Squid Anatomy: Interactive 3D Model with Labeled Diagrams

A squid is a cephalopod mollusc with a long, torpedo-shaped mantle, a pair of fins at the back, a short head with two large eyes, and ten appendages: eight arms and two longer tentacles. Inside, it has a thin internal shell called the pen (gladius), two gills, three hearts, blue copper-based blood, an ink sac, and the famous giant axons that fire its escape jet.

This page models the longfin inshore squid, Doryteuthis pealeii (formerly Loligo pealeii), the squid most often dissected in North American classrooms and a species J. Z. Young worked on at Woods Hole in his pioneering studies of squid giant nerve fibres. Adults are usually 10–20 cm in mantle length. It belongs to the family Loliginidae, the "myopsid" squids, whose eyes are covered by a clear cornea.

Orientation. In cephalopods the arms are treated as anterior (front), the mantle tip and fins as posterior (back), the funnel side as ventral (belly), and the pen side as dorsal (back). Arms are numbered I (dorsal) to IV (ventral) on the left and right sides.

The external anatomy of a squid comprises the mantle, fins, collar, head, eyes, funnel, eight arms with stalked suckers, two tentacles ending in clubs, and the mouth with its beak. The diagram shows the ventral side, which is the side with the funnel.

• Mantle: The long, torpedo-shaped muscular tube that forms the body and encloses the organs and the mantle cavity. Mantle muscles contract to squeeze water out through the funnel (jet propulsion) and relax to draw water in over the gills for breathing.
• Fins: A pair of muscular fins that together form a rhombus (diamond shape) at the posterior end of the mantle. Fine control of position, slow swimming, steering and stabilising during jetting.
• Collar (Mantle opening): The free anterior edge of the mantle, which encircles the 'neck' behind the head. The gap between collar and head is the inhalant opening: water enters the mantle cavity here and is then sealed in when the mantle contracts.
• Head (Cephalic region): The short region between the collar and the arms that carries the eyes and houses the brain. Protects the brain inside the cephalic cartilage and carries the eyes, olfactory organs and arms.
• Funnel (Siphon): A muscular cone on the ventral side through which water is expelled. Jet propulsion and steering, and the exit for exhaled water, ink, faeces, urine, eggs and spermatophores.
• Arms: Eight muscular arms, each with two rows of stalked suckers along its inner (oral) surface. Hold and manipulate prey after the tentacles have caught it, and help steer.
• Arm suckers: Cup-shaped suckers on short stalks, in two alternating rows on each arm. Grip prey by suction and by the toothed ring inside each cup.
• Sucker rings (Chitinous ring teeth): Hard, toothed rings that line the opening of each squid sucker. The teeth dig into the prey's skin so it cannot slip free.
• Tentacles (Tentacle stalks): Two long, retractile feeding tentacles with bare stalks and a sucker-bearing club at the tip. Shoot out in a fraction of a second to seize prey, then pull it back to the arms.
• Chromatophores: Tiny elastic pigment sacs in the skin that make the coloured dots of a squid. Muscles stretch each sac open to show its colour (red, brown, yellow) in a fraction of a second, for camouflage and signalling.

The internal anatomy of a squid is seen by cutting the mantle along the ventral midline. In this classic view the two feathery gills lie along the sides, the three hearts sit near the gill bases, the silvery ink sac lies along the rectum in the middle, the digestive gland lies deeper and to the front, and the gonad fills the posterior end. The pen runs along the dorsal side, under everything else.

Organ systems found inside the mantle, from front to back on the ventral side: funnel and funnel retractor muscles; anus and rectum; ink sac; renal papillae and renal sacs (kidneys); accessory nidamental glands (females); the three hearts at the gill bases; nidamental glands or the male duct system; stomach and caecum; caecal sac; and the gonad.

Male and female squid differ mainly inside: a female has two large white nidamental glands, accessory nidamental glands, an ovary and a single oviduct; a male has a testis and a duct system that packages sperm into spermatophores, stored in Needham's sac. Externally, a mature male's left ventral arm is modified as the hectocotylus.

During mating, large "consort" males hold the female side by side and use the hectocotylus to place spermatophores in her mantle cavity near the oviduct opening. Smaller "sneaker" males dart in head-to-head and attach spermatophores near the seminal receptacle on her buccal membrane. Females store sperm and can lay fertilised eggs for weeks.

Every structure in the 3D model, grouped by organ system. This is the text version of the labeled diagram above.

The squid brain is a ring of fused ganglia around the esophagus, protected by the cephalic cartilage and flanked by two large optic lobes; from it, pallial nerves run to the stellate ganglia, whose giant axons drive the mantle.

• Supraesophageal mass (Upper brain): The dorsal part of the brain, above the esophagus. Higher centres for learning, memory and decision-making (vertical and frontal lobes).
• Subesophageal mass (Lower brain): The ventral part of the brain, below the esophagus. Motor centres for the arms, funnel, mantle and chromatophores; its palliovisceral lobe contains the first-order giant neurons that start the escape jet.
• Optic lobes: Two large kidney-shaped lobes of the brain directly behind the eyes. Process visual information; they are the largest parts of the squid brain.
• Stellate ganglia: Two star-shaped nerve centres on the inner wall of the mantle. Relay the brain's commands to the mantle muscles through the stellar nerves; here the second-order giant fibres synapse on the giant-axon cells.
• Giant axon: The largest nerve fibre in the squid: up to about 0.5–1 mm across, inside the longest, most posterior stellar nerve. A wide axon conducts impulses very fast, so all mantle muscles contract together for the escape jet.

The squid giant axon. The giant fibre system has three relays: giant cells in the brain, second-order fibres in the pallial nerve, and third-order giant axons in the stellar nerves. Third-order axons are largest in the longest, most posterior nerves, up to about 0.5–1 mm across, so the far end of the mantle contracts at the same time as the near end. Hodgkin and Huxley used these axons to work out how the action potential works.

The squid eye is a camera-type eye with a cornea, iris, rigid spherical lens and retina; it evolved independently of the vertebrate eye and has no blind spot, because the nerve fibres leave from behind the retina.

• Eyes: Two large camera-type eyes, one on each side of the head. Vision for hunting, schooling and avoiding predators.
• Cornea (Corneal membrane): A clear membrane of skin that covers the eye of loliginid squids. Seals the eye from seawater and protects the lens.
• Lens: A hard, spherical lens that focuses light on the retina. Focuses images; moves forward and back to change focus.
• Statocysts: A pair of fluid-filled balance organs inside the cephalic cartilage. Sense gravity, acceleration and rotation, like the vertebrate inner ear.

Loliginid squids such as Doryteuthis have the eye sealed behind a transparent corneal membrane (myopsid eye). Open-ocean (oegopsid) squids, including the giant and Humboldt squids, have no cornea, so seawater bathes the lens.

A squid has three hearts: two branchial hearts pump blood through the gills, and the systemic heart pumps the oxygenated blood through the anterior and posterior aortas to the body. The blood is blue because it carries oxygen on copper-based haemocyanin.

• Systemic heart: The main heart, which pumps oxygenated blood from the gills to the body. Drives blood through the aortas to the head, mantle and organs.
• Branchial hearts (Gill hearts): Two small hearts, one at the base of each gill. Pump deoxygenated blood from the veins through the gills.
• Branchial heart appendages (Pericardial glands): Small glandular caps on each branchial heart. Filter the blood to make primary urine, which passes into the excretory system.
• Vena cava: The main vein, which brings blood back from the head and splits towards the branchial hearts. Returns deoxygenated blood to the branchial hearts; its branches carry the renal appendages.
• Gills (Ctenidia): Two long, feather-like gills in the mantle cavity. Take up oxygen from water pumped over them by the mantle.

The squid mouth sits at the centre of the arms and contains a parrot-like beak of two chitinous jaws and a toothed radula; food passes through the esophagus, which threads through the brain, to the stomach and caecum, helped by the digestive gland.

• Upper beak: The smaller, dorsal jaw of the beak, with a hooked tip. Bites prey into small pieces with the lower beak, like a parrot's beak upside down.
• Lower beak: The larger, ventral jaw of the beak, which wraps around the upper beak. Cuts food with the upper beak.
• Radula: A ribbon of tiny backward-pointing teeth on a tongue-like support. Pulls bitten food into the esophagus.
• Esophagus: A narrow tube that carries food from the buccal mass to the stomach. Moves food by muscular contractions.
• Stomach: A muscular sac where food is ground and digestion begins. Mixes food with enzymes from the digestive gland.
• Caecum: A chamber beside the stomach where most nutrients are absorbed. Sorts food particles and absorbs digested nutrients.
• Digestive gland (Liver, hepatopancreas): A large, elongated brown-orange gland in the anterior visceral mass. Makes digestive enzymes, absorbs nutrients and stores reserves.
• Ink sac: A silvery, pear-shaped sac filled with black ink (melanin). Stores ink that is squirted out to confuse predators.

The squid pen, or gladius, is a feather-shaped internal shell of chitin and protein that runs the length of the dorsal mantle. It is all that is left of the external shell of the squid's ancestors, and it gives the mantle muscles a stiff support.

Squid have eight arms with suckers along their whole length and two longer tentacles with suckers only on the club. The club has three regions: the carpus (base), manus (hand) and dactylus (tip). Every squid sucker sits on a short stalk and is reinforced by a hard, toothed sucker ring.

• Carpus (Wrist of the tentacular club): The base of the tentacular club, with small suckers and knobs. Its small suckers and knobs can lock the two clubs together so they strike as one unit.
• Manus (Hand of the tentacular club): The broad middle part of the club, with four rows of suckers; the middle rows are enlarged. Main gripping surface that holds struck prey.
• Dactylus (Finger of the tentacular club): The tapering tip of the club with small suckers in four rows. Adds extra grip at the tip of the strike.
• Hectocotylus: The modified left ventral arm (arm IV) of the male, used to transfer spermatophores. Picks up spermatophores from the penis and places them inside the female's mantle cavity near the oviduct opening.

Squid and octopuses are both cephalopods with three hearts, blue blood, a beak, ink and a ring-shaped brain, but their bodies are built for different lives. See the octopus anatomy model to compare.

The giant squid, Architeuthis dux, has the same basic body plan as the longfin squid but on a vast scale: specimens of around 12–13 m total length are reported, most of it the two very long feeding tentacles. It has the largest eyes known in any animal, about 27 cm across, used to spot sperm whales in the dark. Its suckers have finely serrated rings, its tissues contain ammonium ions that help with buoyancy, and, as an oegopsid squid, its eyes have no cornea.

The colossal squid, Mesonychoteuthis hamiltoni, is shorter than the giant squid but much heavier, making it the heaviest known invertebrate (the Te Papa specimen weighed about 495 kg). Its tentacle clubs carry two rows of sharp, swivelling hooks as well as suckers, its arms have hooks too, and its beak is the largest of any squid. It is a glass squid (family Cranchiidae) of the Southern Ocean.

The Humboldt or jumbo squid, Dosidicus gigas, is a flying squid (family Ommastrephidae) of the eastern Pacific, reaching over 1 m in mantle length. Compared with the longfin squid it has an open (oegopsid) eye, an inverted-T-shaped funnel locking cartilage, sharply toothed suckers, small light organs in its skin, and a chromatophore system that flashes the whole body red and white.

The vampire squid, Vampyroteuthis infernalis, is not a true squid. It is the only living member of the order Vampyromorphida, more closely related to octopuses. It has eight webbed arms lined with soft cirri, two thin retractile filaments stored in pockets, large ear-like fins, light organs, no ink sac, and an internal gladius. Adults are only about 20 cm in mantle length.

Breathing and jetting use the same pump: the mantle expands to draw water in at the collar, then contracts; the collar locks onto the funnel cartilages so the water passes over the gills and out of the funnel. Escape is triggered by the giant fibre system, which makes the whole mantle contract at once. Feeding starts with the tentacles striking prey, the arms holding it and the beak biting it into pieces small enough to pass through the brain ring. Excretion happens as the branchial heart appendages filter blood and the renal appendages secrete ammonia into the renal sacs. Camouflage comes from nerve-controlled chromatophores, and ink provides a last defence.

Test yourself, then try the full worksheet with answer key on the squid dissection guide.