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Animals 2Name______
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 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 ArmsTentacles
On the AnteriorPosterior part of the foot the tentacles are attached between arms.
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 TrueFalse
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
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 HardSoft
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 anteriorposterior side of the mantle.
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? MaleFemale
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.
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
spermatophoricnidamental
glandsgland
testisovary
penisoviduct
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.
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
spermatophoricnidamental
glandsgland
testisovary
penisoviduct
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!