| Objectives:
As a result of this lesson, students will be able to:
- Locate and identify major external and internal features and organs of a squid.
- Understand and use basic dissection techniques and terms.
- Critically examine the functions of several squid features and organs.
| Teaching Notes: This lab is a very thorough dissection of a squid and can be adapted to different grade levels. Teachers should try the lessons, considering which parts are most appropriate for their students and curriculum. The descriptions use complex dissection terminology. Be certain students understand the vocabulary of dissection prior to beginning the lab.
These lessons were tested with middle school students ages 11 to 13. They followed procedures and understood concepts well. The skills necessary to do all steps in the dissection are within the normal ability range of middle school students. |
Materials:
- squid*
- scissors
- toothpicks (for probes and pointers)
- drawing paper
- forceps
- hand lens (5x recommended)
- small cups (ketchup cups work well)
- dissecting pan (or lunch trays)
- paper towels
- diagram of squid
- wash bottle
- microscope (optional)
- dissecting scopes (optional)
- slides (optional)
- slide covers (optional)
| *Look for squid at the local supermarket in the seafood and frozen foods sections. You may have to order it in advance. For areas that have them, you can also go to the local fish market or oriental food stores, or you can deal directly with fishermen. |
| Teaching Notes: Squid specimens tend to deteriorate rapidly. Keep all squid frozen until the morning before dissection. Thaw the squid in the refrigerator. If the entire dissection cannot be completed in one day, do the external activities while the specimens are still partly frozen, and the internal activities the next day after squid are thawed.
Squid may have tentacles or arms missing. Individual squid vary internally, and their relative maturity determines which organs are formed well enough to be seen clearly, and which have lost (or have yet to gain) their shape and coloration. Please advise students that they may not see everything shown in the enclosed diagram. Tissue shrinks and organs become misshapen quickly. To help maintain the freshness of the specimen, cover it with a wet paper towel as you work so it does not dry out so quickly.
Finally, this lesson is a tactile experience. You may want to explore this aspect through sensory activities, written descriptions, poetry, and/or artwork. Encourage students to experience the many textures found inside and outside the squid’s body. Moving fingertips along the suckers is suggested as well – the suckers do not scrape or hurt if you are gentle with them. |
Procedure
Place the squid with the dorsal (back) side up in the dissecting pan. This means put the side with the funnel down and the fin side up. Make sure the tentacles and arms are towards you. Locate the head, eyes, beaks (mouth), arms (8), two longer feeding tentacles, fins, mantle, and skin. Use the hand lens to examine the suckers on the tentacles and arms as well as the spots on the skin, which are chromatophores.
What are the differences between arm and tentacle suckers? Where are the suckers located on the feeding tentacles as compared to the location of the suckers on the arms?
How do you account for the different locations of the suckers on the tentacles and the arms? What are chromatophores?
- The Mouth and Beaks: Locate the dark beaks in the center of the mouth.
Open and close the beaks, noting how the ventral beak overlaps the dorsal beak. How is this different from a parrot’s beak? Before you pull out the beaks, imagine what they will look like on the inside. With tweezers, remove the beaks and place beaks together with dark pointed parts opposite one another. Manipulate them (open and close) as if the squid were eating. What makes them work in this way?
In order to remove the radula (a ribbon with rows of teeth on a tongue-like muscle) from inside the mouth, make small incisions in the edge of the mouth. With tweezers, locate the small, folded, plastic-like radula between beaks and remove it. It is usually very small, yellow or white in color. What is the radula’s function? Store the radula and the beaks in water in a small cup if you are going to do a microscopic examination.
- Funnel:
- Turn the body over, ventral side up, and locate the funnel (a deflated fleshy tube located at the base of the head). A squid swims by squirting water from the mantle through the funnel. The direction it swims depends on which way the funnel is aimed. Move the funnel and note its flexibility.
- External Anatomy:
- Orient the squid so that the tentacles are away from you, at the top of the dissection tray. Spread out the arms, tentacles, and fins. Draw and label the external parts of the squid: arms, tentacles (have suckers only at the tips), head, eyes, fins, mantle, funnel, tail, suckers, beaks (where each would be found on an intact squid) and mouth. If something cannot be seen, draw an arrow to show where it should be.
If you have time, slice open an eyeball and locate the lens, pupil, retina, and iris (colored part of the eye). Look for the creamy white brain between the eyeballs. For assistance in identifying these parts, refer to the illustration below.
- Opening the Mantle:
- Keep the squid on its back (the side opposite the funnel). Using forceps, lift up the opening to the mantle behind the funnel (near the head) and separate the mantle from the internal organs. Close the forceps firmly so as to “pinch” the mantle flesh to keep it taut, cut along the ventral midline of the mantle, from its opening all the way to the tail. Be careful to keep the scissors lifted away from the internal organs so they are not damaged.
- Locating and Removing Reproductive Organs:
- Locate the gonad (reproductive organ) in the posterior end (refer to diagram for shape and location).
Upon opening female specimens, the large, firm, white nidamental glands are seen first. Males do not have nidamental glands. The glands lay on top of the other internal organs. These glands create the gelatinous matrix that envelops the eggs. In order to proceed further, carefully remove these glands. In females the eggs are jelly-like in a conical sac at the posterior end of the mantle. The male genital duct is a white, fluid-filled sac in the posterior end of the mantle. The sperm are stored in thin tubes in an elongated sac behind and along one gill.
- Gills:
- Find the gills. These are the long, feather-shaped organs that are attached to the sides of the mantle and extend along the anterior half of the mantle. Identify the gill hearts, one on the posterior end of each gill (these are small, flat and white). Questions: Why are they white and our hearts are red or purple? The squid has a third heart (the systemic heart) that pumps blood to the rest of the body.
Challenge: Why does it have separate hearts for the gills alone?
- Digestive Tract:
The long, silvery dark tube on the bottom of the liver (but appearing to be on top of the liver because of the squid’s inverted position) is the ink sac. Be careful not to break it open. Locate the stomach and caecum. These lie together as one white, silky-looking tube, like a deflated bladder and a coiled sack. The bunched up organs that look like human intestines are digestive ducts for the squid. If you are curious about the liver, wait to cut it open until the end of the dissection. It contains a lot of brown, oily liquid which may obscure other organs. If possible, open the stomach and examine its contents. Many squid will have bits of partially digested crustaceans (pink and white pieces), or tiny fish scales and bones.
- Removing the Ink Sac:
- Find and carefully remove the silvery-black ink sac that lies connected to the intestine. To do this, pinch the opening of the sac (near the back of the funnel) with forceps while gently pulling up and cutting the connective membrane along its length. After cutting about 1/3 to 1/2 of it, hold the sac with your fingers and pull the sac off the liver. Be careful not to puncture it. Squid ink stains clothing and skin. Place the sac in a small cup for later use with the gladius (pen).
- Removing the Gladius (Pen):
- The gladius is a long, clear feather-shaped structure used to support the mantle and for organ attachment. It and the cranium, or brain case, make up the “skeleton” of the squid. It feels like plastic and is made of tissue similar to a shrimp shell. There are two ways to remove it: from the tail or from inside the cut-open mantle. To remove it from inside the open mantle, grasp the head and organs firmly, and rotate them to the side with your left hand while holding on to one side of the mantle with your right hand and pulling away gently. Pulling the gladius out is like removing a splinter from your skin. You may need to cut away connective tissues that hold the gladius in place.
The gladius is revealed, lying along the dorsal midline of the mantle.
Grab the forward end of the gladius and pull it carefully from its slot in the mantle. It may be helpful to have one person hold down the lower mantle while the other removes the gladius. To remove from the tail end, rotate the organs to one side, cutting connective tissues. Make sure the mantle is slit along the internal dorsal midline all the way to the tip of the tail. Pry out the tail end of the gladius and pull straight back, away from the body.
- Writing with the Gladius (Pen) and Squid Ink:
Cut one end of the ink sac open and press it against the bottom of the cup with forceps or toothpick. You can also hold one end and push the ink out with your finger, as you would toothpaste from a tube. This will release the ink. Dip the pointed tip (the anterior end) of the gladius into the ink, filling the tip with the dark fluid. Then, using only the ink-filled tip of the gladius, write your name on your squid illustration or paper. If there is enough ink, create and write the name of your dissected squid under its picture. If the ink seems dry and pasty, add one drop of water at a time to create fluid ink. Though this is an unusual way to write, squid ink was actually used to write and draw in ancient times, and it is used today in some cultures. Unfortunately, it tends to fade over time (except from your clothes!).
- Internal Anatomy:
- Draw, label, and identify the function of the following internal parts of the squid:
- stomach
- caecum
- hearts (systemic and gill)
- gills
- reproductive organs
- ink sac
- liver (digestive gland)
- gladius
- brain
- eyeball
- Microscope Slide Option:
The following parts of the squid make excellent specimens for microscopic study:
- eggs from the ovaries
- suckers
- nidamental glands
- tips of arms and tentacles
- spermatophores
- connective membranes (thinly-sliced: mantle, fin, arm muscle)
- radula
- stomach contents
- liver fluids
- skin and chromatophores
- portions of the eye
- beak
| Teaching Note: Most of these are useful only for a dissecting microscope. |
Questions for further Investigation:
- Identify the differences between the tentacles and the arms. Why are they different?
- How are squid mouths and beaks like your jaw and teeth? How are they different?
- How does the squid use the funnel and mantle for locomotion?
- How does the squid obtain oxygen from the water?
- How do squid reproduce?
- Why are the chromatophores important to the squid?
- What are the relatives of the squid?
- What are the characteristics of cephalopods and of mollusks?
- Why is it difficult to identify stomach contents?
- What is the function of the fins?
- What organ systems are the same or different from vertebrates?
When finished, clean your area completely. Return all equipment and wash your hands. The squid odor will remain for a little while. Lemon juice will alleviate the odor if you find it offensive. To dispose of your specimen, wrap it in plastic or a zip-lock bag and throw it away. You may want to feed it to your cat, cut it up for fish bait, or even serve it as tonight’s calamari. Bon Appetite! Visit Clyde’s Kitchen on this website for tasty squid recipes!
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Squid Dissection
