The Anatomy of the Earthworm

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1 Carolina Biological Supply Company presents The Anatomy of the Earthworm 2700 York Road Box 187 Burlington, North Carolina Gladstone, Oregon 97027

2 Abstract. This program facilitates a study of the anatomy of the earthworm. The optional overhead transparencies, 35-mm transparencies, and Bioreview sheets are available as ancillaries to the dissection process. Running Time: total minutes (The External Anatomy: 2:20 minutes, The Internal Anatomy: 2:45 minutes, The Digestive System: 1:45 minutes, The Circulatory System: 1:40 minutes, The Excretory System: 1.00 minutes, The Reproductive System: 2:25 minutes, The Nervous System: 2:15 minutes). Under no circumstances may any part of this transcript or program be reproduced in any manner without the express written consent of Carolina Biological Supply Company. For information about reproduction, please contact: Carolina Biological Supply Company Attn: Audiovisual Development 2700 York Road Burlington, NC Carolina Biological Supply Company Printed in USA

3 The Anatomy of the Earthworm Narrative Earthworms are often studied in biology classes. They are common, well-known animals, and they illustrate some basic anatomical features typical of most complex animals. For example, earthworms have a well-developed central body cavity, the coelom, which contains the internal organs. They also demonstrate segmentation, or the serial repetition of body parts. These and other important anatomical features make the earthworm an excellent model for studying the basic pattern of organization of many evolutionarily advanced animals. Earthworms represent the phylum Annelida, which includes more than 12,000 species of segmented worms living in both terrestrial and aquatic habitats. Terrestrial annelids burrow in soil or decaying organic matter. Aquatic annelids include both freshwater and marine species. The most specialized annelids, however, are the leeches. Although some species of leeches suck the blood of certain species of vertebrates, and others are active predators of smaller aquatic animals, most are scavengers that feed on dead or decaying organic matter. THE EXTERNAL ANATOMY Earthworms live in tunnels they make in soil or decaying organic matter. The external anatomy of earthworms is well adapted to their underground life. The most obvious adaptations are the lack of a head and appendages. There are no visible eyes, antennae, or other sense organs at the anterior end of the body, nor any appendages attached to the exterior of the body. The cylindrical body is tapered at both ends, aiding the worm in burrowing through the soil. The anterior end of a living worm is easy to identify by its movements. On a preserved earthworm, the anterior end can be identified by referencing the clitellum, a swollen region a few segments back from the anterior end. The body of the earthworm is divided into many segments indicated by distinct rings encircling the body. The mouth is found on the first segment. Above the mouth is a small flap of tissue, the prostomium. The exterior surface of the body is covered by a thin, waxy, protective cuticle that helps reduce water loss. Each body segment except the first and last bears small chitinous bristles, or setae, that extend outward from the body surface and 3

4 help anchor the worm as it burrows. As seen in a cross section of the body, the setae are arranged in eight rows. Four parallel rows are located on the flattened ventral surface of the body, and two paired rows run along each side. Muscles in the body extend and retract the setae. The clitellum is a swollen, glandular part of the body wall that secretes a cocoon during the reproductive season. Fertilized eggs are deposited and incubated in the cocoon. Openings on the external surface of the body include one pair of excretory pores on most body segments and several pairs of reproductive pores. THE INTERNAL ANATOMY To study the internal anatomy, place the specimen on a dissecting pad and carefully pin it down with the dorsal surface up. Cut through the thin body wall behind the clitellum and make a longitudinal cut slightly to the side of the dorsal midline. Continue this dorsal incision forward to the mouth region. Take care to avoid damaging the internal organs. When the dorsal incision is complete, spread apart the body wall, carefully separating it from the internal organs, and pin it to the dissecting pad. We can now observe the large body cavity or coelom that contains the internal organs. The coelom is divided into many chambers or segments by thin partitions, the septa. In an intact worm, each segment is filled with coelomic fluid. The coelom is also completely lined by specialized mesodermal tissues, the basic feature of a true coelom as it is found in annelids and most higher animals. The wall of the digestive tract is covered by layers of circular and longitudinal muscles and a thin layer of peritoneum. The outer body wall is also lined by circular and longitudinal muscles. A fluid-filled coelom allows further specialization among animals and serves many functions in earthworms and other coleomate animals: 1) it provides space for a specialized, long digestive tract that increases the surface area for absorption; 2) it permits the circulation of fluids, facilitating gas exchange, nutrient supply, waste removal, and the chemical defense of body tissues; 3) it aids in maintaining the proper balance of water and ions; 4) it provides space for the development of complex sex organs and for the storage of gametes; and 5) it also can act as a hydrostatic skeleton. 4

5 Support for the body is provided by contraction of the circular and longitudinal muscles of the body wall against the fluid-filled segments. Peristaltic contractions passing down the body can force coelomic fluid from one segment to another through small pores in the septa. These contractions play an important role in locomotion. THE DIGESTIVE SYSTEM The digestive tract of the earthworm consists of a straight tube extending from the mouth to the anus. The digestive system includes the mouth, located in the first segment, which opens into a small buccal cavity. Behind the buccal cavity is a muscular, expandable pharynx. Expansion of the pharynx helps to suck moist soil, which contains food materials, through the mouth and into the buccal cavity. Behind the pharynx is a thin-walled esophagus that carries food posteriorly to the crop. Along the sides of the esophagus are two or more pairs of calciferous glands that help regulate the amount of calcium, the carbonate ions, and the ph of the blood. Food passes through the esophagus into the crop where it is temporarily stored. From the crop, food enters the muscular gizzard where it is ground into smaller pieces before entering the intestine. Food is digested by enzymes in the intestine, and nutrients are absorbed. Undigested materials are passed on to the anus for elimination. Most of the intestine and some of the large blood vessels are covered by a layer of chlorogogue tissue. The chlorogogue tissue is not directly involved in digestion, but it does store glycogen and lipids. It also aids in chemical defense and has several other functions similar to the liver of vertebrate animals. THE CIRCULATORY SYSTEM Earthworms have a closed circulatory system of tubular blood vessels. The red blood of earthworms contains hemoglobin, a respiratory pigment similar to the hemoglobin of vertebrate blood. The most apparent vessel of the earthworm is the large dorsal blood vessel, which has thick, muscular walls and carries blood posteriorly. The rhythmic peristaltic contractions of the dorsal blood vessel can be seen through the body wall of a living specimen. Extending laterally from the dorsal blood vessel are numerous lateral vessels, which connect the dorsal vessel with the ventral vessel in certain anterior segments. Five of these lateral vessels are greatly enlarged and are commonly called hearts. 5

6 The earthworm has no external gills or other respiratory organs. Instead, the body wall has many capillaries and serves in the removal of carbon dioxide and the uptake of oxygen. The pattern of blood circulation in the earthworm can conveniently be traced from the hearts. The hearts beat with waves of contraction of the smooth muscles within their walls. These rhythmic waves force blood upward to the dorsal blood vessel where further peristalsis of the vessel wall moves the blood posteriorly. Valves in the hearts and in the dorsal blood vessel prevent backflow. THE EXCRETORY SYSTEM The excretory system of the earthworm consists of many pairs of segmental nephridia; most body segments contain one pair of nephridia. Each nephridium has a ciliated funnel connected with a coiled tubule leading to an external excretory pore. A nephridial funnel attaches to the anterior septum and thus drains waste fluids from the adjacent segment. Cilia lining the tubule move the wastes and fluid through the nephridium and expel them through the excretory pores or nephridiopores in the ventral body wall. Most body segments have two nephridia and two nephridiopores. (Scientists believe that the primitive ancestors of modern annelids actually had many similar segments, arranged one behind the other. In such hypothetical ancestral annelids, each segment had paired nephridia, paired sex organs, paired blood vessels, etc. Later descendents became more specialized, and certain segments became adapted for specific functions and lost some of the primitive paired organs.) THE REPRODUCTIVE SYSTEM Earthworms contain both male and female sex organs within a single individual, and are termed monoecious. Animals in which the male and female organs occur in different individuals, such as humans, are dioecious. Even though each worm has both male and female sex organs, cross-fertilization is most common. Such exchange of gametes and of hereditary characters is believed to increase genetic diversity and thus enhance adaptability and the chances of survival among the offspring. The reproductive organs are located in the anterior segments and are easily identified in a dissected specimen. The presence, number, and location of specific reproductive structures differ in various species of earthworms. The male reproductive organs are more conspicuous than the female reproductive organs. Most apparent are three large pairs of seminal vesicles that store sperm while they mature. Two pairs of 6

7 small testes embedded in the tissue of the seminal vesicles can only be observed by dissection of the seminal vesicles. Adjacent to the seminal vesicles are two small ciliated sperm funnels that receive sperm after release from the seminal vesicles. Connecting with the sperm funnels are two tiny sperm ducts, which carry the sperm to the external male genital pores in segment 15. The female reproductive organs are smaller and more difficult to find. The female organs include one pair of ovaries. These produce eggs, which mature and are released into the coelom. The eggs pass from the coelom through two ciliated egg funnels and enter two oviducts, which terminate at external female genital pores on segment 14. Other important female organs are two pairs of seminal receptacles in segments nine and ten which receive sperm during copulation and store it. The stored sperm and eggs are later released into cocoons where fertilization and development occur. THE NERVOUS SYSTEM The nervous system of the earthworm, like its circulatory and excretory systems, also shows definite segmentation. A pair of dorsal ganglia located above the pharynx serves as the brain. From the brain, two lateral nerves pass around the pharynx and connect with the bilobed first ventral ganglion lying beneath the pharynx. The ventral nerve cord extends posteriorly with a bilobed ganglion in each body segment. Three pairs of segmental nerves branch off from each segmental ganglion and connect with sensory cells in the body wall. The segmental ganglia coordinate the alternate contraction and relaxation of the circular and longitudinal muscles necessary for locomotion. The anterior ganglia, located above and below the pharynx, receive sensory impulses through several nerves of the anterior segments. The anterior ganglia also play a role in the control of feeding and burrowing behavior. The nerve cord and the nerve ganglia have two lobes, as seen in microscopic cross sections. Other annelids show similar nervous organization, and some primitive species have two separate nerve cords with lateral nerve connections. For this reason, the annelid nervous system is often thought to be derived from a ventral ladder nervous system with two parallel nerve cords, which is similar to that seen in flatworms. Earthworms are familiar to every gardener, farmer, fisherman, and student. They play an important role in aerating and mixing the soil, aiding the decomposition of organic matter, and recycling the elements. They also can give us important insights into the structure and relationships of other animals. For these reasons, earthworm present a valuable learning experience in the classroom. 7

8 GLOSSARY Annelid. Any member of the phylum Annelida. Annelids have a true coelom and a conspicuously segmented body. Buccal cavity. The oral cavity. An opening immediately outside the mouth. Chlorogogue. A yellow-to-brown tissue found lining the exterior surface of the digestive tract and many large blood vessels of the earthworm and certain other annelids. The chlorogogue is a modified peritoneum that functions in lipid and nitrogen metabolism and has other functions similar to the liver of vertebrates. Clitellum. A swollen glandular region of the body wall in the anterior segments of the earthworm. The clitellum secretes mucus and material that forms the cocoon for holding the eggs. Cocoon. A protective structure secreted in the earthworm after mating. The cocoon contains the fertilized eggs and developing embryos. Coelom. The large central cavity. The coelom develops within the mesoderm and is completely lined with mesodermal tissue. Copulation. The mating process. Copulation among earthworms results in the mutual exchange of sperm. Crop. An enlarged region of the digestive tract in which food is stored. Cuticle. The noncellular waxy material covering the outer body surface. The cuticle is secreted by epidermal cells. Dioecious. Having male reproductive organs in one individual and female reproductive organs in another. Dorsal ganglion. The brain of the earthworm. The dorsal ganglion consists primarily of nerve cell bodies that coordinate feeding, locomotion, and other activities. Esophagus. The tube leading from the buccal cavity to the crop. Genital pore. One of the external openings of the male and female reproductive systems. Gizzard. A thick-walled muscular chamber in the digestive tract between the crop and the intestine. Food is ground into smaller bits by contractions of the muscular walls. Intestine. The portion of the digestive tract extending from the gizzard to the anus. Monoecious. Having both male and female sex organs in the same individual. Nephridium. A simple, branched, tubular structure that functions as an excretory organ. Nephridipore. An external opening of the nephridium. 8

9 Ovary. The female gonad. The ovaries produce eggs. Oviduct. The duct that conveys eggs from the ovary to the genital pore. Pharynx. The muscular anterior chamber of the digestive tract connecting the buccal cavity and the esophagus. Prostomium. The small, first anterior segment that projects forward over the mouth. Segmental ganglion. A mass of nerve cell bodies located along the ventral nerve cord in each body segment. The segmental ganglion serves as a local center for nervous coordination. Seminal receptacle. A saclike structure that stores sperm received from another worm during copulation. Seminal vesicle. A saclike structure connected with the sperm duct that stores sperm before their discharge from the body. Septum. A sheet of tissue that separates adjacent body segments. Seta. A stiff, bristlelike structure made of chitin. Setae are embedded in the epidermis and aid in locomotion of the earthworm. Testis. The male gonad. The testes produce sperm. Typhlosole. A longitudinal fold of tissue extending into the intestine of the earthworm and certain other invertebrates. The typhlosole serves to increase the area of the intestine for absorption. 9

10 The Anatomy of the Earthworm 10 Written and directed by Charles F. Lytle, Ph.D. North Carolina State University, Raleigh, NC Produced by Roger E. Phillips, Jr. Production Assistants Thomas A. Martin Robert E. Mize Thomas E. Powell, IV Videography by Thom Cox, AMI Video/Post Thomas A. Martin Videomicroscopy by Robert E. Mize William R. West, FBPA Illustrations by Beverly Benner Script Editors Robert E. Mize Phillip L. Owens, Ph.D. Roger E. Phillips, Jr. Content Review by Phyllis Bradbury, Ph.D. Raymond O. Flagg, Ph.D. Geoffrey L. Leister, Ph.D. Charles F. Lytle, Ph.D. Robert E. Mize, M.S. Phillip L. Owens, Ph.D. Kenneth W. Perkins, Ph.D. William R. Surver, Ph.D. Narration by Laurie Wolf Video Postproduction Facilities AMI Video/Post Dedicated to the late Clarence M. Flaten, Ph.D. Indiana University

11 Other Programs Available in this Series are: The Anatomy of the Freshwater Mussel The Anatomy of the Starfish The Anatomy of the Crayfish The Anatomy of the Grasshopper The Anatomy of the Shark The Anatomy of the Perch The Anatomy of the Frog The Anatomy of the Fetal Pig The Anatomy of the Cat A Production of Carolina Biological Supply Company 2700 York Road Burlington, North Carolina (NC ) 2700 York Road Box 187 Burlington, North Carolina Gladstone, Oregon CB

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