A school of mackerel swim together as they migrate to a new feeding ground. Suddenly from above and below, torpedo shaped animals jet among the fish and strike. The mackerel quickly scatter in different directions and the attack is over in seconds. In the end, two of the fish have been captured by a pair of squid that quickly subdue them with a poisonous bite. Squid are swift and agile marine predators that are, in turn, preyed upon by larger predators such as whales. Squid have well developed eyes and a brain with the capacity to learn. They represent one extreme of mollusc development and, to the uneducated, it is hard to believe that snails and clams are related to this predator of the deep.
Squid belong to the class of molluscs, the Cephalopoda. This class also contains octopuses, nautiluses and cuttlefish. Cephalopods represent the most advanced class in the phylum Mollusca. Molluscs are the first phylum to have a true coelomic body cavity. The shared characteristics of squid with other mollusc groups are not at all readily apparent because of the advanced cephalopod features. All molluscs, however, share the following body features:
1. An unsegmented body that is bilatreally symmetric with a reduced or absent body cavity and a specialized muscular foot.
2. Body covered by a thin tissue layer fold of the dorsal wall called the mantle (or pallium). The mantle secretes the exoskeleton or the shell (which may be well developed as found in clams, reduced as found in squid, or totally absent as in octopuses).
3. Usually an open circulatory system (closed in cephalopods) with a well devloped pumping heart consisting of one or two atria and one ventricle.
4. Presence of a radula, a rasp-like organ used for food scraping in snails and food manipulation in squid. Molluscs that are bivalves, such as clams, do not have a radula.
Cephalopods (means head-footed) are characterized by possessing the following advanced features:
1. Distinct head with large well-developed eyes and brain.
2. Head projects into a crown or group of flexible, muscled arms.
3. Mouth with well developed jaws and a hard parrot-like beak for tearing off food pieces.
4. All are active, quick, predators found only in marine habitats.
5. All cephalopods can swim rapidly by expelling water from their mantle cavity.
Squid Dissecting video (abt 10 mins), start and stop it as needed to view the external and internal structures of the squid as you complete your worksheet and questions that follow.
Position your squid specimen on your dissection tray so that it is in the position shown in Figure 1 above, so the funnel is on the top. This is the posterior view of the squid based on morphology. Note how the squid is divided into three regions, the head, neck, and body trunk. Find and identify the body features listed in Figure 1 on your specimen.
Arms: There are four pairs (8 ) arms on the squid. Note the two rows of stalked suckers that line the underside of each arm. Arms manipulate and hold food while it is being eaten. In males, the 4th arm is also used for the transfer of spermatophores.
Tenacles: These are two (1 pair) long appendages with suckers restricted to pads on the distal ends. Tentacles are used to grasp prey and pull it to the arms. Tentacles are a modification of the foot found in other molluscs.
Mouth: The mouth is located in the center of the circle of arms. You will have to spread the arms apart to view the mouth. Note the hard horny jaws of the squid beak which are used to tear off pieces of food for swallowing. The mouth is surrounded by the peristomial membrane that is surrounded by a buccal membrane. The buccal membrane has projections with suckers on them. See Figure 2 below.
Eye: Squid possess very advanced eyes which are more similar to vertebrate eyes than they are to any invertebrate eye. This is a good example of convergent evolution. Note the black pupil and light colored iris.
Suckers: Take a tentacle of your squid and pin it to the dissection tray bottom so that you can examine the suckers with a hand lens or dissectiong microscope. Compare the tentacles of your specimen with the suckers. Note how each sucker has a set of teeth. These teeth grab the skin of the prey or attach on an object and by pulling with the arms a suction is created. This suction aids in holding the object. The sucker stalk connects the sucker to the tentacle or arm. It is fexible and muscular to aid in securing prey.
Funnel: This muscular tube extends out from the mantle collar. The funnel directs water that is expelled from the mantle cavity. Water is expelled with such force that the squid can propel itself with jets of water. In addition, the funnel is flexible and can be aimed in many directions. If the funnel is directed backward the squid is propelled forward. Conversely the squid is propelled backward when the funnel is directed forward. Wastes and ink are also expelled through the funnel valve.
Mantle: A thick cone of muscular tissue that covers and protects the visceral mass and mantle cavity. At the apex or tip of the mantel are two fins the squid uses for swimming. Currents of water enter the mantle cavity through the mantle collar. The mantle is covered with chromatophores, cellular mechanisms that allows the mantle to change colors and blend in with its environment.
Use the unlabelled diagram above and match the letters to the correct structures. You will select the incorrect answer.
B (pointing to individual cells in this area) mantle
Continued from previous question: Use the unlabelled diagram above and match the letters to the correct structures. You will select the incorrect answer.
F suction cups
H (pointing to the entire area) chromatophores
Skills Assessment 10 pts: After you have identified all the external features, verify your work by identifying the structures with your teacher.
A Yes, I verified my work with my teacher.
B No, I did not verify my work with my teacher.
Turn the squid ventral side up. Pull the mantle up with the scissors where the water jet is, it should be loose and easy to pull up. Use scissors to cut from the water jet to the fins. Open the mantle to expose the structures inside. See Figure 4.
Use a dissection tray to pin down the two mantle flaps on either side of the visceral mass. Compare your specimen with the illustrated figures. Find and familiarize yourself with the organs identified and listed here:
a. Find the ink sac, this is a small dark sac near the water jet; located underneath the intestine, this dark pouch holds a
quantity of ink that the squid can release into the surrounding water to obscure itself and escape from enemies.
b. Find the esophagus, this is best found by looking into the mouth and seeing where it leads, use a prove to poke within the
mouth; this muscular tube transports food from the mouth through the liver and pancreas to the stomach by muscle
contractions. Look at the mouth and find the chiltinous jaws and the tongue-shaped cartilage with the radula
c. To find the stomach, follow the esophagus toward the posterior; it is a large round organ that lies between the two mantle
d. The anus empties into the water jet, use scissors to cut the water jet down the center so you can see the small opening of
the anus; The anus and rectum collects food wastes from the intestine, compacts them and expels them via the anus. The
fecal mass is expelled with water through the funnel.
--The intestine is located in the upper end of the rectum.
--The kidneys are two small, whitish triangular organs located on either side of the upper intestine
--Located between each kidney, the anterior aorta carried blood from the heart to the anterior body regions and head
e. Locate the gills, these are feathery structures that may be hidden under other things, there are two of them on each side;
The afferent and efferent vessels transport blood to and from the gills.
f. Follow the gills toward the interior to find an enlarged structure at their base, this is the gill heart also called branchial
hearts; one is located at the top end of each gill, they are round circular organs that pump blood into the gills;
--The systemic heart is located between the two bronchial hearts
--The posterior vena cava are triangular shaped vessels and are quite large and dark colored. They carry collected blood
from the posterior body regions into the branchial hearts
--Posterior aorta is located directly behind the systemic heart
g. All the way toward the fin is a whitish or yellowish structure, this is the gonad. The male gonad is genreally white, the
female gonad is usually more yellow to clear. They usually fill the entire posterior mantle cavity. Squid used for
dissection are generally caught during the reproductive season resulting in the large size of this organ.
--The cecum is a yellowish organ found underneath the gonad and stomach. It is a digestive organ directly connected to
h. Find the hard point at the end of the fin and gently grip it with forceps pulling away from the squid. In this way you
should be able to remove the pen in one piece
Using the word bank above with the unlabelled internal structures of the squid, label the structures correctly. #1 is ....
Using the word bank above with the unlabelled internal structures of the squid, label the structures correctly. #2 is ....
Using the word bank above with the unlabelled internal structures of the squid, label the structures correctly. #3 is ....
Using the word bank above with the unlabelled internal structures of the squid, label the structures correctly. #4 is ....
Using the word bank above with the unlabelled internal structures of the squid, label the structures correctly. #5 is ....
Using the word bank above with the unlabelled internal structures of the squid, label the structures correctly. #6 is ....
Using the word bank above with the unlabelled internal structures of the squid, label the structures correctly. #7 is ....
Using the word bank above with the unlabelled internal structures of the squid, label the structures correctly. #8 is ....
Using the word bank above with the unlabelled internal structures of the squid, label the structures correctly. #9 is ....
Using the word bank above with the unlabelled internal structures of the squid, label the structures correctly. #10 is ....
Is your specimen a male or female?
Skills Assessment 10 pts: Once you have located and identified the internal structures, demonstrate your knowledge with your teacher.
A Yes, I have completed sharing my knowledge with my teacher.
B No, I selected to not complete this part of the lab.
The squid has a well developed closed circulatory system. It includes a vast capiliary system that reaches all body tissues. In addition, all vessels are lined with epithelial tissue. This circulatory system allows for rapid replenishment of nutrients and gas exchange that the squid requires for its active predatory existence. The blood pumping system of the squid consists of three separate hearts, the systemic heart and two gill hearts. Blood returning from the body tissues is pumped into the gills by the gill hearts. This forces the blood under high pressure into the thin capillaries of the gill by the mantle muscles. Gas exchange, carbon dioxide passing into the water and oxygen passing into the blood, occurs here. Oxygenated blood is collected and passed to the systemic heart, where it is pumped to body tissues via the arterial system.
Food is bitten into pieces by the parrot-like beak in the buccal cavity. One pair of salivary glands produces digestive enzymes and mucus, the other pair produces a poison. When the squid bites its prey, poison flows into the prey to subdue it. A bite of food is mixed with saliva and is passed to the stomach by the long esophagus. The stomach is a muscular organ that kneads the food with digestive secretions produced by the pancreas and liver. After the food has been mixed and kneaded, it passes into the cecal sac. Here digested food particles are sorted from non-digestible particles. The digestible items are absorbed through the cecal walls. Nondigestible waste is returned to the stomach where it is passed into the intestine. Some absorption takes place in the intestine before the waste is expelled through the anus and out the funnel. The digestive modifications found in the squid are adaptations for rapid digestion of food that supports the squids' active carnivore existence.
There are two compact, sac-like kidneys in the squid. The kidneys are enclosed in a sac that also includes the pancreas. Through the middle of each kidney is a large systemic vein that leads to one of the gill hearts. The interface of the kidney with this vein is where waste is transferred from the blood. The waste is primarily ammonia and is excreted into the mantle cavity through the renopericardial canal. There is no bladder and waste passes out the squid with water being expelled out through the mantle funnel.
(see figure 6 above) Squid have a highly developed nervous system, the complexity of which is attained only in a few invertebrate groups such as bees and spiders. The squid brain is the largest invertebrate brain and is composed of the typical molluscan ganglia that have fused and concentrated together to form a rather large brain mass. The brain is segregated ito various ganglion regions that encircle the esophagus. These ganglia regions are so well differentiated that scientists hav been able to distinguish various centers responsible for various functions. For example feeding, swimming, and sucker control is all under the direction of the cerebral ganglion. The cerebral ganlion is the largest and most prominent brain region.
Other brain regions include the brachial ganglion that receives nerve inputs from the arms and tentacles, the visceral ganglion that receives and processes input from body organs and the pedal ganglion. The optic nerves from both eyes are also large and serve as conduits to the cerebral ganglion for optical input. The long nerves of the mantle form giant neurons that are so large they are used extensively by scientists to study nerve function. Experiments with squid have demonstrated that they have a surprising amount of learning ability that is unique to invertebrates. This learning is credited to their sophisticated brain. The brain is protected with a cartilage "skull" or capsule.
The squid eyes are also highly developed. Squid possess eyes that are similar to vertebrate eyes where a single transparent lens is used to focus light onto a light sensitive retina. The focus can be rapidly controlled by ciliary muscles that move the lens back and forth (a system also found in some vertebrates). The eyes are surrounded and protected by a series of cartilage plates that form a protective capsule. External muscles attached to the eye provide movement. Squid eyes and vertebrate eyes are a good example of convergent evolution.
Squid are dioecious, males and females are separate individuals. Males have a sac-like testis that produces sperm. Sperm is transferred to a seminal vesicle that encase the sperm into a packet called a spermatophore. Spermatophores are stored together in a separate reservoir that empties into the mantle cavity.
Females have a single ovary that produces eggs. Eggs travel through the oviduct, which is spiral and loops to an oviductal gland. Here a gelatinous covering is created for eggs as they are released for fertilization.
During mating, the male squid uses a specialized arm to pluck a spermatophore from its reservoir and inserts it into the female's opening. This transfer is known as hectocotyly. One arm of the male, called a hectocotyly, is modified as an intromittent organ for sperm transfer. The spermatophore wall disintegrates, releasing the sperm into the female's mantle cavity.
Eggs leaving the mantle cavity are grasped by the female's arms and are fertilized by sperm exiting the females funnel. The female attaches clusters of eggs in a mass to rock surfaces. There can be an many as fifty eggs in a cluster. Upon contact with sea water, the gel-coating hardens providing a protective egg covering.
As you answer the next questions, 20 thru 24, you will identify either the clam or the squid with your answer. By doing this, you will automatically identify the structure for the other specimen.
I am a filter feeder compared to being an active predator.
I have no external skeleton.
I have an open circulation system compared to having a closed system.
I have a head compared to organisms that do not have heads.
I use jet propulsion to move around quickly compard to having a slow creeping foot.
I have 3 separate ganglia compared to having a brain and nerves.
Continue Lab #15 Diversity of Animals by going to Part 3 of 4: Arthropods
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