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Squid Dissection
Phylum: Mollusca
Class: Cephalopoda
Introduction:
Mollusks are soft-bodied animals that are bilaterally symmetrical, with welldeveloped digestive, circulatory, excretory, and respiratory systems.
Cephalopods are characterized by possessing a variety of advanced
characteristics. They have a distinct head with large well-developed eyes and a
well-developed brain. The head projects into a group of flexible, well-muscled
arms. Cephalopods also contain well-developed jaws and a hard parrot-like
beak for tearing off food pieces. All cephalopods are active, quick predators that
can swim rapidly by expelling water from their mantle cavity.
External Anatomy: (Use Figure 1 as a reference)
1. Position your squid on your dissection tray so that the apex (pointed end) is
farthest away and the arms are closest to you. Turn the animal so that the
funnel (or siphon) is facing you.
2. Locate the dorsal, ventral, posterior, and anterior surfaces. Note how the
squid is divided into three regions, the head, neck, and the body trunk.
3. The main part of the body containing all the organs is covered by a thick
epidermal-circular sheet of skin called the mantle. The mantle secrets the
hard calcareous skeleton, the mantle cavity houses the gills and
reproductive organs. Look closely at the mantle; notice that it is covered in
pigment cells called chromatophores. The expansion and contraction of
these cells result from the action of tiny muscles that are attached to the
periphery of each pigment cell. When the muscles contract, the
chromatophore cell is drawn out into a large flat plate causing a display of
color. When the muscles relax, the pigment is concentrated into a small,
inconspicuous dot. Squid can change color rapidly and use this to
camouflage themselves, attract mates, and to communicate with each other.
4. Locate the two fins on the mantle near the apex of the body. The fins are
used as stabilizers and to propel the squid with controlled motions at
relatively slow speeds and to guide sudden turns.
5. The siphon (or funnel) is a short tube with one opening near the eyes and
the other end just under the mantle collar. The siphon directs the water that
is expelled from the mantle cavity and works to propel the squid through the
water in the opposite direction to which the siphon is pointing, much like jet
propulsion. The water comes out with enough force to propel the squid
through the water at about 20 mph. The siphon is also used for the excretion
of waste.
6. Locate the eight arms on the ventral side of the squid. Note the two rows of
suckers that line the underside of each arm. The arms manipulate and hold
food while it is being eaten. In males, the fourth arm is specialized for the
transfer of sperm cells.
7. Look at the suckers closely. Note the small teeth in a ring around the
suckers that are used in holding prey. Each suction cup has set of hooks or
barbs of chitin. These hooks grab the skin of prey and when the arm
muscles pull, suction is created which aids in holding prey.
8. Locate the two longer tentacles also on the ventral side of the squid.
Tentacles are used to grasp prey and pull it to the arms.
9. Look inside the circle of arms and tentacles. The small black dot is the beak.
Use your dissection tools to push the tissue back around it. Gently squeeze
the beak from the surrounding tissue to observe both halves. You may be
able to see the radula, which is a file-like tongue used to shred the pieces of
food before they are swallowed.
10. Locate the eyes. They are more similar to vertebrate eyes than to any other
vertebrate eye. They are much like our own, but the lens is shaped like a
football while ours is round. Note the black pupil and light colored iris. This
type of eye gives cephalopods an ability to detect visual images accurately.
If you carefully snip open one eye, you can remove the hard lens with your
finger or dissection tool.
Internal Anatomy: (Use Figure 2 as a reference)
1. Make an incision through the posterior view of the mantle. Begin at the base
of the funnel and continue cutting to the apex. Use care when making the
incision to prevent cutting any of the internal organs. Spread back the sides
of the mantle exposing the inside of the body cavity.
2. Find the esophagus, a muscular tube that transports food from the mouth
through the liver and pancreas to the stomach by muscle contraction. Look
at the base of the esophagus to observe the beak and radula from an internal
view.
3. Trace the thin-walled esophagus (surrounded by the liver) from the mouth to
the thick-walled stomach. The stomach is an oval structure about 1/2” long
hooked to the side and near the top of the cecum. The stomach is the major
site of digestion and the cecum increases the surface area available for
digestion.
4. The ink sac is located on the rectum and looks much like a small silver fish
or thin black line depending on how full the sac is. The ink is the pigment
melanin, which is the pigment found within our skin cells and causes tanning
and different skin tones. Squid expel the ink when they are threatened
causing confusion to the predator while the squid escapes.
5. Locate the gills, two white feathery structures found within the mantle cavity.
At the top of each gill is a brachial heart which pumps blood from the body
up to the gills to be oxygenated. Each of these hearts is quite small and
slightly yellowish in color.
6. Squid actually have three hearts. Locate the third systemic heart found
between the two branchial hearts. This heart pumps oxygenated blood from
the gills to the rest of the body.
7. Examine your specimen and trace the pathway of circulation. The blood
flows from the systemic heart to the body via the anterior and posterior
aorta, which give rise to various arteries supplying the head, mantle, and
visceral (internal) organs. Blood drains from the body regions in the veins
and pools in large anterior and posterior vena cava, passes to the gills,
and then returns to the systemic heart.
8. Locate the kidneys, two small whitish triangular organs located on either side
of the upper intestine. In the kidneys, nitrogenous wastes are extracted from
the blood and removed from the body by the siphon.
9. Determine if your squid is male or female. The gonads are located from near
the tip of the body to about the midpoint of the mantle.
10. In females, the ovaries containing the eggs are light yellow in color; they look
and feel like gelatin.
11. In males, the sperm is white in color and more watery than the eggs. Locate
the white testis at the apex. The sperm pass through the coiled tube called
the vas deferens and into the spermatophoric gland, which looks like a small
sac with many intertwining circles within it. This gland adds substance to the
sperm to make it into a sperm packet. The penis is the mechanism by
which the sperm packet is transmitted to the female. It is not a muscular
organ but simply the end of the vas deferens.
12. The squid is supported as it speeds through the water by a structure called a
pen. This is the evolutionary remnant of the mollusk shell. The pen is as
long as the mantle and shaped like a transparent feather. The pen is located
dorsally, just under the mantle. To remove the pen grasp the tip and start to
pull until it comes free of the mantle.