Download Squid Dissection A squid is in the phylum Mollusca

Squid Dissection
A squid is in the phylum Mollusca and it belongs to the class Cephalopoda, meaning “head-foot”. This class also includes the
octopus, cuttlefish and ancient nautilus. Squid can be as small as a thumbnail, or as large as a house. The squid you will be
dissecting is found the in genus Logio.
All mollusks have a soft body with a special covering called the mantle, which encloses all of the body organs. Squid have a large
mantle, eight arms with two longer feeding tentacles, a beak and mouth, a siphon, a large head (with a brain), two large eyes, and
three hearts. The tentacles are long and retractable and have suckers only at the tips. The shell has been reduced to a chitinous pen
that is embedded in the upper surface of the mantle.
Squid are built for speed and move by squirting water from the mantle through the siphon, using a type of movement called jet
propulsion. They can move both backward and forward just by changing the direction of the water flow through siphon. Its
streamlined body and jet propulsion make the squid a fast, active predator. Squid can change the color of their skin to mimic their
environment and hide from predators. When in danger, squid can release a cloud of dark ink from their ink sac in order to confuse
their attacker and allow the squid to escape. These fast-moving carnivores catch prey with their two feeding tentacles, then hold the
prey with the eight arms and bite it into small pieces using a parrot-like beak. The esophagus runs through the brain, so the food
must be in small pieces before swallowing. Squid feed on small crustaceans, fish, marine worms, and even their own kind.
Squid reproduce sexually with internal fertilization. Male squid have a modified arm, which is used to insert a packet of sperm,
known as a spermatophore into the female’s mantle cavity. After mating, a female squid will produce 10-50 elongated egg strings,
which contain hundreds of eggs in each string. These string of eggs will then be released into the water. In many species, the
parents will soon die after leaving the spawning ground. The egg strings are attached to the ocean floor, are left to develop on their
own, and hatch approximately ten days later.
Squid are an important part of the ocean food web. Squid are a major food source for many fishes, birds and marine mammals.
Squid are gaining popularity as a food source for humans around the world (calamari).
Part I External Anatomy
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Place the squid on the dissection tray dorsal side up. Notice how the dorsal side is darker than the ventral side. This is
called counter shading.
The main part of the squid body containing its internal organs is called the mantle. Squid have no external shell so this soft,
dotted and colored tube encloses the main part of the body. All the small dots you can see are pigment cells called
chromatophores. Muscles controlled by the nervous system surround each chromatophore. The chromatophore muscles
expand or contract to reveal the colors of the pigments within. When the muscles tighten the squid will darken in color,
when they relax the squid becomes a lighter color. If you had a fresh squid, you could rub them and possible see a color change.
At the posterior (apex) end of the squid you will notice the two fins on the mantle. These fins allow the squid to steer
themselves, help stabilize their position and propel the squid at slow speeds.
Squid have big eyes compared to their head. In comparison, humans’ eyes would be the size of dinner plates if the
proportions were the same. Squid eyes are also positioned on the sides of their heads, giving them more peripheral vision.
Squid eyes are much like a humans except the lens of a squid eye is more football shaped whereas a human’s lens is round.
The lens serves the same purpose in both animals and that is to focus light to be able to see images at various distances.
Carefully nip open the cornea, which is the soft protective covering of eyes and use your fingers to search for the only hard
part of the eye, the lens. Squid can tell the difference between light and dark, but not color. They can also receive polarized
light, allowing them to reduce the glare given off by reflective structures such as fish scales.
Observe the foot of the squid which has evolved to become the tentacles and arms. The tentacles are longer than the arms
and only have suckers on the end. The swollen ends of the tentacles are called clubs. The tentacles are used to capture
prey and pass food to the shorter arms, which are used to hold and direct food to the mouth. Arms have suction cups all
the way down. Notice all the small teeth in a ring around each sucker.
Find the funnel, which acts as siphon. This is a short tube located just under the eyes and on the underside of the mantle.
The funnel allows the squid to move through the water by jet propulsion. The squid will take water into the mantle cavity,
the large opening around the front of the mantle. Muscles will close off the mantle and the water can only leave the squid
through the funnel. The mantle’s muscles contract and water will jet through the siphon with a high force. This force is
called jet propulsion and allows squid to be one of the fastest moving animals in the ocean, swimming up to 30 mph.
Located within the circle of arms is the squid’s mouth which is a beak. The beak is a hard, dark brownish color. Use
tweezers to carefully remove the round, muscular encasing around the beak known as the buccal mass. When the buccal
mass is removed a small long tube may be attached. This is the esophagus which is connected to the stomach. The beak
has two halves and is much like a parrot’s beak. The beak tears food into small pieces before it is swallowed. Each half the
beak can be removed and a tongue with a toothed ribbon known as the radula will be found. The radula shreds the small
pieces of foods to be easily digested.
Part II – Internal Anatomy
Place your squid on in the dissection pan so the squid’s ventral side (funnel side) is facing up. Use scissors to cut down the
middle of the squid’s mantle starting from the funnel and cutting towards the apex of the squid. Be careful to cut only the
mantle and not the underlying organs. Once the squid is open, spread and pin back the sides of the mantle.
Reproductive System
Inside the mantle cavity of the squid, down by the fins is the squid’s gonads or reproductive organs. If your squid is a male,
the gonad will be a cloudy, white-ish mound. Female gonads may have eggs inside the ovaries that are slightly yellowish in
color and they look and feel like jelly. Depending on the reproductive stage, the eggs may be plentiful and look like tiny
spheres in gelatin. Females also have a large organ above their gonad called the nidamental gland. This is used to harden
the eggs before they are released. Next to the gonads is the caecum which is part of the digestive system.
Digestive System, Ink sac and Retractor muscles
This part of the squid starts with the squid’s beak and radula, food travels down into the esophagus, which passes through
the center of the squid’s brain and runs midway down the squid’s mantle cavity. Squids must tear off and swallow small
bites so food does not get caught in the brain as it passes through. Next locate the stomach. It is small an oval structure
connected to the anterior part of the caecum. It is below the esophagus and can be difficult to locate. The stomach is the
main part of digestion and the caecum increase the surface area for digestion and is the primary site of nutrient absorption.
The digestive gland can also be called the liver and secretes digestive enzymes into the caecum. It is oval in shape and can
be found underneath the esophagus. Waste products leave the caecum through the long intestine, eventually reaching the
terminal end called the anus. The anus runs into the funnel where waste is released.
Attached to the intestine is a small, silver structure containing a thin black line. This is the ink sac and its size will depend
on how much ink is inside. The ink is used as a defense against predators. Squid squirt ink from their body through their
funnel into the surrounding water. This confuses the predator and allows the squid to escape with its jet propulsion. The
squid can also camouflage itself by changing its body color to match the cloud of dark ink in the water. The ink may also
numb some predator’s sense of smell, which makes it harder to pursue the squid.
The two large masses surrounding the organs, attached to the mantle are muscles, named the funnel retractor muscles.
These two muscles allow the squid to move, direct and create jets streams of water through its funnel. They feel different
than the rest of the squid’s soft body and feel like strong tendons.
Respiratory, Circulatory and excretory system
Locate the two feathery gills on each side of the mantle cavity. The gills are used to extract oxygen from the water. The
water that enters the mantle cavity to be used for jet propulsion also bathes the gills with oxygenated water. The feathery
structure increases the amount of surface area potential for gas exchange.
Squid have three hearts. The two smaller branchial hearts are located at the base of each gill and are used to pump blood
to the gills. The systematic heart is larger and located between the branchial hearts. It is used to circulate oxygen rich
blood around the entire body. It may be covered by the kidney. Squid have blood that has a bluish tint. This is due to a
pigment containing copper rather than the iron found in humans. The kidney is located above the systematic heart. It is
used to remove metabolic waste products from the blood.
Nervous System
The brain is located right between the eyes and consists of many bundles of nerves fused together. It is surrounded by a
“skull” made of cartilage and surrounds the esophagus. You will have to carefully cut in‐between the squid’s eyes through
the protective cartilage of the brain case to see the brain. Many large nerves radiate out from the brain through the mantle
wall. The nerves are highly developed and consist of giant axons to control the squid’s actions.
Support System
The squid is supported by a stiff, slender, internal structure called a pen. The pen can be found by shallowly cutting the
length of the dorsal midline, underneath the muscles and organs, starting near the reproductive organs. Grasp the tip of the
pen and gently tug until it is removed. The pen looks like a thin flexible piece of plastic or transparent feather. Unlike its
external shelled relatives (clams, snails, Nautilus), whose shell is made up of calcium carbonate, the pen of a squid is made
up chitin. Chitin is the same substance that makes up the exoskeletons of arthropods.
Label the following external structures on the squid below: arm, chromatophore, club, eye, fin, funnel, mantle, tentacle, suction
cups
Label the following internal structures on the squid below: anus, branchial heart, caecum, digestive gland, esophagus, gill, gonad,
ink sac, intestine, systemic heart, kidney, pen, stomach
Part III – Analysis questions
1.
What is a spermatophore?
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How many eggs can a female squid lay?
3.
What role do squid play in the ocean food web?
4.
What is counter shading?
5.
How many arms does a squid have?
6.
How many tentacles does a squid have?
7.
The enlarged ends of the tentacles are called what?
8.
Describe what the suction cups look like.
9.
How is a squid’s eye similar to a human eye?
10. How is a squid’s eye different than a human eye?
11. Briefly describe how chromatophores function.
12. Briefly describe jet propulsion in a squid.
13. Describe how the ink sac is used as a defense for squid.
14. Why are the gills feathery?
15. What color is a squid’s blood?
16. Why is it better for a squid to have a pen of chitin rather than calcium carbonate?
17. List and describe 5 adaptations that allow squid to be an efficient predator.