Squid Dissection: Labeled Virtual Lab and Step-by-Step Guide

A squid dissection takes about 45 minutes: you examine and sex the squid from the outside, remove the beak, open the mantle along the ventral (funnel) side, identify the gills, three hearts, ink sac and reproductive organs, then remove the ink sac and the pen. The usual lab specimen is the longfin inshore squid, sold frozen or preserved. If you cannot take part in a hands-on dissection, the 3D model above follows exactly the same steps as a virtual squid dissection.

For the anatomy behind each structure, see the interactive squid anatomy page.

• One squid per pair of students (fresh-frozen market squid or a preserved specimen), thawed
• Dissecting tray lined with paper towel
• Small sharp scissors (fine-pointed) and a scalpel or craft knife
• Forceps and a blunt probe; 2–4 dissecting pins
• Hand lens or dissecting microscope (for sucker rings and the lens)
• Ruler (for mantle length), white paper for the ink-writing activity
• Disposable nitrile gloves, safety glasses, apron; paper towels and a waste bag

Squid for dissection: whole, uncleaned squid with the head and arms on. Market squid from the seafood counter work as well as supply-house specimens and do not contain preservative.

• Wear gloves and eye protection. Preserved specimens may contain fixative: work in a ventilated room and rinse the squid first.
• Cut away from your body and your partner's hands. Lift the mantle with forceps before cutting so the scissor tip stays shallow.
• Anyone with a shellfish or seafood allergy should not handle the specimen; use the virtual dissection instead.
• Do not eat any part of a lab specimen. Bag all tissue for disposal and wash hands and tools afterwards.

These are the same steps as the guided panel next to the 3D model. Structures to find in each step are listed in bold.

1. Orient the squid and measure it

   • Rinse the squid and lay it in the tray. The darker side with the fins lying flat is dorsal (back); the paler side with the funnel is ventral (belly).
   • Identify the three body regions: the long mantle, the short head with the eyes, and the ring of arms and tentacles.
   • Find the funnel (siphon) under the head and the collar, the free edge of the mantle.
   • Measure mantle length (ML) along the dorsal midline from the collar to the tip of the mantle. Scientists use ML, not total length, because tentacles stretch.
   • Use a hand lens to look at the coloured dots on the skin: these are chromatophores.

   Find: Mantle, Fins, Head, Funnel, Collar, Chromatophores

2. Count the arms and tentacles

   • Spread the appendages out. Count eight arms and two longer tentacles (10 in total).
   • Number the arms I (dorsal) to IV (ventral) on each side. The tentacles arise between arms III and IV.
   • Compare an arm and a tentacle: arms have suckers along their whole length; tentacles have a bare stalk and suckers only on the club.
   • On one club, find the carpus (base, small suckers and knobs), the manus (wide middle, four rows of suckers) and the dactylus (tip).
   • Pull off one large sucker and look at it under a hand lens: the hard, toothed sucker ring sits inside the cup on a short stalk.

   Find: Arms, Arm suckers, Sucker rings, Tentacles, Carpus, Manus, Dactylus

3. Examine the head and eyes

   • Find the two large eyes. In this squid each eye is covered by a clear corneal membrane (a 'closed', myopsid eye).
   • Look just behind and below each eye for the small olfactory pit.
   • Spread the arms to see the mouth at the centre, surrounded by the thin, pointed buccal membrane.

   Find: Eyes, Cornea, Olfactory organs, Buccal membrane

4. Sex the squid from the outside

   • Look at the left ventral arm (left arm IV). In a mature male the outer third or so is the hectocotylus: its suckers are tiny or replaced by fleshy papillae.
   • In a female, look on the ventral buccal membrane, just below the beak, for the seminal receptacle where sperm may be stored.
   • Record your prediction (male or female). You will check it when you open the mantle. Use the Female/Male switch on the model to compare.

   Caution: Young or immature squid may show no clear external difference; the internal organs are the reliable test.

   Find: Hectocotylus, Seminal receptacle, Buccal membrane

5. Remove the beak and buccal mass

   • Push the arms back to expose the round buccal mass at the centre of the arm crown.
   • Cut around the base of the arms to free the buccal mass, or squeeze it until the beak pops out.
   • Separate the two halves of the beak: the larger lower beak wraps around the smaller upper beak. It looks like a parrot's beak upside down.
   • Inside the buccal mass, behind the beak, find the radula, a tiny ribbon of teeth. Use a hand lens.

   Caution: The beak is sharp at its tip. Rinse and keep it as a specimen; it dries hard.

   Find: Buccal mass, Upper beak, Lower beak, Radula

6. Open the mantle (ventral incision)

   • Turn the squid ventral side up (funnel on top), head towards you.
   • Find where the mantle snaps onto the funnel. Unhook the funnel locking cartilages from the mantle locking cartilages with your fingers.
   • Lift the collar with forceps, insert the scissor tip just under the mantle, and cut along the ventral midline from the collar to the tip. Keep the blade shallow so you do not cut the organs.
   • Fold the mantle flaps out to each side and pin them. You are now looking into the mantle cavity.
   • Note the funnel retractor muscles running back from the funnel.

   Caution: Cut slightly to one side of the midline if your squid is a female: the large white nidamental glands lie right under the ventral mantle.

   Find: Funnel locking cartilages, Mantle locking cartilages, Mantle cavity, Funnel retractor muscles

7. Identify the sex from the reproductive organs

   • Female: two large, white, oval nidamental glands lie on the ventral side; in front of them are the accessory nidamental glands (mottled red-orange when mature). The ovary is at the posterior end and the coiled oviduct, with the oviducal gland, runs forward on the left.
   • Male: a long, white Needham's sac on the animal's left side is packed with thread-like spermatophores; it leads to the penis near the left gill. Behind it is the coiled spermatophoric organ, and the compact white testis is at the posterior end.
   • Was your external prediction correct? Swap squid with a group that has the other sex. Use the Female/Male switch to see both.

   Find: Nidamental glands, Accessory nidamental glands, Ovary, Oviduct, Oviducal gland, Needham's sac, Spermatophores, Spermatophoric organ, Penis, Testis

8. Find the gills and the three hearts

   • Find the two feathery gills along the sides of the mantle cavity. Their bases are posterior; their tips point towards the head.
   • At the base of each gill is a small branchial (gill) heart, often with a pale appendage attached.
   • Between the two branchial hearts, on the midline, find the systemic heart. Females: gently move the accessory nidamental glands aside.
   • Trace the vessels: the vena cava and its branches bring blood to the branchial hearts; efferent vessels take oxygenated blood from the gills to the systemic heart.

   Find: Gills, Branchial hearts, Branchial heart appendages, Systemic heart, Vena cava, Afferent branchial vessels, Efferent branchial vessels

9. Find the anus, kidneys and ink sac

   • Follow the rectum forward along the ventral midline to the anus, just behind the funnel.
   • On each side of the rectum, a little behind the anus, look for the small renal papillae (kidney openings). The thin renal sacs lie behind them.
   • Find the silvery-black ink sac lying along the rectum on the digestive gland. Its duct joins the rectum near the anus.
   • Free the ink sac with forceps and lift it out without bursting it. Pierce it over a dish and dip the tip of the pen (step 12) or a toothpick in the ink to write your name.

   Caution: Ink stains clothes and skin. Keep the sac over the tray.

   Find: Intestine & rectum, Anus, Renal papillae, Renal sacs, Renal appendages, Ink sac, Ink duct

10. Trace the digestive system

    • Find the large digestive gland ('liver'), brown or orange, in the front half of the visceral mass.
    • Behind it, near the gill bases, find the muscular stomach and, beside it, the caecum. The thin-walled caecal sac extends towards the posterior end.
    • At the back of the digestive gland look for the small digestive duct appendages ('pancreas').
    • Follow the narrow esophagus forward along the dorsal side of the digestive gland toward the head. It passes through the brain.

    Find: Digestive gland, Stomach, Caecum, Caecal sac, Digestive duct appendages, Esophagus, Posterior salivary gland

11. Find the stellate ganglia and giant axons

    • Push the viscera aside and look on the inner surface of the mantle, just behind the collar, near the dorsal side. On each side is a star-shaped stellate ganglion.
    • Nerves fan out from it across the mantle wall. The last, longest stellar nerve runs back toward the fins; it contains the largest giant axon.
    • Trace the pallial nerve forward from the ganglion towards the head.

    Caution: The nerves are thin and pale. Use a hand lens and good light.

    Find: Stellate ganglia, Stellar nerves, Giant axon, Pallial nerves

12. Remove the pen (gladius)

    • Remove or push aside the remaining organs to see the inner dorsal wall of the mantle.
    • Feel along the dorsal midline for a stiff strip. Make a shallow cut along it near the collar end and grip the tip of the pen with forceps.
    • Pull it out slowly. It is thin, clear and feather-shaped, with a central rib (rachis): the squid's internal shell.
    • Use the pointed end of the pen as a quill with the ink from step 9.

    Find: Pen, Nuchal cartilage, Head retractor muscles

13. Dissect the eye and find the brain

    • Cut one eye out of the head with scissors and make a small cut through its front.
    • Squeeze gently to release the lens: a hard, clear-to-amber ball. Place it on printed text and look through it.
    • Look into the eye cup for the dark retina.
    • Carefully shave the cartilage between the eyes from the dorsal side to find the pale brain and, at the sides, the large optic lobes. The statocysts lie in the cartilage below the brain.

    Find: Eyes, Lens, Cornea, Optic lobes, Supraesophageal mass, Subesophageal mass, Cephalic cartilage, Statocysts

In a dissected female squid, the most obvious organs after opening the mantle are the two large white nidamental glands. The labeled diagram shows what you should see in the ventral view.

In a dissected male squid, look on the animal's left for the long white Needham's sac full of spermatophores, leading to the penis near the left gill.

You can tell a male from a female squid only reliably once the mantle is open. Females have nidamental glands (white), accessory nidamental glands (mottled red-orange in mature animals), an ovary and an oviduct, sometimes full of eggs. Males have a testis, a coiled spermatophoric organ, Needham's sac with spermatophores and a penis, and the left arm IV is a hectocotylus. Compare specimens with another group so everyone sees both sexes.

The ink sac is the silvery-black, pear-shaped sac lying along the rectum on the ventral side of the digestive gland. Lift it out intact, then pierce it over a dish to write with the ink using the squid's own pen as a quill. For the squid eye dissection, cut out one eye, slit the front and squeeze out the hard, spherical lens; it magnifies print and dries into a clear-to-amber bead.

Use these as pre-lab or post-lab questions. Open each answer only after you have written your own.

Giant squid (Architeuthis dux) are only dissected by museum scientists, when a dead specimen washes ashore or is caught by fishers. The procedure is the same in principle: the mantle is opened ventrally to examine the gills, hearts, gonads and stomach contents, and the beak and statoliths are removed for identification and ageing. Classroom dissections of large squid usually use the Humboldt squid (Dosidicus gigas).