Introduction to 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. Is this the rudimentary beginning of the more elaborate coelom found in higher animals? Or did ancestral molluscs abandon a more active life style for a sedentary existence (like the clam), no longer needing a fully developed coelom? Primitive molluscs also show a rudimentary type of segmentation, an important feature of the annelid worms. Are molluscs descended from annelids, or is it the other way around? The evolution of this group remains a source of great controversy.
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.
Within the mantle cavity hang the gills, 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 use the mantle cavity as a rudimentary lung. Squid and octopi use the mantle and mantle cavity as an escape mechanism. 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 the octopus 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. This is the same the way oil circulates in your car. The oil pump collects the oil as it drips into the oil pan, then carries it back to the top of the engine and pumps it out to run down over the motor.
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 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 we will focus on the four most familiar classes of living molluscs.
Molluscs are protostomes, one of the two main evolutionary pathways taken by the eucoelomate 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 refereed to as schizocoels. Contrast this with the deuterostome animals (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.
Class Polyplacophora - chitons; 800 sp. (fr. Gr. poly = many, plax = plate)
Chitons are common in rocky tidal pools. They are believed to retain many characteristics of their remote molluscan ancestors. Chitons have a soft bilaterally symmetric body with a simple tube in a tube body plan, protected by a shell of eight overlapping plates. The body is dorsoventrally flattened, much like their flatworm ancestors. Chitons use their radula to scrape up algae and small animals on rocks and other hard surfaces. When threatened, the chiton creates a vacuum under the shell, almost becoming part of the rock.
Take the body of a chiton. Place your fingers on each side and squeeze it together so the two sides meet. Let it fall over on its side - congratulations, you've made a clam! Unlike chitons, bivalves are laterally flattened. The body is enclosed between two valves (shells), which are opened by a hinge ligament. This wedge-shaped body plan is an adaptation for burrowing in soft sand. The shells are closed and held together by a pair of strong muscles called adductor muscles, located at either end of the shell. It is these adductor muscles that we eat when we eat scallops.
Bivalves form a pair of siphons, sometimes formed by folds of the mantle, which let water in (incurrent siphon) or let water out (excurrent siphon). The flow of water is caused by the beating of the cilia that cover the gills. The water current brings in oxygen, food, and gametes, and carries off waste materials. Bivalves are sedentary filter feeders - they don't move around very much. A coating of mucus on their enlarged gills traps small bits of organic matter as the water passes through the bivalve's shell. The highly mobile trochophore larvae allows these sedentary animals to disperse themselves widely.
Take the chitons body and twist it into a spiral, and you have created a snail. This twisting, or torsion, starts during early development. One side of the larva starts to grow faster than the other, and the snail's body gradually becomes twisted around. Eventually, the visceral mass is rotated a full 180 degrees! The gills in snails are located near the front, a more efficient location for a forward moving animal.
Unlike bivalves, gastropods have a single shell. The twisting of this external shell is actually secondary to the initial twisting of the body mass. Torsion may be an adaptation to improve respiration, or to provide better protection against predators. Snails no longer need to clamp down their shell on a hard surface, as chitons do. They can withdraw into their shell, leaving only a single opening to defend, an opening capped by a shelly plate called an operculum. Torsion also causes a few structural problems. The organs on the right side of the body, such as the gill, nephridium, and the right auricle of the heart, are longer needed, and subsequently disappear. Torsion brings the anus to a rather awkward position directly over the snail's head. The waste stream must pass out the same hole through which the head emerges (bummer!).
At night, we commonly see many snails with no shells, the slugs. Like all snails, slugs secrete a mucus trail from glands in the foot which helps them move efficiently. Slugs actually have a shell, but the shell is reduced to small plates buried within the outer soft tissues of the animal. Terrestrial slugs are not especially attractive, but the marine slugs, the nudibranchs, are vividly colored and patterned. Like some flatworms, nudibranchs can eat cnidarians and place the cnidocytes in their own epidermis. Their vivid colors are probably warning coloration.
Like all animals in motion, snails are highly cephalized. Most have a pair of sensory tentacles on the head, and some have primitive eyes on or near these tentacles. Snails also have a radula, a chitinous tongue which they use to scrape algae or animal tissues off the surfaces they glide over. In whelks, the radula is modified as a little drill, which they can use to drill into the shells of other molluscs to feed on them. In many terrestrial snails, the mantle cavity is enriched with blood vessels, and used as a rudimentary lung. These pulmonate snails can still submerge in water, but must periodically return to the surface in order to breathe.
Stretch the chiton's body vertically, and carve the foot into several tentacles - you've made a cephalopod! Cephalopods are marine predators, feeding on fish, crustaceans, and other molluscs. They are the only molluscs with entirely closed circulatory systems. With the exception of the Nautilus, cephalopod molluscs lack an external shell. The chambered nautilus enlarges its shell as it grows, living only in the largest outer chamber, and using the spiral of smaller inner chambers to store or release air, so that it can easily rise and fall in the water. The ventral foot of the chiton becomes a posterior foot, divided into a highly modified set of tentacles, 10 in the squid, 8 in the octopus. These tentacles are all equipped with large sucker discs, which can be used for defense, as well as for capturing and manipulating prey. The mouth is equipped with poison glands. Cephalopods stun or kill their prey with toxic saliva, carry it to their mouth with their tentacles, and then tear the prey apart with their strong beak and radula. Male cephalopods use a modified tentacle to place sperm into the female's mantle cavity during reproduction.
Giant squid can reach a length of over 60 feet. Giant octopi have been seen in the Sea of Japan with arms up to 45 feet long! Even though they are rarely seen, we know that giant squid exist, because of the titanic battles in the ocean depths between giant squid and the sperm whales that eat them. Sucker scars on the sides of whales can be used to estimate the size of these sea monsters.
The mantle cavity of cephalopods is modified as an escape mechanism. Cephalopods can forcefully expel water from the mantle cavity by quickly closing their mantle and jetting away to a safe place. Cephalopods also squirt dark ink to hide their escape. Octopi also crawl about the ocean floor, using their tentacles. The actively swimming squid uses jets of water from the mantle cavity to propel itself through the sea.
Because of the great length of the squid's body, it uses a single large nerve cell to send the escape message from its brain down to its lower body. This nerve cell is so large that a narrow glass tube can be inserted inside the slender axon to permit experiments and observations on nervous conduction. Study of these giant nerves gave us our first insights into how nerve cells conducted electrical signals. Cephalopods are highly cephalized, with large, complex brains capable of primitive problem solving, and some very advanced sensory organs. The eye of the octopus is very elaborate, with a retina and basic structure very similar to the eyes of vertebrates. It is a marvelous example of convergent evolution.
Phylum Mollusca
Class Bivalvia - mussels, clams, oysters, scallops
Class Gastropoda - snails, slugs, conch, whelk, limpet
Class Cephalopoda - squid, octopus, nautilus
Class Polyplacophora - chitons
Examine slides of the radula of a snail, and think about how this strange tongue operates. Remember you are looking down at it from above.
Observe the live mussels. Can you see the siphons at work? The siphons pump water into and out of the clam for filter feeding and for respiration.
Observe the live snails. Note their cephalization. Can you see the tentacles? Watch how they use their foot to glide along the glass. If you watch carefully, you may see them extrude their radula to scrape algae or small critters from the glass. When disturbed, they retreat into their shell and close the door, a round disc called an operculum.
Observe the slug as it crawls along. Look on the underside of the glass and watch the smooth waves of muscular contraction visible on the base of the foot.
Examine the clam. Find its anterior, posterior, ventral and dorsal sides. Observe the incurrent and excurrent siphons. In this species, the siphons may be visible as spaces along one edge of the mantle. In other species, like the razor clam, these siphons are prominent tubes sticking out of the shell. To open the shell, you must first cut the anterior and posterior adductor muscles. This is the hardest part of the dissection. Hold the shell up and peek in between the two valves. The adductor muscles are visible as two short columns of tissue. Carefully cut through them (they are very tough), trying not to cut yourself in the process. Gently separate the two halves of the shell. As you separate the shell, use your blunt probe to tease the mantle tissue away from the shell. The mantle is the thin brown layer of tissue sticking to the underside of the shell. Be careful not to tear open the coelom, located near the hinge of the shell.
Note how the mantle encloses the visceral mass, and creates a mantle cavity, an open space around the internal organs. In the living clam, this space would be filled with water. Observe the muscular foot. Cut the mantle away from one side to expose the gills. Bivalves use their gills both for filter feeding and for gas exchange. The gills look like tiny sheets of corrugated cardboard. Why is the gill tissue so highly folded? Carefully cut away the gills. Look for the labial palps, which help carry food to the nearby mouth.
Carefully tear open the mesentery or membrane that encloses the coelom or hemocoel. Observe the heart (may be hard to see). The dark material here is dried blood. Note the string-like rectum that passes right through the middle of the heart on its way to the anus. Cut the entire visceral mass in half. Look for small circular openings that are loops of the intestines. The greenish gray tissues you may see are digestive glands, and some of the light brown tissues are gonadal tissue (remember, bivalves are hermaphroditic).
Edible molluscs form the basis of a multi-billion dollar seafood industry.
Mollusc shells are sold as souvenirs, or as jewelry, and oysters produce pearls.
Think of how the various types of molluscs can be shaped by squeezing, stretching or twisting the body of the primitive chiton.
If a coelom is so important, why is it greatly reduced in this phylum?
How do we know that molluscs and earthworms are closely related?
Why are cephalopods the only molluscs that have evolved a closed circulatory system? (Hint: How does their life differ from that of the clam or snail?)
Annelids are eucoelomate, with a simple tube-in-a-tube body plan. The identical segments each contain circular and longtitudinal muscles. The outside of the worm is covered with small stiff bristles called setae. Setae are made of chitin, and each of them is equipped with a tiny retractor muscle. Setae function to anchor the worm in its burrow, and also to help it crawl along. Annelids have a closed circulatory system; the blood is entirely contained in vessels. Annelids have no lungs, although many species have simple gills. Respiration occurs by diffusion through the moist surface of the body. That's why earthworms die so quickly when their epidermis dries up. They literally suffocate! Excretion is handled by tubular nephridia, with one pair of nephridia in each segment. Annelids have a well-developed nervous system, a visible brain consisting of several cerebral ganglia, with smaller ganglia controlling each segment down the length of the nerve cords. While many annelids are tiny, on the order of 1/2 mm, the Australian earthworm stretches an amazing 3 meters!
The annelid worms owe their evolutionary success to segmentation. The coelom becomes divided into a linear series of identical fluid-filled compartments, or segments, that run between the head and the anus. The 8,600 species of annelid worms take their name from the Latin anellus, meaning "little ring". Another word for segmentation is metamerism, from the Greek word meta (=between) and mere (=part, or segment); Segments are literally the "parts between" the anterior and posterior.
Segmentation probably evolved as an adaptation for burrowing. Segments are usually separated by transverse membranes called septae. Coelomic fluid can be shifted from one fluid compartment to the next, allowing a much finer control over the hydrostatic skeleton provided by the coelom. Better muscular control makes annelids excellent swimmers and burrowers.
Segments are formed from the muscles of the body wall and coelomic spaces, which are derived from the mesoderm. Once the mesoderm has segmented, the rest of the animal's "supply systems", such as circulatory, nervous and excretory systems, must adapt accordingly. Some organs, like excretory organs, may be repeated in each segment. But the digestive tract, nerve cords and blood vessels must run continuously through all the segments. Segmentation is a significant evolutionary step, and evolved independently in both annelids and chordates. Segmentation offers many advantages:
1) Segments are identical. If one or more is harmed, the others may be able to survive and repair the damage. Like flatworms, annelids have amazing powers of regeneration.
2) Segments allow for very efficient locomotion over solid surfaces, due to the interplay of the muscles in each segment. The coelomic compartments provide a hydrostatic skeleton. Muscles push against the fluid filled coelom. Waves of muscular contraction ripple down the segments, causing them to expand or contract independently.
3) Worms can burrow through the earth by contracting and expanding the muscles in each segment. By anchoring certain segments to the ground with special bristles, annelids can pull and push themselves through the soil.
4) Segments are free to specialize in various ways, a trend that culminates in the complex bodies of arthropods. The elaborate heads and mouthparts of insects, for example, are formed by fused segments.
The combination of bilateral symmetry, a true coelom, and segmentation created new possibilities for organisms. Further specialization could now take place along the sides of the cephalized and forward-moving animal body. The success of the arthropods and also of the chordates are the end result of this evolutionary breakthrough. Earthworms and polychaetes evolved from a common ancestor, a primitive burrowing marine worm. Leeches probably evolved from earthworms. Like earthworms, leeches lack parapodia and cephalization. They are also hermaphroditic, develop a clitellum and lay eggs in a cocoon.
Class Polychaeta - tubeworms, paddleworms (Nereis), sea mice; 5,400 sp. (fr. Gr. poly = many, khaite=long hair or bristle)
These mostly nocturnal marine worms are the most primitive members of the Phylum Annelida. They are both common and abundant. One study in Tampa Bay found 13,425 polychaetes per square meter of ocean floor! Polychaetes are sometimes called paddleworms, because each segment has a pair of paddle-like appendages called parapodia. These paddles, often covered with setae, are used for swimming, crawling along, and burrowing, and also provide more surface area for respiration. Most polychaetes also have gills to aid in respiration. This extra need for aerated blood probably results from their active life styles. Many polychaetes, however, are filter feeders, living in burrows sunken into the soft sediments of the ocean floor.
They are highly cephalized, with complex sensory organs. Most have eyes, complete with a lens and retina. Burrowing and tube species also have statocysts (balance organs), which use diatom shells, grains of quartz, and sponge spicules as balance weights. Burrowing worms need to know which way is down. Polychaetes have separate sexes, and rely on external fertilization in water. They often congregate in huge mating swarms, which are driven by the phases of the moon. Mating swarms greatly increase the chance of successful external fertilization. This primitive group develops from a trochophore larvae, as do the molluscs, suggesting that these animals are descended from a common ancestor.
Earthworms live in the soil, and also in the bottom debris of all kinds of freshwater habitats. A few species have even reinvaded the ancestral ocean. Like polychaetes, they are common and extremely abundant. One meadow was found to contain over 8,700 oligochaete worms per square meter. Most oligochaetes are detritivores, feeding on dead organic matter, mostly vegetable matter. Freshwater forms eat detritus, algae, and protozoans.
Earthworms are critically important in aerating the soil. They literally eat their way through the earth, digesting small particles of organic matter in the soil. The pharynx draws in food as the worm chews through the soil, and the particles of food are ground up by soil particles in the crop and gizzard. From 22 to 40 metric tons of soil per hectare per year pass through the guts of one or more earthworms, an estimate made by Charles Darwin in his book on earthworms. Darwin was the first person to realize the tremendous importance of earthworms in aerating and churning the soil, and breaking down dead vegetation. Because they burrow through the ground, they have shed many of the features of the more primitive polychaetes. They lack parapodia, and are not highly cephalized. Although they have no eyes, they have many light sensitive organs in some segments. Oligochaetes that live in dryer environments excrete nitrogen wastes as urea, which uses less water to dissolve, in addition to the usual ammonia waste. They have a complex circulatory system, with a row of five muscular blood vessels serving as hearts.
Earthworms can reproduce asexually by transverse fission, much like the flatworms. Earthworms are hermaphroditic, and fertilize one another simultaneously with the help of a special structure called a clitellum. The clitellum is the small bump that forms one of the few external features of the worm. It is really a series of segments swollen by large mucus glands. Mucus secreted by the clitellum helps hold the animals together during mating. A few days after copulation, the fertilized eggs are released into a mucus sac, which slowly sloughs off the end of the worm, and dries into a hardened cocoon, which protects the eggs until they hatch.
Some leeches are predators or scavengers, feeding on worms, snails, and insect larvae. But many species (about 75% of them) suck the blood of mammals and even some crustaceans. They have an anterior and posterior sucker for attaching to the skin of their hosts. They are excellent swimmers, and their suckers also helps attach them to the bottom as they crawl along. Leeches are common in freshwater habitats, but only a few species are marine or terrestrial. One Illinois stream contained 10,000 leeches per square meter!
The coelom is greatly reduced, and not divided into compartments. Because leeches move by swimming or crawling, they have lost these coelomic adaptations for burrowing. The blood meal is stored in special pouches in the digestive tract, so leeches don't need to feed very often. And a good thing, too, because a feeding leech will suck up to five to ten times its own weight in blood! When they attach, leeches secrete a special anticoagulant to keep the host's blood flowing. Because medieval physicians believed that "bad blood" caused diseases, patients were bled with leeches until they often died of anemia. The medicinal leech, Hirudino medicinalis, is enjoying a modern day revival, because its bite is antiseptic, and the anticoagulant that it secretes will dissolve blood clots. Leeches are also used to drain postoperative swelling. Lancing the swelling in order to drain it often leads to infection. The best way to remove a feeding leech is by using the tip of a lit cigarette (or cigarette-like object) or by pouring salt over the leech.
Phylum Annelida
Class Oligochaeta -earthworms (Lumbricus)
Class Polychaeta - tubeworms, paddleworms (Nereis)
Class Hirudinea - leeches
Observe the live earthworms. Watch the way the segments of the earthworm contract and expand as it crawls along. How/why does this body plan allow this type of movement? Notice that as the earthworm contracts the longtitudinal muscles in each segment, the segment gets shorter, and also gets thicker. Remember that water is relatively incompressible. The volume of water stays the same in each segment as it contracts, so the shape of the segment changes, bulging out and getting thinner. When the circular muscles contract, the segments become longer and narrower, pushing the worm forward. Think about how the worm uses its bristles to anchor one section of segments while it contracts or expands other sections.
Identify the clitellum, the large swollen segments near the anterior end of the worm. Glands in the clitellum secrete mucus which holds the worms together during mating. Fertilized eggs are later deposited in a mucus ring, which then dries up and slides off to form a protective cocoon for the developing eggs.
Observe the live leeches. Notice how smoothly and gracefully the leeches swim through the water when they are disturbed (be gentle if you prod them with a blunt probe - poke them too roughly and you will kill them). How does their movement compare with that of the earthworm?
Compare the external anatomy of the three annelid classes on display. How do the features of each class relate to its habitat and ecological niche? Why isn't the earthworm highly cephalized like the polychaetes? Why does it lack parapodia? (Hint: Don't be fooled by sea mice on a lab exam - We sometimes use them as a trick question. Their long hairs are actually thick long setae [the worm's hair, that is, not the lab instructor's] Look on the underside of the worm and you will see the clearly segmented body).
Observe Nereis under the dissecting scope. Identify the parapodia. The head is comprised of two sections, the prostomium and the peristomium. You can see a small "bump" on top, called the prostomium, which has a pair of small tentacles, four tiny eyes, and a pair of sensory palps. Palps are sensory organs for taste or touch. The first segment of the worm, the peristomium, has four pairs of sensory tentacles. The mouth is on the ventral side of the first segment, and the worm sticks its pharynx out through its mouth to feed. The pharynx has a pair of chitinous jaws, little pincers used to grab prey.
Examine cs slides of the earthworm Lumbricus Look for the dorsal blood vessel, the ventral nerve cord, the circular and longtitudinal muscles, the coelom, the peritoneum (the thin mesodermal lining of the coelom), and the intestine. Your cross section might also show one or more setae, complete with the muscles used to evert or retract them.
Examine the external surface of the worm. Although it appears to be smooth, if you run your finger over its surface (oh, go ahead, I know it's gross!), you may feel the numerous small bristles or setae that project from the epidermis. The first trick is to figure out which way is up. Find the clitellum, the large prominent and smooth ring near the anterior end of the worm. Now you know which end is the mouth (closest to the clitellum) and which end is the anus. Notice that that the peristomium (first segment) and the short blunt prostomium overhanging the mouth.
Finding the dorsal or ventral side is a little harder. Count down about 15 segments from the head of the worm, and look for two small pores. These are the male pores, through which sperm emerge. The two long seminal grooves convey the sperm to the female pores on its mating partner. The female pores are smaller and much harder to see. You are now looking at the ventral surface (belly side) of the worm. Turn the worm over, so that the ventral side is on the bottom, and pin the worm lengthwise in the pan, putting a pin through the third segment and through any one of the posterior segments.
Take your scalpel and make a small cut a few segments towards the posterior side of the clitellum. Using scissors or scalpel, cut all the way to the head. Try to keep your cut just to one side of the mid-dorsal line, so that the dorsal blood vessel will remain intact. If you use your scalpel, keep the sharp tip pointed up, rather than down, and apply upward pressure to gently tear the epidermis, rather than sawing downward into the worm's delicate anatomy. Pin the worm out into the pan as you go, so that the length of the internal organs is exposed. From the clitellum on down, the worm is mostly intestinal tract, so focus on the anterior end in your dissection.
Notice the internal structure of the coelom, with its obvious cross-walls or septae. The tiny paired nephridia are visible in each segment, though they are surprisingly hard to find. Notice how the nerve cords, blood vessels etc. pass through each segment. Locate and identify the following structures: pharnyx, crop, gizzard, intestine, hearts, dorsal blood vessel, testes, ovary. Try to find the tiny brain (this is a real challenge). You can recognize the brain by the pair of small nerve cords extending at 45o angles from both sides.
Earthworms are critical in soil aeration and soil fertility.
Leeches have been used as a medical anti-coagulant for hundreds of years.
Worm ranching is a major industry, with sales to both gardeners and fishermen.
How does the body of the leech reflect its parasitic strategy?
Why do we believe that molluscs and annelids are closely related?
How/why is segmentation a useful adaptation for a burrowing animal?
How does segmentation open up a new pathway for evolutionary success?
Molluscs
Lots of links at:
Start crawling at: