Phylum Mollusca – the Molluscs
Molluscs are amazingly diverse, with 110,000 named species, second only to the arthropods among all phyla of animals. Molluscs include such familiar creatures as clams, oysters, snails, and octopi. They share a distant common ancestor with the annelid worms, an evolutionary heritage suggested by their larval form, called a trochophore larva, found in all molluscs and in certain marine annelids called polychaete worms. It's hard to imagine that a clam could be a close cousin to the earthworm, because most familiar molluscs have a highly modified body type. The ancestral mollusc probably resembled a chiton, a flattened worm like animal protected by a dorsal shell.
Both molluscs and annelids probably evolved from free-living flatworms. Both flatworms and molluscs are triploblastic, bilaterally symmetric, and cephalized. But molluscs have developed a true coelom, an internal body cavity enclosed by mesodermal membranes. The coelom in molluscs, however, is strangely reduced to a small space around the heart, sometimes called a hemocoel. Primitive molluscs also show a rudimentary type of segmentation, an important feature of the annelid worms.
Molluscs are mostly aquatic, and are named from the Latin molluscus, meaning "soft". Their soft bodies are enclosed in a hard shell made of calcium carbonate (CaCO3), which functions as an exoskeleton. This shell is secreted by a thin sheet of tissue called the mantle, which encloses the internal organs like a glove. The mantle creates a small empty space called a mantle cavity, which is modified for different functions in different groups of molluscs. Squid and octopi use the mantle and mantle cavity as an escape mechanism.
Within the mantle cavity hang the gills or ctenidia, highly complex and greatly folded sheets of tissue. Gills are used to exchange oxygen and carbon dioxide in respiration. Cilia on the gills create a flow of oxygenated water through the mantle cavity, carrying off carbon dioxide and nitrogenous wastes. Bivalves like oysters and clams, have greatly enlarged gills that they use for both respiration and filter feeding. Land snails do not have gills but use the highly vascularized mantle cavity as a rudimentary lung.
Molluscs feed by means of a peculiar rasping tongue called a radula, a tiny little chainsaw-like structure made of chitin. Chitin is basically a cellulose polymer with an added nitrogenous group, and is widely found as a structural element in nature, for example in the cell walls of fungi and the exoskeleton of arthropods.
Within the body of the mollusc, the internal organs are embedded in a solid mass of tissue called the visceral mass. Protruding from the bottom of the animal is a muscular foot, used by the bivalve to dig in the sand, used by the snail to creep along rocks, and (divided into tentacles) used by molluscs like the octopus and squid to catch prey.
Molluscs have an open circulatory system - only part of the blood flow is contained in vessels. Molluscs have a three-chambered heart. Two auricles collect oxygenated blood from the gills, and the ventricle forces it from the aorta into small vessels which finally bathe the tissues directly. The blood pools in small chambers or sinuses, where it is collected and carried back to the gills. The oxygenated blood is then returned to the auricles.
Molluscs also have a well-developed excretory system, using tubular nephridia organized as kidneys, that collect liquid wastes from the coelom and dump them in the mantle cavity, where they are pumped out of the shell.
Sexes are separate (dioecious), except for bivalves and some snails, which are hermaphroditic.
The molluscan nervous system consists of a pair of ganglia and nerve cords, with statocysts(balance organs) and eyes as major sense organs.
Molluscs are protostomes, one of the two main evolutionary pathways taken by the coelomate animals. Remember that protostome means "first mouth". The small opening into the embryonic ball of cells that appears early in animal development is called the blastopore. In protostomes, the blastopore becomes the mouth, and the anus appears later on the opposite side. Protostomes like molluscs, annelids, and arthropods develop by spiral cleavage, and their embryonic cells are determinate, the fate of the embryonic cells is fixed very early on in development. The protostome coelom forms from a split within the mesoderm tissue, so they are sometimes referred to as schizocoels. Contrast this with the deuterostomeanimals (starfish, chordates), in which the blastopore becomes the anus and the mouth opens elsewhere. Deuterostomes have radial cleavage and their embryonic cells are indeterminate. Deuterostomes are enterocoels, their coelom forms as out-pockets along the gut.
Molluscs include the largest invertebrates (giant squid) and the smartest invertebrates (the octopus). There are eight or more classes of molluscs, and many fossil classes, but the three most familiar classes of living molluscs are the Gastropods, the Bivalves and the Cephalopods.
In this section of the lab, you will dissect a member of Class Bivalvia – the clam
Lab Materials
Model – snail anatomy
Live snails
Preserved samples – gastropods
-bivalves
-chitons
-barnacle
-mussel
-cephalopods
Clam Dissection
Clam Taxonomy: Kingdom - Animalia Phylum - Mollusca Class - Bivalvia or Pelecypoda
Materials Dissecting pan, dissecting kit, screwdriver, lab apron, plastic gloves, safety glasses, preserved clam
Procedure
- Place a clam in a dissecting tray and identify the anterior and posterior ends of the clam as well as the dorsal, ventral, & lateral surfaces.
2. Locate the umbo, the bump at the anterior end of the valve. This is the oldest part of the clam shell. Find the hinge ligament, which hinges the valves together and observe the growth rings.
3. Turn the clam with its dorsal side down and insert a screwdriver or dissection probe between the ventral edges of the valves. Carefully work the tip of the screwdriver or probe between the valves so you do not jab your hand. . Turn the screwdriver so that the valves are about a centimeter apart. Leave the tip of the probe between the valves and place the clam in the pan with the left valve up.
4. Locate the adductor muscles. With your blade pointing toward the dorsal edge, slide your scalpel between the upper valve & the top tissue layer. Cut through the anterior adductor muscle, cutting as close to the shell as possible.
5. Repeat step 4 in cutting the posterior adductor muscle. Bend the left valve back so it lies flat in the tray.
6. Run your fingers along the outside and the inside of the left valve and compare the texture of the two surfaces.
7. Examine the inner dorsal edges of both valves near the umbo and locate the toothlike projections. Close the valves & notice how the toothlike projections interlock.
8. Locate the muscle "scars" on the inner surface of the left valve. The adductor muscles were attached here to hold the clam closed.
9. Identify the mantle, the tissue that lines both valves & covers the soft body of the clam. Find the mantle cavity, the space inside the mantle.
10. Locate two openings on the posterior end of the clam. The more ventral opening is the incurrent siphon that carries water into the clam and the more dorsal opening is the excurrent siphon where wastes & water leave.
11. With scissors, carefully cut away the half of the mantle that lined the left valve. After removing this partof the mantle, you can see the gills, respiratory structures.
12. Observe the muscular foot of the clam, which is ventral to the gills. Note the hatchet shape of the foot used to burrow into mud or sand.
13. Locate the palps, flaplike structures that surround & guide food into the clam's mouth. The palps are anterior to the gills & ventral to the anterior adductor muscle. Beneath the palps, find the mouth.
14. With scissors, cut off the ventral portion of the foot as indicated in the figure below. Use the scalpel to carefully cut the muscle at the top of the foot into right and left halves.
15. Carefully peel away the muscle layer to view the internal organs.
16. Locate the spongy, yellowish reproductive organs.
17. Ventral to the umbo, find the digestive gland, a greenish structure that surrounds the stomach.
18. Locate the long, coiled intestine extending from the stomach.
19. Follow the intestine through the calm. Find the area near the dorsal surfacethat the intestine passes through called the pericardial area. Find the clam's heart in this area.
20. Continue following the intestine toward the posterior end of the clam. Find the anus just behind the posterior adductor muscle.
Lab Questions:
1. What is the oldest part of a clam's shell called and how can it be located?
2. What do the rings on the clam's shell indicate?
3. Name the clam's siphons.
4. What holds the two shells together?
5. What muscles open & close the clam?
6. Describe the inside lining of the shell.
7. What is the function of the tooth-like projections at the dorsal edge of the clam's valves?
8. Where is the mantle located in the clam? What is its function?
9. Describe the clam's foot.
10. What is the mantle cavity?
11. How do clams breathe?
12. What helps direct water over the gills?
13. Where are the palps found and what is their function?
14. Describe the movement of food from the current siphon through the digestive system of the clam.
15. Where is the clam's heart located?
16. What are the parts of the clam's nervous system?
17. Why are clam's referred to as "filter feeders"?
18. Based on your dissection. Label the figure below.
Phylum Arthropoda – the Arthropods
There are over 800,000 named species in the Phylum Arthropoda, named from the Greek arthros (= jointed) and poda (= foot), including the familiar arachnids, crustaceans, and insects, together with a host of less familiar critters, like centipedes, millipedes and sea spiders. There are about 1018 (10 billion billion) arthropods alive at any one time.
All arthropods have jointed appendages. There are several evolutionary innovations that were the key to the stunning success of this diverse group and jointed appendages are one of them. Others include segmentation and the presence of an outer protective exoskeleton.
Arthropods do everything with their jointed appendages or legs. They walk, they swim, they creep and crawl, they use legs to sense with (the antennae), to bite and sting with, and even to chew with. That's one reason arthropods look so alien when we see them up close. They chew sideways, and it's all done with legs!
Their bodies are protected by a tough cuticle made of proteins and chitin, a polysaccharide with added nitrogen groups. A cuticle is a tough outer layer of non-living organic material. The cuticle of arthropods acts as an exoskeleton. Terrestrial arthropods remain small primarily because of the limitation imposed by their exoskeleton. A large insect would need such a thick exoskeleton to withstand its strong muscles that the weight of the cuticle would be too great for the animal to carry around.
For small animals, having your skeleton on the outside is as logical as having it on the inside. But it poses a fundamental problem for arthropods. They must shed their exoskeleton, or molt, in order to grow. This is known asecdysis and it places the arthropods into the Clade Ecdysozoans along with the Nematode roundworms. During molting, the exoskeleton must split open. The animal emerges and swells to a larger size until the newer, larger exoskeleton is hardened. While the animal molts, it is especially vulnerable - just ask a plate of soft-shelled crabs!
Arthropods have segmented bodies, like the annelid worms. These segments, or metamers, have become specialized, however, with one pair of jointed appendages added to each segment. Arthropod segments have also fused together into functional units called tagma. This fusion usually results in an arthropod body that consists of three major sections, a head, thorax, and abdomen. Sometimes the head and thorax are fused together into a cephalothorax. Each of these body sections still bear the appendages that went with it, though these appendages are often highly modified.
Arthropods are very highly cephalized, often with intricate mouthparts and elaborate sensory organs, including statocysts, antennae, simple eyes and compound eyes located at a well-defined head. Sensitive hairs or bristles known assensilla on the surface of the body can detect touch, vibration, water currents, or chemicals. Their nervous systems are highly developed, with chains of ganglia serving various parts of the body, and three fused pairs of cerebral ganglia forming a brain.
Aquatic arthropods respire with gills. Terrestrial forms rely on diffusion through tiny tubes called trachea. Trachea are cuticle-lined air ducts that branch throughout the body, and open in tiny holes called spiracles, located along the abdomen. Insects can open and close these spiracles, to conserve water that would otherwise be lost to evaporation from the open tubes. Their reliance on diffusion for respiration is one of the reasons that insects are small.
Terrestrial arthropods, including the insects, excrete by means of Malphigian tubules, projections of the digestive tract that help conserve water. They excrete nitrogen as uric acid, as do birds. Their waste is nearly dry, a superb adaptation to life on land. Other arthropods use Coxal glands to rid themselves of their nitrogenous wastes. The green gland of the crustacean is a kind of coxal gland.
Arthropods have an open circulatory system, and separate sexes. Fertilization is usually internal, another adaptation for terrestrial life. Although crustaceans may undergo external fertilization as the females fertilize their eggs as they are laying them. Males and females often show pronounced sexual dimorphism.
Arthropods are coelomate protostomes, dominating the protostome branch of the animal tree, just as vertebrates dominate the deuterostome branch. Arthropods share a common ancestor with polychaete worms, and may even be a direct descendant of polychaetes. But unlike other coelomate invertebrates, the arthropod coelom is greatly reduced in the adult animal. However, they still have one. The millipede is likely to represent what the typical ancestral arthropod used to look like.
While the taxonomy of Phylum Arthropoda can be large and complex, some of the major groups you are expected to know are Subphylum Chelicerata, Subphylum Crustacea and Subphylum Hexapoda.
Lab Materials
Preserved arthropod dry mounts – insect dry mounts
-horsehoe crab dry mount
Models – grasshopper
-crayfish
Dissected models – lobster
-crayfish
-grasshopper
Dissection – crayfish
-grasshopper
Demo slides – drosophila life cycle
Dissection of the Crayfish
Prelab Questions:
1. Crayfish belong to the Kingdom ______, the Phylum ______,
and the subphylum ______
2. List three characteristics that all arthropods share.
3. Name two other animals in the same phylum as a crayfish (related).
The Head:
Place the crayfish ventral side up so the mouthparts can be observed.
4. Locate the 1st, 2nd, and 3rd maxillipeds. These appendages are used for manipulating food. (The 3rd maxilliped is the largest and topmost one, the 2nd is underneath, and the 1st is underneath the 2nd)
5. Locate the mandible which lies underneath the maxillipeds. This structure should be hard and difficult to move. The mandible of arthropods opens differently than the jaws of humans. Describe the difference. ______
6. Locate the two large antennae and the smaller antennules that branch from the base. The antennae are sense organs (touch, taste, equilibrium)
7. Locate the eyes, which extend from two stalks called pedicles.
The Body
8. The body of the crayfish consists of a fused head and thorax: the cephalothorax. The cephalothorax is covered by a thick armor called a carapace. Extending from the carapace is a pointy structure called the rostrum. Locate the cephalothorax and rostrum.
9. The abdomen of the crayfish is segmented and flexible. Bend the abdomen back and forth and observe how each segment moves.
10. Count the number of segments on the abdomen. Hint: bending it will show you were the segments are separated. How many segments are on your crayfish? ______
Compare this number to other crayfish, are they all the same? ______
The Appendages
11. Locate the chelipeds (the claws). Gently manipulate the cheliped to determine the direction in which the appendage can bend. How many joints are there on a single cheliped? ______
12. Cut the end of the cheliped off and use the forceps to find the connective tissue inside. Pulling on this tissue will make the claw open and close. Try it!
13. Behind the cheliped are four pairs of walking legs or periopods. How many joints are on each leg? _____
14. Locate the swimmerets (appendages attached to each segment of the abdomen). Are the swimmerets jointed? ______How many pairs of swimmerets are there? ______
15. The last segment of the abdomen (the 7th segment) is called the telson, and it is specialized for swimming. Locate the two uropods extend from either side of the telson.