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The most highly evolved class of the phylum Mollusca. It consists of squids, cuttlefishes, octopuses, and the chambered nautiluses. The earliest known cephalopods are small, shelled fossils from the Upper Cambrian rocks of northeast China that are 500 million years old. Cephalopods always have been marine, never fresh-water or land, animals. Most fossil cephalopods, among them the subclasses Nautiloidea and Ammonoidea, had external shells and generally were shallow-living, slow-moving animals. Of the thousands of species of such shelled cephalopods that evolved, all are extinct except for four species of the only surviving genus, Nautilus. All other recent cephalopods belong to four orders of the subclass Coleoidea, which also contains five extinct orders.
Living cephalopods are bilaterally symmetrical mollusks with a conspicuously developed head that has a crown of 8–10 appendages (8 arms and 2 tentacles) around the mouth. These appendages are lined with one to several rows of suckers or hooks. Nautilus is exceptional in having many simple arms. The mouth contains a pair of hard chitinous jaws that resemble a parrot's beak and a tonguelike, toothed radula (a uniquely molluscan organ). Eyes are lateral on the head; they are large and well developed. The “cranium” contains the highly developed brain, the center of the extensive, proliferated nervous system. The shell of ancestral cephalopods has become, in living forms, internal, highly modified, reduced, or absent; and is contained in the sac- or tubelike, soft muscular body, the mantle. A pair of fins may occur on the mantle as an aid to locomotion, but primary movement is achieved through jet propulsion in which water is drawn into the mantle cavity and then forcibly expelled through the nozzlelike funnel. Fewer than 1000 species of living cephalopods inhabit all oceans and seas.
The classification given here concentrates on the living groups and lists only the major fossil groups.
- Class Cephalopoda
- Subclass Nautiloidea
- Subclass Ammonoidea
- Subclass Coleoidea
- Order Belemnoidea
- Order Sepioidea
- Order Teuthoidea
- Suborder Myopsida
- Suborder Oegopsida
- Order Vampyromorpha
- Order Octopoda
- Suborder Cirrata
- Suborder Incirrata
Species of cephalopods inhabit most marine habitats. Cephalopods inhabit tide pools, rocky patches, sandy bottoms, coral reefs, grass beds, mangrove swamps, coastal waters, and the open ocean from the surface through the water column to depths on the abyssal bottom at over 16, 000 ft (5000 m). See Nervous system (invertebrate)
Cephalopods are high-level, active predators that feed on a variety of invertebrates, fishes, and even other cephalopods. The relatively sluggish nautiluses feed primarily on slow-moving prey such as reed shrimps, and even are scavengers of the cast-off 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. Salivary glands secrete toxins that subdue the prey and, in octopuses, begin digestion.
To protect themselves from predators cephalopods would rather hide than fight. To this end they have become masters of camouflage and escape. Benthic forms especially (for example, Sepia and Octopus) have evolved an intricate, complex system of rapid changes in color and patterns via thousands of individually innervated chromatophores (pigment cells) that allow precise matching to the color and pattern of the background. In addition, they regulate the texture of their skins by erecting papillae, flaps, and knobs that simulate the texture of the background. Many midwater oceanic squids camouflage against predation from below by turning on photophores (light organs) that match the light intensity from the surface and eliminate their silhouettes. See Chromatophore, Protective coloration
Cephalopods have perfected jet propulsion for many modes of locomotion, from hovering motionless, to normal cruising, to extremely rapid escape swimming. Water enters the mantle cavity through an opening around the neck when the muscular mantle (body) expands. The mantle opening seals shut as the mantle contracts and jets the water out through the hoselike funnel, driving the cephalopod tail-first through the water.
The sexes are separate in cephalopods, and many species display complex courtship, mating, spawning, and parental care behavior. At mating, the male of most species transfers the spermatophores to the female with a specially modified arm, the hectocotylus. The spermatophores are implanted into the female's mantle cavity, around the neck, under the eyes, or around the mouth, depending on the species. Incubation takes a few weeks to a few months depending on the species.
Cephalopods are extremely important in the diets of toothed whales (sperm whales, dolphins), pinnipeds (seals, sea lions), pelagic birds (petrels, albatrosses), and predatory fishes (tunas, billfishes, groupers). For example, pilot whales in the North Atlantic feed almost exclusively on one species of squid, Illex illecebrosus, that aggregates for spawning in the summer. See Mollusca
a class of invertebrate animals belonging to the phylum Mollusca. The vascular system, the brain (which is encased in a cartilaginous cranium), and the sense organs (particularly the eyes) are highly developed in the Cephalopoda. The body of the cephalopod is bilaterally symmetrical, the head is separate, and a ring of eight to ten tentacles (arms) surrounds the mouth. The tentacles are a part of the foot (whence the name), which was altered and displaced toward the head, and serve to grasp prey and for locomotion. In most members of this class, the tentacles are muscular organs covered with suckers or, in some cases, with horny claws. A fold of skin, the mantle, surrounds the mantle cavity on the ventral surface. A muscular organ, called the funnel, is located at the narrow opening into the mantle cavity. This organ, which has the appearance of an inverted funnel, is also derived from the foot. Water entering the mantle cavity is forcefully expelled through the funnel owing to contraction of the mantle muscles. This gives the body a push and propels it posteriorly, much like a rocket. Of modern cephalopods, only the nautilus has an external shell. Although other cephalopods do not have an external shell, some have an internal vestigial shell in the form of calcareous or horny plates. Cephalopods range in size from 1 cm to 18 m.
The sexes are separate in cephalopods. The male germ cells are contained in capsules called spermatophores. The male grasps the spermatophore with a specialized arm, called the hectocotylus, and transfers it into the mantle cavity of the female. The female attaches the fertilized eggs, which are enclosed in a thick membrane, to underwater objects. The young hatch as small but already developed animals.
Cephalopods dwell in seas, primarily in warm water. They are found both close to shore and at great depths. For example, the octopus is a bottom-dweller, living among rocks, cliffs, and seaweed; the cuttlefish prefers a sandy bottom; and squid swim rapidly in deep water. Cephalopods are predators, feeding primarily on fish, although the bottom-dwellers also eat crustaceans and mollusks. Cephalopods, in turn, are eaten primarily by mammals, especially by the sperm whale (which occasionally eats even giant squid), and also by marine birds. Many cephalopods, such as squid, cuttlefish, and octopus, are eaten by man and are an item of commerce. There are about 600 modern species of cephalopods. They comprise two subclasses, the Tetrabran-chia and the Dibranchia. Of the living cephalopods, only the Pacific genus Nautilus belongs to the Tetrabranchia; the large extinct group of ammonites may also belong to this subclass. The Dibranchia include the modern cuttlefish, squid, and octopus, as well as the extinct belemnites.
I. M. LIKHAREV
The oldest known cephalopod fossils are from Cambrian deposits; certain groups reached a peak in the Ordovician and were already beginning to die out. Cephalopods with an external shell (nautiloids, actinoceratoids, bactritoids, endoceratids, and ammonites) predominated in the Paleozoic. Belemnites, the oldest cephalopods with an internal shell, are known from the early Carboniferous. Most of the early cephalopod groups became extinct by the end of the Paleozoic. Nautiloids, ammonites, and belemnites are characteristic of the Mesozoic; the immediate predecessors of the modern cephalopods also appear. About 10,000 extinct species of cephalopods are known. Paleontologists generally divide the class Cephalopoda into two subclasses—the Ectocochlia and the Endocochlia. The extinct cephalopods are of interest in understanding the phylogenetic development of large groups of organisms. Because they were widespread, represented by a large number of species, and subject to rapid evolution, the cephalopods constitute one of the most important groups in the stratigraphy of Paleozoic and Mesozoic deposits.
V. N. SHIMANSKII
REFERENCESRukovodstvo po zoologii, vol. 2. Moscow-Leningrad, 1940.
Akimushkin, I. I. Golovonogie molliuski morei SSSR. Moscow, 1963.
Dogel’, V. A. Zoologiia bespozvonochnykh, 5th ed. Moscow, 1959.
Zhizn’ zhivotnykh, vol. 2. Moscow, 1968.
Osnovy paleontologii: Molliuski—golovonogie, vols. I—II. Moscow, 1958–62.