each of their eight arms, allowing those to largely function independently. The eyes of most cephalopods are also highly developed. Apart from the pinhole camera eyes of nautiluses, most living cephalopods possess sophisticated lens eyes, and those of the Giant and Colossal Squid rank among the largest eyes in the entire animal kingdom. Many cephalopods are also capable of deliberately changing their colour by means of specialised pigment cells in the skin, known as chromatophores. They use this ability not only for camouflage, but also for communication and even to confuse prey.

Cephalopods have been of great importance to humans since the earliest times. Many species, particularly squids and octopuses, are fished as food. Cephalopods are also studied scientifically in order to better understand their remarkable abilities. Octopuses are generally considered the most intelligent invertebrates and have demonstrated the ability to solve problems. As a result, they are popular, albeit not necessarily voluntary inhabitants of aquaria and zoological collections. Human fascination with cephalopods, however, is not purely economic or scientific. Since the earliest recorded history, numerous legends involving cephalopods have existed. Although many of these stories can now be explained through scientific research and technological advances, there remains a great deal that is still unknown about these remarkable animals.

Auralis und Natural World Facts: Why Octopuses Haven't Taken Over The World... Yet. ( YouTube Video).

Various Groups and Differences

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Class Cephalopoda Cuvier, 1795 Subclass Endoceratoidea † Order Endoceratida † Family Endoceratidae †: z.B. Cameroceras trentonense Conrad, 1842 † Unterklasse Nautiloidea Order Nautilida Superfamily Nautiloidea Family Nautilidae: z.B. Nautilus pompilius Linnaeus, 1758 Unterklasse Coleoidea Superorder Ammonoida † Superorder Decapodiformes Order Bathyteuthida Order Belemnitida † Order Idiosepida Order Myopsida Family Loliginidae: z.B. Loligo vulgaris Lamarck, 1798 Order Oegopsida Superfamily Cranchoidea Family Cranchiidae: z.B. Mesonychoteuthis hamiltoni G.C. Robson, 1925 Family Ommastrephidae: z.B. Dosidicus gigas (A. d'Orbigny, 1835) Superfamily Architeuthoidea Family Architeuthidae: z.B. Architeuthis dux Steenstrup, 1857 Order Sepiida Suborder Sepiina Superfamily Sepioidea Family Sepiidae: z.B. Sepia officinalis Linnaeus, 1758 Order Sepiolida Order Spirulida Family Spirulidae: z.B. Spirula spirula Linnaeus, 1758 Superorder Octopodiformes Order Vampyromorpha Genus Nanaimoteuthis: z.B. Nanaimoteuthis haggarti † Family Vampyroteuthidae: z.B. Vampyroteuthis infernalis Chun, 1903 Order Octopoda Suborder Cirrata Suborder Incirrata Superfamily Argonautoidea Family Argonautidae: z.B: Argonauta argo Linnaeus, 1758 Superfamily Octopodoidea Family Eledonidae: z.B. Eledone cirrhosa (Lamarck, 1789) Family Enteroctopodidae: z.B. Enteroctopus dofleini (Wülker, 1910) Family Octopodidae: z.B. Octopus vulgaris Cuvier, 1797. Source: MolluscaBase (2025): Cephalopoda Cuvier, 1795, vereinfacht.

Several specimens of Lesson's Reef Squid (Sepioteuthis lessoniana): Seychelles. Photo: David Kimr (iNaturalist) Enlarge Image!

Cephalopods (Cephalopoda) are an exclusively sea-living (marine) class of molluscs which, compared to the snails (Gastropoda) and clams (Bivalvia), comprise only a relatively small number of species. Worldwide, currently about 900 to 1,000 species of cephalopods (see above) are estimated to be present, representing only around 1% of all known recent molluscs (Mollusca).

The name "cephalopod" derives from the characteristic body structure of this animal group: The body of a cephalopod consists of a head and a visceral mass contained by a mantle, bearing several arm-like appendages, feet or tentacles. These appendages arise directly from the head, which explains the name, derived from the Greek κεφαλή (kephalē): head, and ποδ- (pod-): foot. While Aristoteles still used the name Polypus (πολύπους, polýpous: "many-footed") for these animals, the term "polyp", derived from this name, later came to be used exclusively for the juvenile or sessile stages of cnidarians, such as jellyfish. Finally, Georges Cuvier introduced the term Cephalopoda for the group in 1795.

The number of tentacles characteristic of the different cephalopod groups varies considerably. While the primitive nautiluses possess a large number of up to 90 tentacles, octopuses have only eight, equipped with suckers. Cuttlefish and squids, by contrast, maintain two additional and distinctly longer feeding tentacles, while the remaining eight shorter arms are primarily used for grasping.

In the past, cephalopods were traditionally divided into two "modern" groups, unsurprisingly according to the number of their appendages: the Decabrachia (ten-armed cephalopods), including squids and cuttlefish, and the Octobrachia (eight-armed cephalopods), comprising octopuses and related forms. These were contrasted with the obviously ancient group of nautiluses and their relatives.

As the simplified classification shown on the right demonstrates, however, this arrangement is no longer considered concurrent with relations between groups. Therefore, cephalopods today are classified according to phylogenetic relationships. Although cephalopod systematics is, as of today, still not yet completely resolved, the classification presented here follows the modern view according to MolluscaBase (2025). However, it should also be noted that MolluscaBase is a dynamic database that may change in response to new scientific findings.

The following pages therefore focus in greater detail on four major groups within the cephalopods: the octopuses (Octopoda), the squids (Myopsida), the cuttlefish (Sepiida), and the nautiluses (Nautilida). These groups differ considerably both in their evolutionary history and in their mode of life. While octopuses are primarily benthic and live mainly on the ocean floor, cuttlefish also inhabit near-bottom habitats but are generally much more mobile. Squids, on the other hand, live pelagically in the open water, in a manner somewhat comparable to many fishes.

Subclass Nautiloidea

Nautilus (Nautilus pompilius),frontal view: Brugelet- te, Belgium. Photo: Hans Hillewaert (Source).

A part of the ancient subclass Nautiloidea, with its only living order Nautilida, is, for example the Chambered Nautilus (Nautilus pompilius). Because of many features regarded as primitive - such as its large number of tentacles lacking suckers, the presence of four gills, and its external shell, unique among living cephalopods - the nautilus is often considered a living fossil.

In fact however, the lineage of the Nautilida is thought to extend much further back in time than that of the most famous fossil shell-bearing cephalopods, such as ammonites and belemnites, which occurred in great abundance in the seas of the Jurassic Period (see Geological Timescale).

Unterklasse Coleoidea

In contrast stands the subclass Coleoidea, the dominant group among the Cephalopods living today, divided into the ten-armed Decapodiformes and the eight-armed Octopodiformes.

Externally, the two principal groups of the Coleoidea differ mainly in the number of their arms or tentacles, apart from differences in lifestyle. Octopuses, as their name Octopus (Greek: "eight-footed") already indicates, possess eight arms, all equipped with suckers. Squids and cuttlefish, by contrast, possess ten appendages: eight shorter sucker-bearing arms used primarily for grasping, and two much longer feeding tentacles that are club-shaped at the tip and bear suckers only at their ends.

Superorder Decapodiformes

The Decapodiformes comprise the ten-armed cephalopods. Among them is the order Sepiida, containing the cuttlefish or "true" "inkfish" (Sepiidae). The order Myopsida, on the other hand, includes the "true" squids (Loliginidae). Although both groups share the presence of two elongated feeding tentacles in addition to the eight shorter arms, they differ greatly in both appearance and lifestyle.

Common Cuttlefish (Sepia officinalis): Zakynthos, Greece. Photo: Falk Viczian (iNaturalist) Enlarge Image!

Common Cuttlefish (Sepia officinalis):  Girona, Catalonia, Spain. Photo: Ealcaniz (iNaturalist) Enlarge Image!

Cuttlefish are generally relatively slow-moving swimmers that inhabit shallow shelf seas near the ocean floor, often extremely well camouflaged (see Colouration, Camouflage and Defence). For locomotion they rely mainly on the broad fin margin running along the sides of the mantle.

Caribbean Reef Squid (Sepioteuthis sepioidea): Virgin Islands. Photo: Monica Schandel (iNaturalist) Enlarge Image!
Flying Squid (Ommastrephes bartramii): Southeast of Kyushu, Japan. Photo: Maksim Stefanovich (iNaturalist) Enlarge Image!

Squids, by contrast, are usually fast-moving animals, often living in schools in the open ocean. They move primarily by jet propulsion and possess terminal fins on the mantle that provide stability and are also used for slower swimming movements (see Cephalopod Locomotion). Reef Squids (Sepioteuthis), however, although true squids, resemble cuttlefish more closely in their way of life. Their fins are greatly expanded and almost resemble the characteristic fin margin of cuttlefish.

The order Oegopsida includes, on the one hand, the largest known living cephalopods, such as the Giant Squid (Architeuthis dux, family Architeuthidae) and the Colossal Squid (Mesonychoteuthis hamiltoni, family Cranchiidae). Giant Squids, which may descend into the deepest regions of the oceans, can exceed 25 metres in total length, although the majority of this length is due to the feeding tentacles, while the body itself measures only around 6 to 8 metres.

On the other hand, the Oegopsida also include the fast-moving Flying Squids (Ommastrephidae), which are capable of travelling through the water at great speed in schools and, while pursuing prey or escaping predators, may even glide through the air for distances of up to 30 metres. For this reason they are commonly referred to in English as "flying squids" (see Locomotion). The Flying Squids, however, also include another very large squid species, the Humboldt Squid (Dosidicus gigas).

From among the recent eight-armed cephalopods (Octopodiformes), by contrast, only the so-called Giant Octopuses (Enteroctopodidae) have attained particularly large sizes. These include the Giant Pacific Octopus (Enteroctopus dofleini), found among other places along the coast of California, which may reach an arm span of up to 9.5 metres.

Superorder Octopodiformes

The Octopodiformes group finally consists of the eight-armed cephalopods, which themselves are divided into several distinct groups:

The "Vampire Squid From Hell" Vampyroteuthis infernalis. Photo: Monterey Bay Aquarium (MBARI). Despite its scientific name, which literally means "vampire squid from hell", this cephalopod is none of these things: Vampyroteuthis is not a vampire, but instead it feeds on so-called detritus: decaying organic matter. Also, it is neither a squid nor an octopus, but it rather represents a living fossil situated evolutionarily between these groups. Vampyroteuthis inhabits the deep waters of the Pacific Ocean and is capable of producing bioluminescent light at the tips of its eight tentacles. Source: Mandy Reid: "Scary by name but not by nature", Australian Museum Research Institute (2020).

The Vampire Squids (Vampyromorpha) firstly represent a highly interesting transitional group between squids and octopuses, with only a single living representative, the Vampire Squid (Vampyroteuthis infernalis, family Vampyroteuthidae). The nomenclature of this group is also noteworthy: names of genera ending in -teuthis are usually associated with squids.

The common name "Vampire Squid", however, is misleading, since this animal is not a squid at all (see box on the right), but rather an evolutionarily ancient transitional form between squids and octopuses. It is also the only known living cephalopod that does not feed as a predator, instead subsisting on detritus - decaying organic material in the deep sea. Since all known relatives are extinct and the species retains many primitive features, it is sometimes regarded as a living fossil, much like the Chambered Nautilus (Nautilus pompilius).

EV Nautilus: Close Encounter With a Vampire Squid. ( YouTube Video).
Mandy Reid: "Scary by name but not by nature", Australian Museum Research Institute (2020).
Natural World Facts: The Vampire Squid, a Living Fossil of the Abyss. ( YouTube Video).
Science Friday: The Vampire Squid From Hell. ( YouTube Video).

The fossil evidence, however, paints a different picture: Only recently (2025), a very large fossil cephalopod, Nanaimoteuthis haggarti from the Late Cretaceous (approximately 100–72 million years ago; see geological timescale) was discovered. Of these fossil cephalopods, only the beak is known. Extrapolating from the size of this structure, Nanaimoteuthis may have grown to a mantle length of up to 19 metres, potentially making it even larger than the modern Giant Squid (Architeuthis dux). While it was initially thought to represent a fossil giant octopus, it is now considered more likely that it was not a cirrate octopus (Cirrata), but rather a representative of the Vampire Squids, although one that differed greatly in lifestyle from today's members of that group and may have been among the apex predators of its time.

		Ikegami, S. et al. (2026): "Earliest octopuses were giant top predators in Cretaceous oceans". Science 392 (6796), p. 406-410. (Abstract).
		Dr. Polaris: "Cretaceous Kraken? Nanaimoteuthis, The Largest Octopus To Ever Live". ( YouTube Video).

Deep sea octopus (Graneledone boreopacifica) in 2500 m depth on the Galapagos ridge in the Pacific. Photo: NOAA (NOAA Photo Library).

Finally, the Octopoda themselves include both the Cirrate Octopuses (Cirrata) and the Incirrate octopuses (Incirrata), such as the Enteroctopodidae with the Giant Pacific Octopus (Enteroctopus dofleini), the largest recent octopus species, and the Octopodidae containing the "true" octopuses, including the Common Octopus (Octopus vulgaris), which occurs in European seas, the Day Octopus (Octopus cyanea, see below), and the iconic venomous Blue-ringed Octopuses (e.g. Hapalochlaena lunulata).

Also a part of the Octopoda is the remarkable group of the Paper Nautiluses (Argonautidae). Although they have only eight arms, female paper nautiluses nevertheless produce an external shell-like eggcase. However, unlike the shell of the nautilus and many other molluscs, this egg-case is formed secondarily by specialised arms and does not develop directly during the animal's embryonal development.

Several species of octopuses inhabit the deep ocean: Two examples are the Deep-sea Octopus (Graneledone boreopacifica), which has been observed at depths exceeding 2,000 metres and is notable for its exceptionally prolonged parental care, and the Hydrothermal Vent Octopus (Vulcanoctopus hydrothermalis), which, as its name suggests, was discovered at even greater depths (approximately 2,600–2,837 m) around hydrothermal vents ("hot vents") along the East Pacific Rise, much like the Scaly-foot Snail (Chrysomallon squamiferum).

There, it preys upon the various other animals that have likewise adapted to what may be among the most hostile habitats on Earth. Systematically, it belongs to the Enteroctopodidae family, together with the largest living octopus species, the Giant Pacific Octopus (Enteroctopus dofleini).

Auralis und Natural World Facts: The Most Extreme Ecosystem on Earth. ( YouTube Video).
EV Nautilus: Up Close with a Graneledone Octopus. ( YouTube Video).
MolluscaBase: Vulcanoctopus González & Guerra, 1998.
OctoNation: Vulcanoctopus.

Body Parts and Morphology

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Common Cuttlefish (Sepia officinalis): Marseille, France. Photo: Floroux (iNaturalist) Enlarge Image!

Although most cephalopods show little similarity to other molluscs at first, because of their highly specialised and different adaptation to their way of life, their basic body plan nonetheless clearly identifies them as molluscs. In the same way like in other molluscs, the body of a cephalopod is divided into a head, a visceral mass and a foot, while the mantle encloses and protects the visceral mass. Unlike in snails, for example, the foot in cephalopods has evolved into two distinct structures: firstly the tentacles or arms, present in varying numbers, and secondly the funnel, which is used for the characteristic jet propulsion of most cephalopods.

Like other molluscs, cephalopods have neither an internal nor an external skeleton composed of bones or skeletal plates. Instead, they possess what is known as a hydrostatic skeleton (hydroskeleton). In this system, the body wall together with the fluid-filled body cavity acts as a supporting framework against which the muscles operate, allowing movement and changes in body shape. Combined with their highly developed decentralised nervous system, this gives cephalopods remarkable flexibility and extraordinary abilities for camouflage.

Wikipedia: Hydrostatic Skeleton.

The Shell

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The composition of the shell is one of the defining features, by which the different classes of molluscs, such as snails, bivalves and, of course, cephalopods can best be distinguished. Since the earliest known fossil cephalopods from the Middle Cambrian approximately 500 million years ago (see geological timescale), cephalopod shells have been characterised by internal chambers that provided buoyancy (see Fossil History). The cephalopods' survival of repeated global mass extinction events throughout Earth's history and, apart from that, their repeatedly occurring abundance during different geological periods demonstrates the evolutionary success of this strategy.

Around 30,000 fossil cephalopod species are known from the geological record, most of them shell-bearing forms. These include the iconic ammonites with their coiled shells, which populated the seas of the Jurassic Period in enormous numbers and species diversity, as well as the belemnites from the same geological periods, parts of whose straight, uncoiled shells became known in folklore as "thunderbolts", referring to the lightning bolts hurled by Zeus in Greek mythology. Other straight-shelled (orthoconic) fossil cephalopods include the gigantic Cameroceras from the much earlier Ordovician Period (see geological timescale).

However, it should also be remembered that cephalopods with a shell are usually far more commonly preserved as fossils than their relatives with little or no shells. In those, like in the recent Coleoids, there is naturally very little that can fossilise, as soon as the organic body has decomposed, with the notable exception of the parrot-like beak (see Nutrition). Often, this is the only proof we have that a prehistoric shell-less cephalopod, such as an octopod, existed at all.

Nautiluses (Nautilidae)

Cyrthoceratites sp., a fossil Nautiloid from the middle Ordovician to the middle Devonian (ca. 470 - 370 mio. years ago).
Cenoceras lineatum, a fossil Nautiloid from the middle Jurassic (ca. 200 mio. years ago). Images: Nobu Tamura (Source).

An important exception today are the nautiluses (Nautilidae, order Nautilida), an evolutionarily ancient group whose origins extend back to the Late Cambrian and Early Ordovician, although the family Nautilidae itself is only known with certainty since the Eocene. Today, however, only two living genera remain, Nautilus and Allonautilus, while all related groups and ancestral lineages have become extinct. For this reason, and because of the many primitive features retained by living nautiluses - not least their shell - they are regarded as so-called living fossils.

Above: The head of a Chambered Nautilus (Nautilus pompilius) with the leathery head cap and one pinhole camera eye below. Photo: Hans Hillewaert (Source). Right: Opened shell of Nautilus pompilius with opened chambers and siphuncle channel. Photo: Anne Laudisoit (iNaturalist).

The coiled shell of the nautilus does, however, differ significantly from the likewise coiled shell of snails (Gastropoda) in several respects: Firstly, the shell of a nautilus is coiled in a single plane, whereas the shell of a snail typically forms an asymmetrical spire. In addition, the shell of a nautilus is internally divided into individual chambers. A tubular extension of the soft body, the siphuncle, passes through those chambers and allows the animal to regulate the gas content within, thus controlling buoyancy in a manner somewhat comparable to modern submarines.

In fact, it appears that the chambered shell was one of the earliest features distinguishing the first cephalopods from other molluscs during the Cambrian, approximately 500 million years ago. It provided them with the ability to regulate buoyancy and thus freed them from permanent association with the sea floor.

		PBS Eons: How the Squid Lost Its Shell. ( YouTube Video).
		PBS Eons: Nautiloids Thrived For 500 Million Years Until These Guys Showed Up. ( YouTube Video).

Ram's Horn Squids (Spirulidae)

The second recent group of shell-bearing cephalopods living today are the Ram's Horn Squids (Spirulidae), represented only by one single species, Spirula spirula. Like the freshwater ramshorn snails (Planorbidae), they owe their name to the shape of their coiled shell, resembling the horn of a ram.

Unlike in a nautilus, the shell of Spirula is almost completely enclosed by the mantle, similar to the condition seen in certain terrestrial glass snails (Vitrinidae). Also contrary to the nautilus, Spirula has ten arms: eight shorter ones and two elongated feeding tentacles, the same like squids and cuttlefish. Thus, like them, it belongs to the Coleoidea.

The shell of a Ram's Horn Squid (Spirula spirula), washed up in Western Australia. Photo: Matt Berger (iNaturalist).

Ram's Horn Squids are mesopelagic animals. During the daytime they inhabit depths between approximately 500 and 1,000 metres, at night ascending to depths of around 300 metres in search of food. Like the nautilus, they use their chambered shell to regulate buoyancy. However, they do not rely upon buoyancy alone for vertical movement, but instead mainly use the fins at the posterior end of the mantle for active swimming. In addition, Ram's Horn Squids possess a bioluminescent organ at the tip of the tail capable of emitting green light. Although their deep-sea lifestyle makes direct observation in their natural habitat extremely difficult, large numbers of their shells are frequently washed ashore in the tropical and subtropical regions where they occur.

Schmidt Ocean Institute: Ram’s Horn Squid (Spirula spirula). ( YouTube Short): First live footage of a ram's horn squid.

Unlike the Nautiloidea, the Ram's Horn Squids belong to the Coleoidea together with most modern cephalopods, although within this group they form a distinct order of their own. All related groups apart from the family Spirulidae, however, are extinct. Fossil relatives of the Ram's Horn Squids are known at least since the Late Cretaceous, approximately 100 million years ago, while even older Carboniferous fossils may also represent related forms. The best-known fossils originate from the Tertiary deposits of northern Germany, where they are relatively common (see geological timescale).

Hoffmann, R.; Warnke, K.M. (2014): "Spirula - das unbekannte Wesen aus der Tiefsee". Denisia. 32, p. 33-46. (Link).
NIWA (Earth Sciences New Zealand): Critter of the week: Spirula spirula.
Wikipedia: Spirula.

Shell Reduction in Cuttlefish, Squids and Octopuses

A tendency towards shell reduction has existed among cephalopods for a very long time. A fossil beak discovered in 2025, belonging to a gigantic octopus-like cephalopod from the Cretaceous Period (approximately 100–72 million years ago) demonstrated that shell-less cephalopods had already become not only very large, but were also rather prominent within marine food chains. The possible advantage is obvious: over time, cephalopods reduced the shell and sacrificed its protection in favour of greater energy efficiency due to reduced weight, as well as increased mobility. Modern squids, for example, are capable of extremely rapid movement, while octopuses can hide within the narrowest crevices and rock cavities.

Cuttlebone of a Common Cuttlefish (Sepia officinalis): Thessaloniki, Greece. Photo: Kostas Zontanos (iNaturalist).

Among recent cephalopods — with the exception of the examples already discussed — shell reduction has progressed to varying degrees, similar to developments seen among snails:

Paper Nautiluses (Argonautidae)

The small Paper Nautiluses or Argonauts belong to the octopuses (Octopoda): Like the "true" octopuses, they also possess eight arms. Unlike most octopuses, however, they live epipelagically, swimming in the open ocean instead of foraging mainly on the sea floor. They also display a pronounced sexual dimorphism: unlike the female, the male is dwarfed and reaches only about one fifth of the female's size. The English term "Paper Nautilus", however, is highly misleading, since the two groups are only distantly related, insofar as both are cephalopods.

Winged Paper Nautilus (Argonauta hians, likely a young female without an egg case) on a pelagic jellyfish: Mabini, Batangas, Philippines. Photo: Xenomatt (iNaturalist) Enlarge Image!

Robert Stansfield: A crazy night of argonauts. (Facebook Video).

The tips of two arms in the female Paper Nautilus are expanded into sail-like appendages. Shortly before reaching sexual maturity, the female starts building a coiled eggcase using glands in those specialised arm tips. That eggcase consists of calcite rather than aragonite and is extremely thin-walled, almost paper-like in appearance, hence the group's vernacular name: Paper Nautiluses.

Unlike the shells of bivalves or snails, the eggcase is not firmly attached to the soft body by muscles. Instead, the female holds it with several of her arms. Consequently, she is can also leave the eggcase, unlike a snail, which remains permanently attached to its shell and generally dies if the shell is lost.

Bizarre Beasts: The Only Octopus With a  Shell. ( YouTube Video).
Em Gems: The Paper Nautilus, An Octopus in Disguise. ( YouTube Video).

Unlike the shells of other molluscs, however, the eggcase of a Paper Nautilus is not there to protect the animal itself, but is rather needed as a container for its eggs, allowing the female to carry the brood with her while searching for food in the open ocean. The eggcase also has no role in locomotion. Although air is enclosed within the upper portion of the egg case and contributes to its buoyancy, no gas exchange occurs between it and the animal's soft body. Instead, the Paper Nautilus moves by means of the jet propulsion mechanism characteristic of most cephalopods.

As a conclusion, the eggcase of the Argonaut is not homologous with the other molluscs' shell, but represents a completely different and independently evolved structure. The evolution of cephalopods does, however, demonstrate impressively that even a fundamentally important characteristic such as the molluscan shell may be repeatedly modified, reduced, or re-evolved in entirely new forms during evolutionary history.

Wikipedia: Argonauta argo.

The Eyes

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Squids (in this case Lesson's Reef Squid, Sepioteuthis lessoniana) have giant eyes, compared to their head: Red Sea, Western Coast, Egypt. Photo: Jean-Paul Cassez (iNaturalist).  View of entire animal.

Alfred Brehm once wrote of the cephalopods that "their eyes are extraordinarily large, not only in proportion to the body, but also in themselves.". While he was far from alone in making this observation, the eyes of cephalopods indeed rank among the most highly developed invertebrate visual organs.

There is, however, a varying degree of eye development among cephalopods: For example, the rather primitive nautiluses still use so-called pinhole camera eyes, which operate based on the principle of a camera obscura. In this system, the eye opening is reduced to such a small aperture that an image can be projected onto the retina even without a lens. In the relevant episode of the BBC series "Walking with Dinosaurs - Sea Monsters", such eyes are also postulated for the gigantic cephalopod Cameroceras from the Ordovician (see geological timescale), which Nigel Marven is able to defend himself against in the film by shining a flashlight into its eye.

The "modern" cephalopods on the other hand: cuttlefish, squids and octopuses, have much more highly developed lens eyes. The eyes of a Giant Squid (Architeuthis dux), whose existence could not been confirmed until relatively recently, are among the largest known eyes in the animal kingdom, with a diameter of up to 25 - 30 cm. Giant Squids inhabit the dark depths of the ocean, where bioluminescence is virtually the only available source of light. A Giant Squid is capable of detecting a sperm whale at over 100 metres away. A sperm whale's sonar may reach farther, but its eyesight nonetheless gives the squid a reasonable chance of escape.

The Colossal Squid (Mesonychoteuthis hamiltoni), on the other hand, might even be able to make use of sight enhanced by its own bioluminescence in order to detect both threats and prey in the darkness of the Antarctic deep sea.

Nilsson, D.; Warrant, E.; Johnsen, S.; Hanlon, R.; Shashar, N. (2012): A Unique Advantage for Giant Eyes in Giant Squid. Current Biology 22, p. 683 - 688.

Unlike the eyes of vertebrates, however, cephalopod eyes are so-called everted eyes. Contrary to the inverted eyes of vertebrates, the cephalopod eye develops from an infolding of the embryonic outer tissue and only later becomes connected to the brain through nerve cells.

The eye of a Common Cuttlefish (Sepia officinalis): Setubal, Portugal. Photo: João Pedro Silva (iNaturalist).

While the retina in vertebrate eyes is inverted, so incoming light needs to first pass through several cellular layers before reaching the photoreceptor cells, in cephalopods, on the other hand, the retina is not inverted, and so the photoreceptor cells directly face the incoming light. Also, the lens of the cephalopod eye develops differently than in a vertebrate eye. Thus, the evolution of eyes in molluscs and vertebrates makes for a classic example of convergent evolution.

Wikipedia: Cephalopod Eye.
Hanke, F.D.; Kelber, A. (2020): The Eye of the Common Octopus (Octopus vulgaris). Frontiers in Physiology 10:1637. (Link).

Within the molluscs, an evolutionary progression is visible, from the simple pit eyes of primitive snails, through the more advanced pinhole camera eyes of the nautilus, to the simple lens eyes of terrestrial pulmonate snails and finally the highly sophisticated lens eyes of modern cephalopods

Snail Morphology: The Eye.

For a long time, it remained a mystery how cephalopods were able to change their colours to hide their environment, although, judging by their photoreceptor cells, they are unable to see colours. One possible explanation may lie in the unusual shape of many species' pupils: Unlike the round pupils of vertebrates, many cephalopods possess U-, W-, or dumbbell-shaped pupils. These enhance an optical phenomenon known as chromatic aberration, where different wavelengths of light are refracted to different degrees. Whereas most animal eyes attempt to minimise this effect, cephalopods might actively exploit it. By making subtle adjustments to focus, objects of different colours would appear with differing degrees of sharpness. In this way, cephalopods might be capable of indirectly telling colours apart, even without having true colour vision in the conventional sense. This hypothesis, however, remains the subject of ongoing scientific discussion.

Robert Sanders (2016): "Weird pupils let octopuses see their colorful gardens". UC Berkeley News.
UC Berkeley News: Are octopuses really colorblind?. ( YouTube Video).

The Soft Body

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Common Squid (Loligo vulgaris): St. Raphaël, Côte d'Azur, France. Photo: Thomas Menut (iNaturalist).

Like other molluscs, cephalopods also display the typical basic molluscan body plan consisting of a head, foot, visceral mass and mantle (compare, for example, the body structure of snails). Like other molluscs such as snails and bivalves, however, cephalopods have developed a highly characteristic body structure of their own, optimally adapted to their way of life.

Since Georges Cuvier coined the name in 1795, cephalopods have been referred to as Cephalopoda, a name derived from the Greek words κεφαλή (kephalē): head, and ποδ- (pod-): foot. Indeed, cephalopods possess a number of arms or tentacles attached directly to the head. This, however, tells only part of the story, since not only the tentacles but also the so-called funnel or siphon evolved from the original molluscan foot and are therefore homologous structures. While in many marine snails, such als whelks, the siphon is merely an elongated extension of the mantle cavity opening, in cephalopods the funnel also serves in locomotion: Out of the mantle cavity, water is forcefully expelled through the funnel, propelling the animal backwards by a natural form of jet propulsion. Squids in particular are capable of achieving remarkable speeds in this way.

An older scientific name formerly used for the cephalopods was Siphonopoda, reflecting the fact that both the funnel and the arms of cephalopods originated evolutionarily from the ancestral molluscan foot.

Body Plan of a Cuttlefish.

The Arms and Tentacles

Broadclub Cuttlefish (Ascarosepion latimanus): Sulawesi, Indonesia. Photo: Steve Cappell (iNaturalist Enlarge Image!

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The number of arms or tentacles, is different in various subclasses of Cephalopods (see above):

The most primitive cephalopods, such as the nautiluses (Nautilida), are equipped with a large number of up to 90 tentacles without suckers. A similar arrangement has also been assumed for fossil cephalopods such as the gigantic Cameroceras from the Ordovician Period (see geological timescale), though the soft tissues of fossil molluscs are evidently hardly ever preserved.

By contrast, cuttlefish (Sepiida) and squids (Loliginidae, Myopsida) possess ten appendages, two of which are greatly elongated and function as feeding tentacles. These are used to seize prey initially and draw it closer, whereupon the prey is grasped with the remaining eight much shorter arms and brought towards the mouth. Octopuses (Octopoda), as their name already suggests, possess only eight arms, the two elongated feeding tentacles having been lost.

Tentacle with suckers of a Giant Pacific Octopus (Enteroctopus dofleini): Triton, Washington, USA.

Enlarged view of the suckers. Photos: Daniel Hershman (iNaturalist).

Unlike the primitive Nautilida, arms and tentacles within the Coleoidea are equipped with suckers: In ten-armed cephalopods, the elongated feeding tentacles end in a terminal tentacular club, which is the only part of the tentacle bearing suckers. The eight shorter grasping arms, by contrast, are lined with suckers along almost their entire inner surface.

The function of these suckers is not only due to the suction force generated by the cephalopod itself, but is additionally reinforced by the pressure of the surrounding water acting from outside.

Also, the arms and tentacles possess sensory cells arranged around the suckers, providing the cephalopod with information about the nature of prey or its surroundings. This can be observed particularly well in octopuses when they "walk" across the sea floor while simultaneously exploring their environment with their arms.

Deep Look: If Your Hands Could Smell, You'd Be an Octopus. ( YouTube Video).

In the Oegopsida group of squids, including the mysterious Giant Squids (Architeuthis), Colossal Squids (Mesonychoteuthis, see Cephalopod Systematics!) and the Flying Squids (Ommastrephidae), the suckers on arms and tentacles are additionally armed with rings of hooks or claws, which provide a firmer grip on prey. The largest suckers on the terminal clubs of the feeding tentacles may even carry four large hooks. Long before Giant Squids themselves became as well known as today, the scars left by those hooks on the skin of sperm whales, which are the main predators of Giant Squids, were already familiar to whalers.

Another specialised form of cephalopod arm is the hectocotylus. This modified arm serves as a copulatory organ for the male and is used to transfer sperm cells packaged within a spermatophora to the female.

The nervous system of cephalopods is not only highly developed, but also partly decentralised. The arms and tentacles possess their own nerve centres (ganglia), so they are able to a certain extent to function independently and autonomously from the main body, at least on the basis of reflexes.

Respiration and Circulation

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Curled or Lesser Octopus (Eledone cirrhosa): Rhossili, Swansea, Wales, UK. Photo: Tom Green (iNaturalist) Enlarge Image!

Cephalopods are an exclusively marine group and, as far as fossil evidence indicates, unlike snails (Gastropoda) and bivalves (Bivalvia), they have never permanently left the sea. Octopuses, however, are certainly capable of moving over short distances on land, for example in order to travel from one tidal pool to another. The Algae Octopus (Abdopus aculeatus), found in northern Australia, Indonesia and the Philippines, does this regularly. It is also well known for its remarkable camouflage abilities, often mimicking an algae-covered marine snail.

BBC Earth: Extraordinary Octopus Takes To Land. ( YouTube Video).
Sophie Ellis, Discover Wildlife (BBC): " This is no ordinary octopus.".
Porsche Indrisie: Octopus gets crabby in Yallingup (Western Australia).

Lesser Octopus (Eledone cirrhosa): Pembrokeshire, Wales, UK. Note the funnel (siphon). Photo: Phil Newman/a> (iNaturalist) Enlarge Image!

Cephalopods breathe primarily through gills located within the mantle cavity. In squids and octopuses, however, a limited amount of cutaneous respiration also take place. Water enters the mantle cavity through openings between the mantle and the head and is subsequently expelled again through the funnel (siphon or hyponome). The number of gills differs among cephalopods (see above): The primitive nautiluses (Nautilidae), have four gills and thus are different from the "modern" living cephalopods, such as cuttlefish, squids and octopuses, which only have two.

The gills of cephalopods evolved from the ctenidia of the ancestral molluscs. Their greatly enlarged surface area is covered by a thin respiratory epithelium, through which oxygen is absorbed from the water while carbon dioxide is released. Movements of the mantle serve both respiration and locomotion. Particularly forceful and sustained jet-propulsion, however, is energetically costly and so usually is not maintained for a prolongued period of time.

As in most other molluscs, the respiratory pigment used in respiration is haemocyanin, containing copper ions as oxygen-binding centres. However, the haemocyanins are not contained in specialised blood cells but instead circulate freely within the blood plasma. This is because haemocyanin molecules are considerably larger than haemoglobin molecules, since they are built out of numerous peptide subunits each capable of binding large quantities of oxygen. In its deoxygenated state, haemocyanin is colourless, while in its oxygenated state it appears blue. Thus, the oxygen-rich blood of cephalopods is blue.

Cephalopods are the only living molluscs to possess a closed circulatory system. In addition, they have multiple hearts: In modern cephalopods, two additional branchial hearts supply the gills directly with blood. The resulting high blood pressure supports their comparatively elevated metabolic rate. The closed circulatory system is regarded as one of the major prerequisites for the active and predatory lifestyle of cephalopods.

Wikipedia: Hemocyanin.

The Nervous System

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Southern Red Octopus (Enteroctopus megalocyathus): Drake-Passage, Chile. Photo: Pat Webster (iNaturalist) Enlarge Image!

Common Octopus (Octopus vulgaris) in its lair with remains of its prey: Girona, Catalonia, Spain. Photo: Manelor (iNaturalist). Enlarge Image!

The nervous system of cephalopods represents the most advanced system not only among the molluscs, but probably among all invertebrates. On the one hand, the large central nerve ganglia (the cerebral, pedal and pleural ganglia) are fused into a highly complex structure that can quite legitimately be described as a brain. This cephalopod brain is arranged in a ring around the upper oesophagus. Consequently, cephalopods must sufficiently break down their food before swallowing it.

On the other hand, the arms of modern cephalopods also possess autonomous nerve ganglia of their own, capable of triggering specialised reflexes involved in locomotion, prey capture and hunting behaviour. Cephalopods additionally possess chemosensory cells on their arms, concentrated especially around the suckers. As a result, a cephalopod is capable of exploring its surroundings not only by touch, but in a sense also by taste and smell.

Actually, in octopuses the majority of nerve cells are located not within the central brain, but within the peripheral nerve pathways and the eight ganglia of the arms. Also, octopuses have intermuscular nerve pathways connecting the arms directly without passing through the central brain.

		Nature News Feature: Drew, L. (2026): "Do octopus brains work like humans’ - or is there another way to be smart?" (Link).
		Auralis and Natural World Facts: Why Octopuses Haven't Taken Over The World... Yet. ( YouTube Video).

As a consequence, the nervous system of cephalopods might be able to give remarkable insight into decentral neural networks, possibly even into automated systems of the technology of the future.

		Bennice, C.O.; Buresch, K.C.; Grossman, J.H. et al. (2025): "Octopus arm flexibility facilitates complex behaviors in diverse natural environments". Sci. Rep. 15, 31875 (Link).
		Mr. Science: Nothing About Octopus Is Normal… Here’s Why. ( YouTube Video).
		Science Friday: The Distributed Mind: Octopus Neurology. ( YouTube Video).

Swimming octopus (Octopus vulgaris): Sète, Hérault, France. Photo: Thomas Menut (iNaturalist) Enlarge Image!

Another characteristic feature of at least the modern cephalopods is the presence of so-called giant axons: exceptionally thick extensions of nerve cells. The significance of these structures lies in the fact that molluscan nerve cells lack the insulating myelinated Schwann sheaths characteristic of vertebrates, making saltatory conduction impossible. By greatly increasing the diameter of the axons, however, cephalopods achieve conduction velocities approaching those of vertebrate nerve cells.

Wikipedia: Squid Giant Axon.
Wikipedia: Saltatory Conduction.

Since the eyes of modern cephalopods are highly developed, these animals also possess large visual processing centres (optic lobes) within the central brain. In addition, cephalopods possess highly developed equilibrium organs known as statocysts, located beside the brain. These organs detect changes in orientation and acceleration and probably also perceive low-frequency sounds.

		Kaifu, K.; Akamatsu, T.; Segawa, S. (2008): Underwater sound detection by cephalopod statocyst. Fisheries Science, Band 74, p. 781 - 786 (Internet Archive PDF).
		Hu, M.Y.; Yan, H.Y.; Chung, W.S.; Shiao, J.; Hwang, P. (2009): Acoustically evoked potential in two cephalopods infered using the auditory brainstem response (ABR) approach. Comparative Biochemistry and Physiology, Part A: Molecular & Integrative Physiology. Band 153, Nr. 3, 2009, p. 278 - 283, (ResearchGate).

Cephalopods are regarded as the most intelligent invertebrates. Behavioural experiments have demonstrated that the cognitive abilities of octopuses in some respects approach those of dogs. They are capable of abstraction - such as counting up to four or distinguishing different shapes - and of solving complex problems, including opening the screw-top lid of a jar in order to reach its contents.

Intelligence Outside of Vertebrates?

Links and Literature

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Links

• Auralis and Natural World Facts: Why Octopuses Haven't Taken Over The World... Yet. ( YouTube Video).
• EV Nautilus: Nautilus Live Ocean Exploration Trust (Underwater Video from a DSRV: YouTube).
• Natural World Facts: The Unique Biology of Cephalopods. ( YouTube Video).
• OctoNation: Octopedia: From argonaut to wunderpus.
• The Cephalopod Page by Dr. James B. Wood since 1995.
• Tonmo.com: The Octopus News Magazine Online with its own YouTube Channel.

Literature

• Haszprunar, G.; Götting, H.-J.: Cephalopoda, Kopffüßer. p. 352 - 362 in: Westheide, W.; Rieger, R.  (Ed.): Spezielle Zoologie Teil 1: Einzeller und Wirbellose Tiere. 2. Ed. (2007). Spektrum Akademischer Verlag (Elsevier), München.
• Mather, Jennifer A.; Anderson, Roland C.; Wood, James B.: "Octopus: The Ocean's Intelligent Invertebrate ". Timber Press, Portland, 2010. (Link).
• Norman, M.: "Cephalopods - A World Guide. Pacific, Indian Ocean, Red Sea, Atlantic Ocean, Caribbean, Artic, Antarctic". Conchbooks, Harxheim, 2000. (Link).
• Staaf, D.: "Monarchs of the Sea: The Extraordinary 500-Million-Year History of Cephalopods". The Experiment Publishing, New York, 2020. (Link).
• Staaf, D.: "The Lives of Octopuses & Their Relatives. A Natural History of Cephalopods". Princeton University Press, Princeton, New Jersey. 2023. (Link).
• Wiese, V., Janke, K.: "Die Meeresschnecken und Muscheln Deutschlands", p. 551 ff. Quelle und Meyer Verlag Wiebelsheim, 2021. (Link).
  
  
• Van Pelt, Shelby: "Remarkably Bright Creatures". A Novel. Ecco Publishing, New York, 2025. (Link). Now a movie at Netflix (2026).

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Latest Change: 04.06.2026 (Robert Nordsieck).
Latest Link Check: 24.05.2026.