Observing the Structure of a Squid

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1 Name Class Date Chapter 27 Worms and Mollusks Observing the Structure of a Squid Introduction You are probably familiar with clams, snails, slugs, squids, and octupuses, and you probably have noticed how very different they are. Nevertheless, all these are classified in the phylum Mollusca. You will be examining one type of mollusk a squid. Squids have some characteristics in common with other mollusks, but they also have special features that make them unique. Squids are classified in the class Cephalopoda. In this investigation, you will be examining a squid to find out why squids are classified as mollusks. You will also determine what special adaptations make them different from other mollusks. Problem What evidence is there that squids are mollusks? What adaptations make them different from other mollusks? Pre-Lab Discussion Read through the entire investigation. Then, work with a partner to answer the following questions. 1. To what kingdom do mollusks belong? The animal kingdom. 2. Describe the characteristics that organisms classified in the phylum Mollusca share. Mollusks have a foot, a mantle, a radula, and a visceral mass. Most also have a shell. You may want to refer students to Section 27 4 in the textbook for a discussion of mollusks. To familiarize yourself with squid anatomy, you might do the dissection in advance. Time required: 80 minutes 3. Name some organisms that belong to the phylum Mollusca. Responses might include: clams, snails, slugs, squids, and octupuses. 4. Explain the characteristics that squids possess that lead to their classification as cephalopods. A cephalopod has its foot concentrated in the head region, tentacles with suction cups, a more advanced nervous system than other mollusks, a closed circulatory system, separate sexes, and an ink sac. 5. What other organisms belong to the cephalopod class and why? Octopuses, cuttlefishes, and chambered nautiluses. In these organisms, the foot is concentrated in the head region. All are free-swimming predators with tentacles extending from the head. Biology Laboratory Manual A/Chapter

2 Materials (per pair) squid dissecting tray dissecting pins compound microscope microscope slide coverslip dropper pipette metric ruler dissecting probe scissors hand lens forceps unlined white paper A box of frozen squid can be obtained in many supermarkets. Keep frozen until the afternoon before students perform the investigation, and then rinse frozen specimens under cool water to thaw and separate them. Safety Put on a laboratory apron, safety goggles, and plastic gloves. Always handle the microscope with extreme care. You are responsible for its proper care and use. Use caution when handling microscope slides, as they can break easily and cut you. Observe proper laboratory procedures when using electrical equipment. Be careful when handling sharp instruments. Dispose of the squid specimen as instructed by your teacher. Wash your hands thoroughly when you are finished working with the materials. Note all safety symbols in the Procedure and review the meanings of each symbol by referring to Safety Symbols on page 8. Procedure 1. Put on a laboratory apron, plastic gloves and safety goggles. Obtain a squid specimen in a dissecting tray. Count the number of short arms and longer tentacles. Using a hand lens, examine the suckers on the arms. 2. Study the two long, grasping tentacles and the sleek body with its pairs of fins and eyes. Place your squid on its dorsal surface. Locate the mantle, a loose edge of tissue near the eyes, and identify the siphon, noting it can be moved in any direction. In the space provided on page 203, make a careful sketch of the specimen, labeling the mantle, lateral fin, siphon, eye, tentacles, arms, and suckers. 3. Using the metric ruler, measure the length and width of your specimen in centimeters and record these measurements with your sketch. For a shorter laboratory period, omit the microscope section and the sketch of the eye. A squid has eight arms and two long tentacles. 202 Biology Laboratory Manual A/Chapter 27

3 Name Class Date Length of Squid External Anatomy of a Squid 4. With forceps, lift the free end of the mantle just above the siphon, and using scissors, cut through the mantle in a straight line to the pointed end of the body. CAUTION: Be careful when handling sharp instruments. Always cut in a direction away from your hands and body. Spread the mantle, as shown in the illustration below, and with dissecting pins, secure it to the tray. As you examine the squid, refer to the illustration below to locate the internal organs. Trace the siphon backward, using a dissecting probe to move aside the muscles that are attached to and control the siphon. Wastes, ink, and gametes are carried out of the squid by a current of water that leaves through the muscular siphon. Kidney Esophagus Systemic heart Cecum Ovary Siphon muscle Liver Ink sac 5. Examine the digestive system. Using scissors, start cutting at the neck and continue cutting through the head to a point midway between but just past the eyes, where you will reach a rough, muscular organ that surrounds the jaws. Push a probe between the jaws to locate the mouth. Gill Stomach Biology Laboratory Manual A/Chapter

4 6. Find the esophagus, a narrow tube below the jaws. Trace it to the stomach. Food moves through the esophagus, passing briefly through a junction with the liver before entering the stomach. The cecum is an elongated pouch off the stomach where absorption occurs. See if you can identify these structures. Follow the narrow tube, or intestine, from the cecum to the anus. Identify the dark ink sac near the anus. Note: Do not puncture the ink sac at this time. 7. Examine the respiratory system. Locate the gills, which look like curved feathers, one on each side of the body. Using forceps, transfer a tiny piece of gill to a microscope slide. Add a drop of water and a coverslip, and observe the tissue under the low-power and then the high-power objective of your microscope. CAUTION: Observe proper laboratory procedures when using electrical equipment. In the space provided below, sketch what you see under high power. Squid Gill 8. Find the systemic heart of the circulatory system, which is located near the ends of the gills where they attach inside the body. 9. Locate the excretory system, which consists of two small kidneys that are anterior to the heart. 10. To locate the small internal shell, cut through the mantle between the eyes and look for a small, plasticlike structure called a pen. 11. Determine the sex of your specimen by observing the fourth pair of arms. In a female, there is a small horseshoe-shaped pouch or fold between these arms, while in the male, the fourth pair of arms is modified to transfer sperm to the female. Inside the pointed section of the body, locate the sticky gonad (either ovary or testis), posterior to the kidneys. Observe the specimen of another lab group so you have a chance to examine squids of both sexes. Identify the sex of your specimen and record your observations. 204 Biology Laboratory Manual A/Chapter 27

5 Name Class Date 12. Squids have an excellent nervous system, which includes one pair of eyes. Using a hand lens, study one eye, noting the covering, or cornea. Record your observations. 13. Before completing the dissection, carefully puncture the ink sac with a dissecting probe and use the ink and the probe to write your name on an unlined piece of paper. Record your observations. 14. Discard your specimen as instructed by your teacher. Wash your hands thoroughly when you are finished with the materials. Analysis and Conclusions 1. Analyzing Data Based upon your examination of the squid, what features explain its classification as a mollusk? Presence of a foot (soft, muscular structure that contains the mouth and other feeding organs), visceral mass, and mantle. Students should provide descriptions of these features. At the end of the investigation, have students discard dissected parts of squid into a plastic bag and wash out dissecting trays with soapy water to remove fish smell. 2. Analyzing Data What features did you observe that are unusual for a mollusk? What function does each serve? The squid is unusual in that it does not have an external shell, the head is a modified foot, the foot contains arms and tentacles that enable it to capture prey, and it has an ink sac that releases ink to help it escape from predators. 3. Drawing Conclusions Often in biological systems, structure is said to be related to function. Describe the structure of the gills and explain how they are specialized for their function. The gills are feathery in appearance. The large amount of surface area speeds up gas exchange (taking up oxygen and releasing carbon dioxide) that occurs there. 4. Drawing Conclusions Based upon your observations, what structures of a squid are specialized for movement? Students may discuss its streamlined body design, which produces little resistance in moving through water; the absence of an external shell, making the squid lighter; and the muscular mantle, which pumps jets of water through an excurrent siphon. Biology Laboratory Manual A/Chapter

6 5. Inferring How do the external and internal anatomy of a squid reflect its life as a predator? Students may discuss the arms, circle of tentacles with suckers, jaws, advanced nervous system, and eyes. 6. Evaluating What part of this investigation did you find most challenging? Explain. Students responses will vary. Students may have difficulty locating specific organs. Students may also note that it is difficult to cut the squid without damaging internal organs. Going Further Examine the internal organs of a clam. See how many you can identify. Refer to the diagram in Section 27 4 of your textbook. Obtain clams from a supermarket or seafood store. Ask a salesperson to open the clams for you. Refrigerate the clams and have students perform the dissection within 24 hours. 206 Biology Laboratory Manual A/Chapter 27

Name Date: Block: Honors Marine Biology Mr. Conlan - Squid Dissection Lab Objective: Can you identify a squid s structures and their functions?

Name Date: Block: Honors Marine Biology Mr. Conlan - Squid Dissection Lab Objective: Can you identify a squid s structures and their functions? Helpful Hints Everyone must wear safety goggles during the