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Labs:
27-1 MollusksSection Objectives
Members of the phylum Mollusca are known as mollusks. Mollusks evolved in the sea more than 600 million years ago and have experienced a long and successful adaptive radiation. Today there are more than 100,000 mollusk species, which are divided into seven classes. Mollusks live everywhere-from deep ocean trenches to mountain brooks to the tops of trees. They range in size from snails as small as a grain of sand to giant squids that may grow more than 20 meters long. And as you can see in Figure 27-1, mollusks come in a wide range of forms and colors.
Why are animals that look and act so differently grouped in the same phylum? One reason mollusks are classified together is that they share similar developmental patterns. (As you may recall from Chapter 15, many animals are classified on the basis of shared features during early development.). Most mollusks have a special kind of larva called a trochophore, (TROH,koh-for). See Figure 27-2 on page 586. Trochophore larvae swim in open water and feed on tiny floating plants.
Trochophore larvae are also seen in segmented worms, which belong to the phylum Annelida. Biologists believe that this indicates that mollusks and annelids evolved from a common ancestor that existed during the Precambrian Period (more than 580 million years ago) and had a trochophore larva. Because the phyla Mollusca and Annelida are closely related to each other, we shall discuss them both in this chapter.
Another reason mollusks are placed in a single phylum is that their different forms are the results of variations on the same basic body plan. Mollusks are defined as softbodied animals that have an internal or external shell. Their name is derived from the Latin word molluscus, meaning soft. Although a few present-day mollusks lack shells, they are thought to have evolved from shelled ancestors.
As you can see in Figure 27-3, the body plan of almost all mollusks consists of four basic parts: foot, mantle, shell, and visceral mass. The soft muscular foot usually contains the mouth and other structures associated with feeding. The foot takes many different shapes in mollusks: Flat surfaces adapted to crawling, spade-shaped structures for burrowing, and tentacles for capturing prey are a few examples. The mantle is a thin, delicate tissue layer that covers most of a mollusk's body, much like a cloak. The shell, which is found in almost all mollusks, is made by glands in the mantle that secrete calcium carbonate (CaCO,). Just beneath the mantle in most mollusks is the visceral mass, which contains the internal organs.
These basic body parts have taken on different forms as mollusks evolved adaptations to different habitats. The type of foot and the kind of shell that mollusks have are used to group them into classes. Later in this chapter we shall examine the three major classes of mollusks.
FEEDING
Mollusks have evolved many types of feeding mechanisms and feed on many kinds of food. In fact it would be simpler to list the few things these animals do not eat than it would be to describe everything they can feed on! Every mode of feeding is seen in the phylum Mollusca. Most mollusks are herbivores, carnivores, or filter feeders, but a few species are detritus feeders and others are parasites.Many mollusks-snails and slugs, for example-feed with a tongue-shaped structure called a radula (RAJ-oo-lah). The radula is a layer of flexible skin that carries hundreds of tiny teeth, which make it look and feel like sandpaper. Inside the radula is a stiff supporting rod of cartilage. When the mollusk feeds, it places the tip of the radula on the food and pulls the sandpapery skin back and forth over the cartilage. Mollusks that are herbivores use their radula to scrape algae off rocks and twigs in the water or to eat the buds, roots, and flowers of land plants.&4ollusks that are carnivores use their radula to drill through the shells of other animals. Once they have made a hole through the shell, these carnivores extend their mouth and radula into the shell and tear up and swallow the prey's soft tissue. In the carnivorous snails called cone shells, the tiny rasping teeth of the radula have evolved into long hollow darts that are attached to poison glands. A cone shell uses these darts to stab and poison prey such as small fish.
Although they may have a radula, carnivorous mollusks such as octopi and certain sea slugs typically use sharp jaws to eat their prey. Like cone shells, some octopi produce poison to subdue their prey. Although cone shells and octopi generally feed on fish and other small animals, the poisons produced by some species are strong enough to hurt or even kill humans.
Mollusks such as clams, oysters, and scallops are filter feeders. They use their featherly gills to sift food from the water. As these animals pass water over their gills, phytoplankton (tiny photosynthetic organisms) in the water become trapped in a layer of sticky mucus. Cilia on the gills move the mixture of mucus and food into the mouth.
RESPIRATION
Gills serve as organs of respiration as well as filters for food. In fact, in most species gills are used only for breathing. Aquatic mollusks such as snails, clams, and octopi breathe by using gills located inside their mantle cavities. But land snails and slugs breathe by using a specially adapted mantle cavity that is lined with many blood vessels. The mantle is wrinkled or folded to fit a larger surface within the limited space of the cavity. This surface is constantly kept moist so that oxygen can enter the cells. Because the mantle loses water in dry air, most land snails and slugs must live in moist places. They prefer to move around at night, during rainstorms, and at other times when the air is humid.INTERNAL TRANSPORT
Oxygen that is taken in by the respiratory system and nutrients that are the products of digestion are carried by the blood to all parts of a mollusk's body. The blood is pumped by a simple heart through what is called an open circulatory system. "Open" does not mean that blood can spill to the outside of the animal! It means that blood does not always travel inside blood vessels . Instead, blood works its way through body tissues in open spaces called sinuses. These sinuses lead to vessels that pass first through the gills, where oxygen and carbon dioxide are exchanged, and then back to the heart. Open circulatory systems work well for slow-moving or sessile (attached to one spot) mollusks like snails and clams. But the flow of blood through sinuses is not efficient enough for fast-moving octopi and squids. Those animals have closed circulatory systems, in which blood always moves inside blood vessels.EXCRETION
Like other animals, mollusks must eliminate waste products. Undigested food becomes solid waste that leaves through the anus in the form of feces. Cellular metabolism produces nitrogen-containing waste in the form of ammonia. Because ammonia is poisonous, it must be removed from body fluids. Mollusks get rid of ammonia by using simple tube shaped organs called nephridia (neh-FRIHD-ee-ah; singular: nephridium). Nephridia remove ammonia from the blood and release it to the outside.RESPONSE
Mollusks vary greatly in the complexities of their nervous systems and their abilities to respond to environmental conditions. Clams and other two-shelled mollusks, many of which lead basically inactive lives burrowing in mud or sand, have simple nervous systems. They have several small ganglia near the mouth, a few nerve cords, and simple sense organs such as chemical and touch receptors, statocysts (simple organs for balance), and ocelli (eyespots). Octopi and other tentacled mollusks, on the other hand, are active intelligent predators that have the most highly developed nervous system of all members of their phylum. Because of their well-developed brain, these animals can remember things for long periods of time, and they may even be more intelligent then some vertebrates. The numerous complex sense organs these mollusks possess help them distinguish shapes by sight and texture by touch. Octopi can be trained to perform different tasks in order to obtain a reward or avoid punishment. Because of these abilities octopi are often studied by psychologists interested in the way animals learn.REPRODUCTION
As with almost all other essential functions, mollusks accomplish the function of reproduction in different ways. In most mollusks, the sexes are separate and fertilization is external. These mollusks-which include many snails, almost all two-shelled mollusks, and most of the species in the four minor classes of mollusks-release eggs and sperm into open water in enormous numbers. Eggs and sperm find each other by chance, and free-swimming larvae develop from the resulting fertilized eggs. In tentacled mollusks and certain snails, fertilization takes place inside the body of the female. Fertilization is also internal in some hermaphrodites (organisms that have both male and female reproductive organs). For example, many hermaphroditic snails get together in pairs and fertilize each other's eggs at the same time. Some other hermaphroditic mollusks, such as certain oysters, switch from one sex to the other. Sometimes they are male (and thus produce sperm) and sometimes female (and thus produce eggs)!Snails, slugs, and Their Relatives
Members of the class Gastropoda are called gastropods (GAS-troh-pahdz). The name gastropod literally means-stomach foot. This name is quite appropriate because most gastropods move by means of a broad, muscular foot located on their ventral (stomach) side. Gastropods include the familiar pond snails and land slugs as well as more exotic mollusks such as abalone's, sea butterflies, sea hares , and nudibranchs.
Many gastropods have a one-piece shell that protects their soft bodies. This shell may be simple and shield like, as in limpets, or coiled, as in snails. When threatened, many snails can pull up completely into their coiled shells. Some snails are additionally protected by a hard disk on their foot that forms a solid "door" at the Mouth of their shell when they withdraw.
Some gastropods have small shells or, as is the case with slugs, lack shells completely. This would seem to make them easy prey for hungry predators. However, these gastropods are not entirely helpless. Most land slugs are protected by their behavior-they spend the daylight hours hiding under rocks and logs, hidden from birds and other animals that might eat them. Some sea hares have a special ink-producing gland that they use when threatened to squirt ink into the surrounding water. This confuses predators and allows the sea hare to escape under its "smoke screen." Some gastropods, such as sea butterflies, escape predators by swimming rapidly. Many nudibranchs., or sea slugs, have chemicals in their bodies that taste bad or are poisonous. When a predator nibbles on one of these bad-tasting morsels, it gets sick. In addition, some nudibranchs. use nematocysts from the cnidarians they eat to sting predators. The bad-tasting, poisonous, stinging, or otherwise booby trapped nudibranchs. are usually brightly colored. The bright colors warn predators to stay away. It a predator ignores the warning colors and eats a nudibranch, the consequences usually guarantee that the predator will remember the bright colors of the nudibranch and avoid it in the future! (While this does not help the first nudibranch, it does protect others of its kind.) Thus shell-less gastropods do have means of protection.
Members of the class_EivaLvia4bi- means two; valve means shell) have two shells that are hinged together at the back and held together by one or two powerful muscles. Common bivalves include clams, oysters, and scallops. Bivalves may be tiny or as large as the giant clam Tridacna, which has been known to grow as large as 1.9 meters in length.
Although bivalve larvae are free-swimming, they soon settle down to a relatively quiet life on the bottom of a body of water. Some bivalves, such as clams, burrow in mud or sand. Others, such as mussels, secrete sticky threads to attach themselves to rocks. Although most adult bivalves are sessile, some, such as scallops, can move around rapidly by flapping their shells when threatened.
The mantles of bivalves, like those of most other mollusks, contain glands that manufacture the shells. These mantle glands also keep the shell's inside surfaces smooth and comfortable by secreting layers of mother-of-pearl. If a foreign object-a sand grain or small pebble, for example-gets caught between mantle and shell, the mantle glands cover it with this secretion. After many years these objects become completely coated and are called pearls.
Cephalopods
(SEHF-uh-loh-pahdz) - members of the class Cephalopoda - are among the most active and interesting mollusks. This class includes octopi, squids, cuttlefish, and nautiluses. Cephalopoda means head-foot). This name refers to the fact that cephalopod's head is attached to its foot, which is divided into tentacles, or arms. Cephalopods range in size from tiny cuttlefish less than 2 centimeters long to giant squids, which are thought to grow to more than 20 meters long.Most cephalopods have eight flexible tentacles equipped with a number of round sucking-disks that are used to grab and hold fish and other prey. In addition to these tentacles, cuttlefish and squids also have two long, slender arms with suckers on the end. Nautiluses have many more tentacles(38 to 90) than other cephalopods. Their tentacles lack suckers but are made sticky by a mucus like covering.
Although fossil evidence indicates that their ancestors had large external cone-shaped or coiled shells, most modern cephalopods have small internal shells or no shells at all. The only present-day cephalopods with shells are the few species of nautiluses. These cephalopods look much like fossil cephalopods from the beginning of the Cambrian Period, more than 500 million years ago. Cuttlefish have small shells that are found inside their bodies. The shells of some cuttlefish are thin and coiled, whereas others (which serve as the cuttlebone on which pet birds condition their beaks) are flat, platelike, and do not resemble shells at all. In both nautiluses and cuttlefish, gases in the shell help the cephalopod remain upright and allow it to float in the water. A squid's internal shell has evolved into a thin, flexible supporting rod known as a pen. Octopi have lost their shells completely.
Although most cephalopods lack a protective shell, they do have other means of protection. Most cephalopods can move quickly, either by swimming or crawling. They can also move by using a form of jet propulsion. The cephalopods draw water into their mantle cavities and then force that water out through the tube like siphon. By pointing the siphon in different directions, they can shoot out a jet of water that propels them backward, away from danger. In addition, many cephalopods can release large amounts of dark-colored, foul-tasting ink when they are frightened. After squirting out a large cloud of ink, they make a hasty retreat. Perhaps most fascinating of all, octopi can quickly change color to match the colors of their surroundings. The match is often close enough that the octopi are nearly invisible.
How Mollusks Fit into the World
Mollusks play many different roles in living systems. For example, they feed on plants, prey on animals, and "clean up" their surroundings by eating detritus. Some of them are hosts to symbiotic algae or to parasites; others are themselves parasites. In addition, mollusks are an important source of food for many organisms, including humans.
Modern-day scientists have found some new uses for mollusks. Because filter-feeding bivalves concentrate dangerous pollutants and microorganisms in their tissues, careful checks of bivalves can warn biologists and public health officials of health problems long before scientists can detect these dangers in the open water. Besides acting as environmental monitors, mollusks also serve as subjects in biological research. Some current investigations are based on the observation that snails and other mollusks never seem to develop any form of cancer. If scientists can determine what protects the cells of these animals from cancer, they will gain valuable insights into how to fight cancer in humans.
Although mollusks are beneficial in many ways, they do have some negative relationships with humans. For example, land slugs and snails are plant eaters that can do much damage to gardens and crops. The bivalves called shipworms, which use their shells to drill their way slowly through pieces of wood in the water, are sometimes described as the termites of the sea. They settle on wood in large numbers and can reduce a good-sized log to a pile of wet sawdust over the course of a few years. Shipworms cause millions of dollars worth of damage to wooden boats and docks every year. Another problem with mollusks is associated with their use as food. Clams and oysters, which are among the few marine animals that are farmed in the sea, are filter feeders and thus gather and concentrate particles floating in the water-including bacteria, viruses, and the toxic protists that cause red tides. Eating bivalves that contain high concentrations of pathogens (things that cause disease), toxins, or pollutants can result in sickness or even death.
If you have ever dug in a garden, you have probably met the most common terrestrial, or land-dwelling segmented worm. But this species is only one of approximately 9000 species of segmented worms that live in soil, in fresh water, and in the sea. Segmented worms, or annelids, live everywhere in the world except Antarctica and Madagascar (a large island located off the southeastern coast of Africa). But because most segmented worms live in the sea, and many others spend their lives underground, only a few species are familiar to us.
Members of the phylum Annelida are known as annelids, or segmented worms. An annelid is a round, worm like animal that has a long, segmented body. The name Annelida is derived from the Latin word annellus, which means little ring, and refers to the ring like appearance of the body segments.
Annelids range in size from tiny aquatic worms less than half a millimeter long to giant earthworms more than 3 meters long. Although they also vary greatly in color, patterning, number of bristles, and other superficial features, most annelids are quite worm like in appearance.
The many segments of an annelid's body are separated by internal walls called septa (singular: septum). Most of the body segments are virtually identical to one another. However, some segments are modified to perform special functions. For example, the first few segments may carry one or more pairs of eyes, several pairs of antennae, and other sense organs.
FEEDING
The digestive tract, or gut, is a long tube within the body cavity of the worm that extends from the mouth to the anus (in the tip of the "tail"). Food enters through the mouth and travels through the gut, where it is digested. Like mollusks, annelids have evolved structures and behaviors that allow them to use a wide variety of foods.One feeding organ that has evolved many different forms in different groups of annelids is the pharynx, or the muscular front end of the digestive tube. Many annelids can extend the pharynx through the mouth. In carnivorous annelids, this type of pharynx usually has two or more sharp jaws attached to it. When a suitable animal approaches, the worm lunges forward, rapidly extends the pharynx, and grabs the prey with its jaws. Jaws are also present in herbivores, which use them to tear off bits of algae. In some detritus feeders, the pharynx is covered with sticky mucus. When these worms extend the pharynx and press it against the sea-floor sediments, food particles stick to it. When the pharynx returns to its normal position, it carries these food particles back into the gut. In other detritus feeders, such as earthworms, the pharynx acts like a pump. It sucks a mixture of soil and detritus through the mouth and forces it down into the gut. In parasites, such as leeches, the pharynx is used to suck blood and tissue fluids from the host.
Annelids have a number of other structures that are used in feeding. For example, some annelids filter-feed by fanning water through their tube like burrows and catching passing food particles in a mucus bag. In other filter-feeding annelids, such as the plume worm shown in Figure 27-15, the first segment forms feather like structures that sift detritus and plankton from the surrounding water. These feeding structures are also used as gills for respiration.
RESPIRATION
Aquatic annelids often breathe through gills. In some of these annelids, such as feather-duster worms, the large brightly colored feathery gills protrude from the opening of the worm's burrow or tube. In other annelids, small delicate gills are located on the sides of the body. The tube dwelling annelids with this type of gill breathe by fanning water through their tubes. Many annelids take in oxygen and give off carbon dioxide through their skin. Because the skin must stay moist to make gas exchange possible, the worms die if the skin dries out. To help guard against this, terrestrial annelids, such as earthworms, secrete a thin protective coating called a cuticle to hold moisture around them.INTERNAL TRANSPORT
Annelids typically have closed circulatory systems organized around two blood vessels that run the length of their bodies. Blood moves toward the head of the worm in the dorsal (top side) vessel and toward the back of the worm in the ventral (bottom side) vessel. In each body segment is a pair of smaller vessels called ring vessels that connect the dorsal and ventral vessels and supply blood to the internal organs. In annelids such as earthworms, several of the ring vessels near the anterior (front) end of the worm are larger than the other ring vessels and have muscle tissue in their walls. These vessels are often called hearts because they contract rhythmically and help pump blood through the system. In other annelids, blood is moved through the body by muscle contractions when the worm moves.EXCRETION
Like other animals, annelids produce two kinds of wastes. Solid wastes pass out through the anus at the end of the gut. Wastes resulting from cellular metabolism are eliminated by nephridia (simple tube-shaped excretory organs). A pair of nephridia in each body segment removes waste products from the body fluids and carries them to the outside.RESPONSE
Many annelids are active animals with well developed nervous systems. The brain sits on top of the gut at the front end of the body. Two large nerves pass around the gut and connect the brain with a pair of ganglia below. From these ganglia, a ventral nerve cord runs the entire length of the worm. Nerves from each segment of the worm enter and leave the nerve cord at a pair of small ganglia. These nerves help carry messages from sense organs and coordinate the movements of muscles.Sense organs are best developed in the free-living marine species of annelids. Many of these annelids have sensory tentacles, statocysts, chemical receptors, and two or more pairs of eyes. Although the eyes are usually simple light detectors, in a few species the eyes can actually perceive objects. Most tube dwelling species have light-sensitive cells either on their gills or near their mouths. These cells allow the animals to detect the shadows of predators passing overhead. When a shadow is detected, the worm pulls back into the shelter of its tube with amazing speed. In addition to specialized sense organs, these free-living marine worms also have various types of isolated sensory cells scattered along their epidermis. These cells respond to light, chemicals, and vibration.
Many other annelids have much simpler sensory systems. For example, earthworms have no specialized sense organs. They rely on simple sensory cells in the skin that are similar to those found in the skin of marine annelids.
Most free-living annelids do not have body structures that protect them from predators. Many depend on rapidly burrowing or swimming away from danger. Some, like earthworms, grab onto the walls of their burrows to make it harder to pull them out. Others, such as the marine fan worms, secrete protective tubes of calcium carbonate into which they withdraw if frightened. But some annelids do fight back. Several carnivorous annelids use their sharp jaws to attack animals that try to eat them. And the marine fireworms have tufts of poisonous bristles that easily break off and penetrate skin, causing painful sores and a burning sensation.
MOVEMENT
Annelids have two major groups of muscles in their body walls. One group, called longitudinal muscles, runs from the front of the worm to the rear. When these muscles contract, they make the worm shorter. Another group of muscles runs in circles around the body of the worm. When these muscles contract, they make the worm skinnier. Marine annelids can swim by using these muscles to wriggle through the water. Burrowing annelids use their muscles to force their way through heavy sediment-not an easy thing for a softbodied animal to do!REPRODUCTION
Although a few annelids are able to reproduce asexually by budding, most annelids reproduce sexually. Some species have separate sexes and external fertilization. This means that females and males release eggs and sperm, respectively, into the open water where fertilization takes place. Of course, the chances of fertilization taking place are enhanced if many worms in an area release their eggs and sperm at the same time. This is exactly what happens in some species. In the South Pacific, islanders eagerly await the autumn spawning season of the annelids called palolo worms. At a particular phase of the moon, hundreds of thousands of male and female palolo worms swarm at the surface of the water to release their eggs and sperm. Just before sunrise, the sea is literally covered with these worms. The islanders, who consider these worms a great delicacy, join sea birds and fishes that gather to feast on the spawning worms.Some annelids, such as earthworms and leeches, are hermaphrodites that undergo internal fertilization. Although an individual worm produces both sperm and eggs, it rarely fertilizes its own eggs. Instead, worms pair up, attach themselves to each other, and exchange sperm. Each worm stores the sperm it has received in special sacs. When eggs are ready for fertilization, a band of thickened, specialized segments called the clitellum (cligh-TEHL-um) secretes a mucus ring into which eggs and sperm are released. The ring then slips off the worm's body and forms a cocoon that shelters the eggs.
Sandworms, Bloodworms, and Their Relatives
The class Polychaeta (poly- means many; chaeta refers to bristles) contains many common and important marine worms. Polychaetes (PAHL-ee-keets) are characterized by paired paddlelike appendages on their body segments. These appendages are tipped with the bristles that give this class its name. In the sea mouse, shown in Figure 27-19, the bristles are so long that they extend over the back of the worm and look like hair or fur.
Polychaetes live in cracks and crevices in coral reefs, in sand, mud, and piles of rocks, and even out in the open water. Some burrow through or crawl over sediments. Others live almost entirely in tubes they build for themselves. Some polychaetes are dull in color and rather uninteresting; some are brightly colored, iridescent, or even luminescent.
Earthworms and Their Relatives
The class Oligochaeta contains earthworms and related species. Two oligochaetes (AHL-ih-goh-keets) that you might be familiar with are earthworms and tubifex worms. Earthworms are long pink worms that often show up on the surfaces of lawns and sidewalks after it rains, are dug up in gardens, or are sold as fishing bait. Tubifex worms are red threadlike aquatic worms that are sold as tropical-fish food in pet stores. Most oligochaetes live in soil or fresh water, although some species live in the ocean. As the name of the class indicates (oligo- means few), oligochates have fewer bristles than polychaetes. These bristles, which can be felt as a roughness on the ventral (bottom) side of an earthworm, help anchor it in its burrow.
Although earthworms spend most of their lives hidden under ground, an observant person may find evidence of their presence above ground in the form of squiggles of mud known as castings. Recall that an earthworm (which swallows just about anything it can get into its mouth) uses its pharynx to suck a mixture of detritus and soil particles into its mouth. As the mixture of food and soil passes through the intestine, part of it is digested. Sand grains, clay particles, and indigestible organic matter pass out through the anus in large quantities, producing castings. Some tropical earthworms produce enormous castings-as large as 18 centimeters long and 2 centimeters in diameter!
The class Hirudinea contains the leeches, most of which live in moist tropical countries. Leeches are typically no more than 6 centimeters long, but there are some tropical species that are as long as 30 centimeters. Most leeches are freshwater organisms that exist as external parasites, drinking the blood and body fluids of their host. However, there are some marine and terrestrial leeches. And roughly one fourth of all leeches are carnivores rather than parasites. Carnivorous leeches, which feed on soft-bodied invertebrates such as snails, worms, and insect larva, either swallow their prey whole or suck all the soft parts from its body.
All leeches have powerful suckers at both ends of their bodies. These suckers-especially the anterior one, which usually surrounds the mouth-are used to attach a leech to its host. The posterior sucker is also used to anchor a leech to rocks, leaves, and other objects as it waits for a host to come the skin of their host in one of two ways. Some leeches use a muscular proboscis (proh-BAHS-ihs), or tubular organ, that they force into the tissue of their host. Others slice into the skin of their hosts with a razor-sharp pair of jaws. Once the wound has been made, the leech uses its muscular pharynx to suck blood from the area. Both types of leeches release a special secretion from their salivary glands to prevent the blood from clotting as they drink it, Some leeches also produce a substance that anesthetizes the wound-thus keeping the host from knowing it has been bitten!
During feeding, a leech can swallow as much as 10 times its weight in blood. Such a huge meal can take the leech up to 200 days to digest, with the help of symbiotic bacteria that live in its gut. A leech can live for a year before it must feed again.
How Annelids Fit into the World
Annelids are important in many habitats. Small polychaetes and their larvae are members of the ocean plankton, where they are food for many fi§hes, crabs, and lobsters. Bottom-dwelling polychaetes are important items in the diets of food fishes such as flounder.
Oligochaetes, particularly earthworms, perform an essential task in conditioning soil, as Charles Darwin noted in a lengthy and detailed study. By constantly burrowing through the ground, earthworms help aerate the soil. And by grinding and partially digesting the incredible amount of soil and detritus that passes through their guts, earthworms speed the return of nitrogen and other important nutrients from dead organisms to forms that can be used by plants. Without the continual efforts of these annelids, the structure and fertility of farm soils would degenerate quickly, lowering crop yields.