Download Squid Dissection

Animals 2
Name _______________________________
Squid Dissection
Squid are invertebrates in phylum Mollusca, a group that includes chitons, snails, bivalves,
and cephalopods. Squid are classified in Class Cephalopoda (literally head-foot) along with the
chambered nautilus, cuttlefish, and octopus.
Molluscs appear in the fossil record over 600 million years ago and are the second-largest
phylum among the animals. They are one of the more primitive animals to have a coelom, which
surrounds the heart, gonads, and the excretory organs.
Adult molluscs have unsegmented bodies consisting of three parts: the foot, the visceral
mass, and the mantle. In molluscs with shells, the mantle secretes the valve(s). The shell of squid
is reduced to a small cartilaginous pen which is enclosed within the mantle. The foot of squid is
evolved into tentacles and arms to aid in capturing prey for food. In cephalopods, the foot is not
much involved in locomotion. Instead, water taken into the mantle is expelled by muscle
contraction through a siphon to provide water-jet propulsion for the organism. Muscles at the
base of the siphon help orient the water-jet to direct the movement.
Cephalopods are advanced molluscs with well-developed nerve systems and sensory organs.
They exhibit complex directed behavior and have an ability to solve problems. The squid have
large eyes of a lens-based design, providing the animal with excellent vision to detect prey to
attack, predators to avoid, or competitors to deter from potential mates. Their brains make squid
among the most intelligent of invertebrate animals. Their giant axons (nerve fibers) helped
scientists begin to understand neural function and signal transmission. Squid are able to adjust
the color patterns of their skin to communicate to each other and to provide either camouflage for
defense or a stealthy offense. Some squid species have translucent bodies, sometimes with
iridescence and can be very beautiful.
The squid has eight short arms and two long tentacles attached to the ventral part of the body.
All its appendages are equipped with adhesive discs, or suckers, which are used in feeding. The
longer and very agile tentacles shoot out rapidly to grab prey, usually fish or shrimp. Squid are
the fastest swimmers among the invertebrates. Some species of "flying squid" have been known
to leap out of the water for short distances reaching speeds of 16- 24 mph. Squid inflate the
mantle cavity with water through the siphon located on the posterior side of the head. When it is
time to move, the mantle contracts strongly and forces water back out through the siphon. This of
course moves the animal in the dorsal direction, dragging the ventral tentacles behind. While
most animals locomote in an anterior direction, squid move in a dorsal direction!
There are over 300 kinds of squid all of which are marine and are found from the Arctic to
Antarctic in all depths of water. They range in size from perhaps 8 centimeters long to the giant
squid at perhaps 18 meters long. The squid has color sacs called chromatophores on the surface
of the epidermis, which are capable of turning a variety of hues in the red, brown, and yellow.
This marine peacock also has thousands of special cells, called iridiophores, which reflect
iridescent shades of blue, green, pink and gold depending on the species of squid.
Squid are proteinaceous food for other marine organisms. Tuna, salmon, and rockfish, seals,
and whales are known to feed on squid. Squid can use their chromatophores to provide cryptic
coloration as a defense and, if threatened more closely, can release a cloud of dark ink into the
water to provide a kind of “smokescreen” while they use their jet-propulsion to escape.
Squid tend to be gregarious, forming large groups called shoals. Some make excellent
seafood and fishermen often hunt shoals of choice species. One technique used is to mount a
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high-intensity lamp on the fishing boat; the squid are fooled into responding as if it is a full moon
and gather near the surface to reproduce.
Swimming toward a light would be called: photosynthesis phototaxis phototropism
Today we will study squid, Loligo opalescens, that have been harvested from a fishery in
California for use as food. These “market squid” were quick-frozen on the fishing boat, packaged
(42 squid per 3-lb box) on the West Coast, and delivered frozen to a local grocery. We have
thawed them out for study. While they are not alive now for behavioral studies, this is a fairly
large invertebrate for us to dissect and study its structure as a representative. Please note: while
some of us like to play with food, today is a time for careful cutting after thoughtful reading!
Orientation:
Remember always that the tentacles are on the ventral part of the body. The siphon points in
the ventral direction. If you hold the animal with its tentacles and mouth downward, the conical
mantle will taper to its tip in the upward direction. The tapered end is the dorsal part of the
animal. The space between the eyes are on the anterior part of the mantle while the siphon is on
the posterior side of the mantle (again with the siphon opening in the ventral direction).
Lay the organism in the dissection pan with the tentacles pointing to your left and the mantle
tapering toward the right. Roll the body so that its left eye is facing toward the ceiling and the
siphon is just visible on the side facing you. This is the usual view of a squid in the water
column. Make a sketch of your squid and label it with the four words in bold type above.
Also label your sketch with tentacles, arms, eye, siphon, mantle, and fin.
Examination of the Foot:
Now turn the squid with the tentacles and facing you. Spread the tentacles and arms apart to
reveal the beak and mouth. Count the tentacles and arms while noticing the number and
distribution of suckers on the arms vis a vis the tentacles.
There are _______ tentacles and ________arms.
There are more suckers on the Arms Tentacles
On the Anterior Posterior part of the foot the tentacles are attached between arms.
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Examine the beak and work the two parts together to discover how they can tear into prey.
Work carefully as the beaks can be pulled out with minimal force!
The anterior beak overlaps the posterior beak True False
The color of the beak tips is: ________________________
With forceps, remove the two beaks while looking carefully down into the mouth. If needed,
you can make some small cuts through the peristomial membrane at the side of the mouth.
When you remove the anterior beak you should be careful to see if any other parts pull out with
it. Often the radula comes with it! If not, dig deeper to remove it for closer inspection in a Petri
dish.
Make a sketch of this ventral view with radiating arms and tentacles with the beak in the
center in the left portion of the space below. On the right, make another sketch showing a closer
(more magnified) view of the mouth region. In the margin sketch a dissection microscope view
of the radula. Label your sketches with the words in bold above.
tentacles
arms
suckers
beak
mouth
peristomial membrane
radula
Examination of the Head:
Now move up the anterior part of the foot toward the eyes into the head region. On the
ventral side of each eye is the aquiferous pore which probably helps the squid equalize
intraocular pressure. To find the pore, apply some slight pressure to the eyeball; fluid with
particles from inside the eye willl escape through this pore. On the dorsal side of each eye
(toward the mantle) and extending around to the posterior of the eye are the olfactory crest and
olfactory groove. The eyes feature prominent iris and pupil. Make a sketch of this part of the
animal and label with the words in bold above.
aquiferous pore
olfactory crest
olfactory groove
iris
pupil
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Dissection of the Eye:
Carefully cut open one of the squid’s eyes to observe its internal structure. Use forceps to
remove parts or to just move them around for observation. Try to determine where the lens is in
relation the cornea, iris and retina. As you work the eye open some pigmented particles come
out; these are portions of the retina released by freeze damage.
What color is the photoreceptor pigment of the retina? _______________________________
Put the lens in the Petri dish bottom, and press down on it with your finger.
The lens is Hard Soft
Make a sketch showing a slice with cornea to the left and the optic nerve to the right. Label
your sketch as completely as possible.
cornea
iris
pupil
lens
retina
optic nerve
Examination of the Mantle:
Roll the animal so the posterior side is facing up, you will find the siphon with its opening
pointing toward the arms and tentacles and away from the mantle. Gently squeeze the sides of
the siphon to open the siphon’s free end; water jets from this opening. The mantle is conical in
shape extending upward (dorsally) from the tentacles. Where the head joins the mantle, the
mantle is at its widest in a region called the collar. If you look into the space between the collar
and the head you will be able to view the mantle cavity. Projections along the edge of the collar
are called the articular ridges. There is one long medial ridge on the anterior side of the collar;
there are two lateral ridges on the posterior side of the collar. On the anterior side, running from
between the eyes and toward the tapered end is a ridge along the mantle. Under this ridge is the
pen…the “shell” secreted by the mantle. The surface of the mantle is an epidermis with abundant
chromatophores. These membranous sacs of pigment can be enlarged or reduced by muscles
controlled by nerves. A living squid can operate these tiny muscles to exquisitely change their
color and color patterns very quickly. Along the sides of the tapered portion of the squid’s
mantle are fins that assist in a wing-like fashion for slow swimming and in a rudder-like fashion
during water-jet propulsion.
The fins are attached laterally on the anterior posterior side of the mantle.
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Make a sketch showing the posterior side of the animal and label it with the words in bold
above. Orient your sketch with the collar to the left.
siphon
collar articular ridge
mantle cavity pen
chromatophore
fin
Dissection of the Visceral Mass:
Lay the squid on its anterior surface so that the siphon is on the top. Thus the entire posterior
surface is visible. Use scissors to make an incision along the middle of the mantle beginning at
the collar just above the siphon and ending near the tapered tip of the mantle. Be careful to cut as
shallowly as possible so as to avoid cutting any internal organs open! You should now be able to
see the internal organs of the visceral mass. It will be at this point that a careful observer can
determine the gender (sex) of their specimen. Males have very little covering the cecum in the
tapered end of the mantle, females have sometimes-extensive jelly-like ovary tissue there. In
males the yellowish branchial hearts are exposed, in females they are hidden under 2 large white
nidamental glands.
What is the sex of your squid? Male Female
Near the beginning of your cut at the collar and near the base of the siphon you should find
the paired white siphon retractor muscles that help orient the siphon to direct the water-jet
movement. A little deeper (toward the anterior) are cephalic retractor muscles which help
move the head with respect to the mantle. Cut the siphon medially and fold it back to reveal the
siphon valves which help regulate the flow of water through the siphon. Dorsal to the insertion
of the siphon into the collar is the anterior (cephalic) vena cava. This large vein returns blood
from the head into the branchial hearts. Still deeper toward the anterior is the yellowish liver, an
elongate organ. Yet deeper is the esophagus extending from the mouth into the interior of the
mantle cavity.
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Begin a sketch below showing this portion of the visceral mass toward the left, labeling the
structures in bold above. Leave room to the right to continue your sketch toward the tapered end.
siphon retractors cephalic retractors siphon valves liver
spermatophoric
glands
nidamental
gland
testis
ovary
penis
oviduct
gills ink sac branchial heart
stomach pancreas cecum
Continuing dorsally:
As you move toward the tapered end of the mantle (dorsally) you will find paired lateral
gills. These are highly vascularized with a branchial vein running down the middle of each one.
Between the gills (medially) you may find a tubular rectum extending toward the collar and
ending in the rectal paillae surrounding the anus. As the exit of water is in the ventral direction
(toward the collar and head) it is not surprising that the anus opens toward the collar. A bit
deeper (toward the anterior) than the rectum you will find a silvery dark ink sac with a dark ink
duct leading toward the collar. Lift the duct and ink sac with your forceps, remove it, and set it
aside on your dissecting tray for later. This structure is NOT to be cut open yet! If you do cut this
open by accident, you will need to wash out your specimen as the ink will ruin your view and
will permanently stain papers and clothing! So use caution here! Just lateral to the rectum in a
male squid will be the tubular penis; on the female squid, the two large nidamental glands may
obscure the rectum and the ink sac and the oviduct leads to the genital opening. Continue adding
to your sketch on the previous page and use the words in bold contained in this paragraph to
label your sketch.
Again moving dorsally (toward the tapered end of the mantle), on the male squid you will
find the two yellowish branchial hearts appearing to be joined medially, with vessels leading
into and away from them. The systemic heart is smaller and deeper (more anterior) than the
branchial hearts. The major vessels lead into the gills, laterally into the mantle cavity, ventrally
as the anterior vena cava and dorsally as the posterior vena cava. There are medial mantle
arteries and genital arteries supplying the major internal organ systems. Lateral to the branchial
hearts are spermatophoric glands (the seminal vesicles) that extend dorsally as the testis
(produce sperm). Deeper, the digestive system has pouches for stomach (for digestion) and
pancreas (secretes digestive enzymes) and a large central cecum (absorption of nutrients from
digested food) extending dorsally toward the tapered end.
On a female squid, the circulatory and digestive structures are generally obscured by the
paired nidamental glands (produce the shell for the zygote) and the extensive jelly-like ovary
(produces the eggs), so removing these or at least pulling them to one side will help reveal the
deeper tissues.
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Add to your sketch on the previous page the sex-specific structures and the rest of the items.
Be sure to label all of the items in bold on your sketch. Below is a space to make a similar sketch
of a squid of the opposite sex!
siphon retractors cephalic retractors siphon valves liver
spermatophoric
glands
nidamental
gland
testis
ovary
penis
oviduct
gills ink sac branchial heart
stomach pancreas cecum
As you dissect more deeply beneath the branchial hearts toward the systemic heart and
cephalic aorta below, you will find a nephridial sac that surrounds two cone-shaped nephridia.
These serve the excretory functions we associate with kidneys and are drained by ducts to the
paired nephridiopores into the mantle cavity.
Open the digestive system (stomach and cecum) and examine the contents in a petri dish with
the dissection microscope. You may find bits of crustaceans, fish scales, and bones among the
debris. Record what you find:
System Organization:
It is strongly recommended at this point to start removing structures from the visceral mass in
an organized way to separate the various systems from each other. As you remove portions, try
to lay them out in your pan so that you can line them up and connect them spatially to understand
how they work toward the common function. Be very careful with the ink sac, leaving it alone as
long as possible to avoid an inky mess!
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Anterior dissection.
Having removed most of the internal organs from the mantle cavity, turn your squid over so
that the siphon (posterior) side is down and you can see the collar and anterior articular ridge
extending from the collar toward the tapered end of the mantle.
From this perspective you can dissect the head of the squid between the two eyes, open the
covering soft tissues and the more cartilaginous cranium (brain case) lying just dorsal to the
eyes. Use two pairs of forceps to pull apart the two side walls of the cranium. Inside the cranium
you should find the large buccal bulb of the brain and deeper (posterior) to this and between the
two eyes you will find the lateral pedal ganglion and visceral ganglion. Sketch what you find
and label appropriately:
cranium buccal bulb pedal ganglion visceral ganglion
Embedded in the anterior surface of the mantle, the gladius (pen) is a long, clear feathershaped 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 (cartilaginous) and is made of
chitin (as in shrimp exoskeleton). The pen can be removed from inside the open mantle by
excavation. Extracting the pen is somewhat like removing a splinter from your skin. Some gentle
pulling with forceps will help you extract the pen.
If you have been careful to avoid the ink sac so far, now use the pointed tip of the gladius to
puncture the ink sac, dip into the ink, and write your name on the line below with a squid pen in
squid ink.