About Lumbricus

What IS Lumbricus terrestris?

Lumbricus terrestris is a common species of earthworm. What do we mean by “species”? You may think of it as a “kind” of animal, like “cat” or “dog.” A species is a group of living things that are capable of breeding with each other and producing offspring—“children”—that are also capable of reproducing. There are other Lumbricus earthworms, but Lumbricus terrestris is the one you are most likely to have seen, either by digging up some soil or by observing them lying on the street after a rainstorm. Their species is terrestris, which is one of several species of Lumbricus. Our own species, sapiens, is one of a few species—and the only surviving one—of the group Homo. So we are Homo sapiens. There are many kinds of worms, including flatworms, roundworms, ribbon worms, and others. These groups are not closely related to each other. They are different branches of the tree of life. Lumbricus terrestris belongs to a group of worms called annelids. One of the distinguishing characteristics of annelids is that their body is segmented—that is, divided into a long row of compartments separated by walls. If you are trying to look something up and don’t find it with the key word “earthworm,” try looking for “annelid.”

Courtesy of Life: The Science of Biology, Eighth Edition,

c2007 Sinauer Associates, Inc. and W. H. Freeman & Co.

What do we mean by “related”?	The animals most closely related to the annelids are the arthropods, which include the insects, spiders, crabs, and many other animal groups. The arthropods are related to one another by the fact that they have all descended from a common ancestor that had no other descendents. The annelids and arthropods are very closely related because these two groups descended, in turn, from a common ancestor that had no other descendents. In this way the tree of life traces the course of evolution—wherever we see a branch, this marks a common ancestor of the groups that descended from it. In the tree shown here, think of time as going upward.
Which end is up?	As you seek information about anatomy here and elsewhere, you may encounter four terms that refer to directions. Cranial means “toward the head end,” caudal means “toward the tail end,” ventral means “toward the lower side,” and dorsal means “toward the upper side.”
What is an animal body made up of?	Well… let’s build the animal up from its smallest parts: Atoms are the smallest bits of all. Molecules are made up of groups of atoms Cells consist of highly organized collections of enormous numbers of molecules all performing important functions, as we are about to see:
What are cells, and what do they consist of?	The bodies of all animals, even the simplest, are composed of cells, the building blocks of all living things. All animal cells are surrounded by a plasma membrane that controls what ions and molecules may enter or leave the cell. All contain a nucleus, the command center of the cell, which contains the DNA that is the hereditary material. Everything outside of the nucleus makes up the cytoplasm, containing mitochondria (the “powerhouses” of the cell) and other important structures that we will not address here. There are more than 200 distinct kinds of cells in the human body, each kind adapted to appropriate functions. Human cells include red blood cells, red skeletal muscle cells, liver cells, and many others. The earthworm body contains far fewer cell types.
What are tissues, organs, and organ systems?	Tissues are groups of similar cells organized into a functional unit. An example would be epithelium, which consists of several layers of cells that cover the surfaces of organs such as the inner lining of the digestive tract. All animals have organs—body parts composed of different tissues integrated to perform a distinct function. Organs of the human body include the heart, liver, brain, kidneys, eyes, the various glands, and numerous others. Earthworms have fewer organs, but the ones they have are of equal importance to those of humans. Organs are often integrated into organ systems such as the reproductive system, digestive system, excretory system, and others. The earthworm’s circulatory system consists of five pairs of “hearts,” a ventral blood vessel, a dorsal blood vessel, and capillaries, for example.
What are the segments of an earthworm?	Like all annelids, Lumbricus terrestris has a segmented body. Most of the segments may be thought of as modules, each much like the others. For example, most segments contain a pair of excretory organs, and each segment except for the first and the last has setae (bristles). The blood vessels and digestive tract run continuously from the cranial (head) end of the worm to the caudal (tail) end, passing through the walls separating the segments, but muscles attach to the segment walls rather than running the length of the worm.
How does the digestive system of the earthworm function?	Digestion is the breakdown of complex molecules such as starch into smaller molecules such as sugars. No energy is stored in ATP during digestion. The energy transfer steps occur in the subsequent metabolism of the products of digestion. Digestion consists largely of chemical reactions with water as one of the reacting molecules. The earthworm’s digestive system consists of the mouth, pharynx (a muscular tube that expands, drawing soil and food into the mouth), esophagus, crop (where food and other material is stored), gizzard (where the contents get ground up into smaller bits for easier digestion), intestine (where most digestion of food takes place), rectum (another short tube) and anus (where the last of the contents of the digestive system are deposited as castings behind the worm as it proceeds through the soil). Besides being a site for digestion, the intestine is where most of the products of digestion pass out of the digestive tract (through the wall of the intestine) on their way to blood vessels for distribution to the rest of the body. The intestine has a long fold (called the typhlosole) that increases the surface area of the intestine, enhancing the release of digestive products.
What is the respiratory system of the earthworm?	The respiratory system (gas exchange system) allows earthworms to take up oxygen from the soil or atmosphere and release carbon dioxide. In fact, earthworms have no respiratory organs at all—they simply exchange gases directly through their moist integument (“skin”). They lack lungs and “noses.” The integument has a thin (one cell layer thick) epidermis with glands that secrete a moist cuticle that is the outer surface of the body. The integument also possesses bristles (called setae) that play no part in gas exchange but are very important in locomotion. Respiration (gas exchange, “breathing”) refers specifically to oxygen and carbon dioxide. Animals and their cells require oxygen in order to extract energy from food. This energy-transferring process (known as cellular respiration) produces carbon dioxide, which becomes toxic to the animal unless it is carried by the circulatory system to the respiratory system, which releases it to the environment.
How does the circulatory system of the earthworm work?	The circulatory system transports gases (oxygen and carbon dioxide), the products of digestion, and other materials (and even heat) throughout the body. It also carries certain waste materials such as ammonia to the nephridia (excretory tubes), where they can be eliminated via the excretory system. The earthworm has a closed circulatory system—that is, it is confined within tubes. Blood proceeds toward the caudal end of the worm in a ventral blood vessel and returns in a dorsal blood vessel. Blood flow is pumped weakly. Five pairs of aortic arches (“hearts”) near the cranial end of the worm squeeze blood down into the ventral blood vessel, and waves of contraction in the dorsal blood vessel help drive blood back toward the cranial end.
How does the excretory system of the earthworm work?	Excretion is the release of metabolic waste consisting of nitrogen-containing molecules that would be toxic (poisonous) to the animal if not removed from the body. The principal nitrogen-containing waste molecules are ammonia, urea, and uric acid. Ammonia is the most toxic but the most soluble in water; it is commonly produced in aquatic animals that can release it directly into the environment, where it is washed away. Earthworms excrete primarily ammonia, which is dissolved away in the moist soil. The excretory system of the earthworm consists of tubular organs called nephridia. Excretion is basically a three-step process. The first process is filtration. Water and dissolved materials filter out of the ventral blood vessel into the body cavity and thence into the funnel-like openings of the excretory tubes (nephridia). Most body segments contain two nephridia, which pass through the segment wall into the next segment. The nephridia then connect to pores in the integument, allowing fluid in the tube to pass into the environment. While in the tube, the fluid is subject to two other processes: absorption, in which some important substances (often including water) are recovered from the tube and returned to the blood, and secretion, in which some substances (often including toxic ones) are transported from the blood or body fluid into the tube. Both absorption and secretion include energy-requiring steps.
What is the skeletal system of the earthworm?	The skeletal system of worms does not include either bones or the hard outer skeletons found in crabs, clams, insects and some other invertebrates (animals without backbones). The earthworm skeleton is a hydrostatic skeleton, in which the integument (body wall) resists the body’s substantial internal pressure, as a balloon resists the pressure of air within it. If a balloon loses air, its shape deforms and the balloon collapses. In exactly the same way, if an earthworm dries, its body collapses as water is lost—as you likely have seen after a rainstorm. The skeletal system serves to support the body and participates in the worm’s burrowing.
How does the reproductive system of the earthworm function?	The reproductive system of the earthworm is complex, as is the whole mating system. Earthworms are hermaphrodites—that is, each worm has both female and male organs. They must mate in pairs, exchanging sperm. The worms approach each other head to head and continue to crawl until the clitellum (the collar-like structure you see about one-third of the way from the cranial end) of each is adjacent to the region containing the ninth to eleventh segments (from the cranial end) of the other partner. Sperm are produced in structures in that region and released, passing from one worm to the clitellum of the other. Each worm wiggles out of its clitellum, which passes over openings by the ovaries, so that the sticky clitellum ends up containing many eggs and sperm. The clitellum develops into a cocoon, in which the new earthworms develop.
What is the nervous system of the earthworm?	The nervous system of the earthworm consists of a ventral nerve cord and a small, simple brain (cerebral ganglion) in segment #3. Information in each segment is collected by a smaller ganglion (bundle of nerves). The cerebral ganglion controls the worm’s behavior—including both the speed and direction of movement. Among the behaviors are avoidance of light, reaction to vibrations in the soil, and withdrawing from toxic substances.
How is the earthworm muscular system designed?	Muscles contract (shorten), a process that requires energy (delivered as ATP). Muscle contraction is an active process, but the elongation of contracted muscles is passive—they simply relax. The burrowing of earthworms depends on energy-requiring contraction of muscles. The muscular system of the earthworm consists of two important categories of muscles. Circular muscles just inside the integument are perpendicular to the length of the worm, and they form a complete loop, like belts. When the circular muscles contract, the worm becomes thinner at that point; when they relax, the body thickens at that point. Longitudinal muscles, interior to the circular muscles, run parallel to the length of the body and are attached at their ends to the walls separating the worm’s segments. When longitudinal muscles contract in part of the body, the worm shortens and fattens in that region; when they relax, the worm thins and lengthens.
How does the earthworm move forward?	The worm moves through the soil by alternately shortening and lengthening longitudinal muscles. (A picture is also included in this document to help you visualize the process.) The circular muscles help in the process, allowing bristles (setae) to be withdrawn from the soil or inserted into it to anchor the body at that point.

Courtesy of Life: The Science of Biology, Eighth Edition,

c2007 Sinauer Associates, Inc. and W. H. Freeman & Co.

Can the earthworm control its body temperature?	Thermoregulation is the control of body temperature. Animals such as the earthworm have no thermoregulatory mechanisms and must tolerate such high and low temperatures as their environment provides; but living in the soil buffers them somewhat as compared with living exposed to the air.
Do organ systems affect one another?	Organ Systems Are Interdependent “No organ system is an island.” That is, each organ system affects other organ systems and in turn is affected by them. Organ systems must operate in harmony with one another in order for the animal to enjoy maximum health. For example, the muscles require energy in order to contract; they obtain this energy by way of the circulatory system, which carries the energy-rich molecules released by the digestive system. As muscles and the digestive system do their work, they produce waste that must be processed by the excretory system.
What is homeostasis?	Some of the organ systems are concerned with homeostasis: maintaining a steady state, such as a constant salt concentration in blood, or a constant temperature. Homeostasis is sometimes referred to as “constancy of the internal environment.” The excretory system is one of the most important organ systems in maintaining homeostasis. Different kinds of animals differ in the degree to which they can maintain homeostasis—for example, whereas humans can maintain a constant body temperature, earthworms cannot.