*Lab is not needed to complete this assignment* You are simply making notes based on facts as in what will happen in the situations. Answer all questions and make notes for all statements.

Background

Figure 1. Earthworm

(Image source: Wikimedia, author S. Shepard)

Should Worms Wear Sunscreen?

Earthworms (Figure 1) are adapted to underground conditions and have no protection from UV rays in direct sunlight. Scientists know that UV rays generate DNA mutations. Extensive DNA damage can be lethal. Although humans have better protection from UV rays than earthworms, there is still a need for sunscreen to protect skin cells, because DNA mutations can cause skin cancer.

Earthworms

Earthworms are found in the wet soil of most warm to moderate climates. They are long, slender animals that have probably been seen by anyone with a green thumb as well as used extensively by fishermen for bait.

Earthworms are indispensable helpers to farmers and gardeners because they enrich the soil by digging and rotating the soil as they ingest decayed leaves and other dead organic matter. Earthworms digest the organic matter and excrete waste products along with any dirt that was also ingested. During this process, soil is overturned and mineral-rich dirt from the subsoil is excreted into the topsoil, making more nutrients available to plants. The soil is enriched by the waste products of the worms. The tunnels or burrows that earthworms make as they crawl through the soil allow air to permeate the soil and water to drain efficiently.

The common earthworm (common name), also known as Lumbricus terrestris (scientific name), belongs to the family Lumbricidae and phylum Annelida, and is the subject of this lab.

Annelida, the Segmented Worms

The phylum Annelida includes a variety of marine, freshwater, and terrestrial worms including the familiar leeches and earthworms. The most distinctive feature of annelids (members of the phylum Annelida), is the division of their bodies into rings or segments. In Latin, annellus means little ring.

This type of body segmentation is called metamerism. In annelids, metamerism is evident in the external features of the worms, which accounts for their common name, the segmented worms. Most annelids also show internal metamerism, with the body divided into compartments by regularly repeated septa (singular: septum), which are thin walls of fibrous connective tissue, and by the repetitive arrangement of organs and organ systems.

Annelids have a coelom, a body cavity between the digestive tract and musculature. It is fully lined by the peritoneum, a thin layer of epithelium that is attached to the musculature. Annelids have a centralized nervous system and a closed circulatory system. Most annelids also have setae, which are small, bristle-like appendages.

Earthworms belong to class Oligochaeta, one of the four classes of Annelids.

Anatomical Terminology

The study of anatomical structures has its own terminology. To know exactly where a structure lies within an organism, there is a specific set of directional terms (Figure 2).

Figure 2. Anatomical Orientation

• The head is the anterior and the anus is the posterior. The axis running between them is called the anteroposterior axis.
• The left and right sides are lateral to the middle and lie along the left-right axis. The opposite of lateral is medial, meaning lying inward and toward the center.
• The dorsal side is the backside. The opposite side is the ventral side, which also means the front or belly side. The axis running between them is called the dorsoventral axis.

When something is sliced through along or parallel to its longest axis, it creates a longitudinal section. When it is sliced through perpendicular to its longest axis, it creates a cross section.

External Anatomy of the Earthworm

Earthworms consist of a series of segments forming a long, cylindrical, tube-like body (Figure 3a). The length of a common earthworm is 6 to 11 inches. However, giant relatives of the earthworm found in Australia and South America can grow to a length of 11 feet. The head of an earthworm is pointed. The tail has a flattened appearance. The skin is colored pink or brown and has a slippery texture.

Figure 3. Anatomical Features of Earthworms (a) external features (b) longitudinal section of an earthworm showing the internal features near the head

An earthworm’s body is made up of a series of segments, which can total to over 100 in number. The thickened, smooth, patch visible on the earthworm is an organ called the clitellum. The clitellum ranges over 5 segments of the earthworm from approximately the 32nd to 37th segments. It functions in the reproductive process of the earthworm by secreting a mucus that holds two earthworms together as they mate. The mucus then acts as a protective cocoon around the earthworm’s eggs as they develop.

Internal Anatomy of the Earthworm

Earthworms have a hard outermost layer called the cuticle, which helps the worms retain water. Encased within the cuticle is an outer skin called the epidermis, inside of which is the internal anatomy of the earthworm (Figure 3b).

Digestive System

Earthworms have a digestive system consisting of a mouth, a pharynx, an esophagus, a crop, a gizzard, an intestine, and an anus.

The first body segment is a tongue-like, protruding appendage called the prostomium. It is used to help ingest food and often functions as a sensory organ. Food and dirt enter through the mouth, the initial opening to the digestive system, and then pass through the pharynx, a muscular structure that sucks food into the mouth.

Food then passes through the esophagus, a muscular tube leading from the pharynx to the crop, a slightly enlarged cavity. The food is stored temporarily in the crop. The walls of the crop include crop muscles, which contract to force food into the gizzard, a larger cavity, where the food is broken up into small pieces. The gizzard muscles are very large and dense. They are responsible for grinding up the food.

The small pieces of food from the gizzard then travel into the intestine, where they are digested. The typhlosole, an internal fold in the intestine, provides more surface area for absorption of nutrients. Any dirt that has entered the mouth passes through the digestive tract along with other solid wastes to be excreted through the anus, the terminal opening of the digestive system.

Excretory System

Another mode of excretion is performed by the nephridia (singular: nephridium). Nephridia are a series of wavy, convoluted channels within the coelom, on each side of the digestive tube. They pull out the liquid waste from the body and excrete it through nephridiopores, openings in the skin (Figure 4).

Figure 4. Cross Section of an Earthworm

Nervous System

Ventral to the digestive tract is a nerve cord. The anterior side of the nerve cord is enlarged to form the cerebral ganglion, a primitive form of brain found on the dorsal surface of the pharynx. The rest of the nerve cord extends posteriorly for the length of the worm on the ventral side and is called the ventral nerve cord.

Although earthworms have no defined eyes, they do have various light sensitive spots on their bodies that help them to detect the presence or absence of light. Earthworms have well-developed senses of touch, smell, and taste.

Circulatory and Respiratory Systems

Earthworms absorb oxygen through their skin, which then goes into small blood vessels.

Earthworms have a primitive circulatory system consisting of two principal blood vessels: the dorsal and ventral blood vessels. Blood moves along the dorsal blood vessel — located above the digestive tube — by muscular contractions. The dorsal blood vessel then pumps the blood into heart-like blood vessels in the anterior of the earthworm called aortic arches. The aortic arches form five arcs over the esophagus. These five vessels pump the blood to the ventral blood vessel, which is located below the digestive tube. This vessel transports the blood throughout the worm.

Musculoskeletal System

Beneath the epidermis of the worm are two sets of muscular tissue layered one on top of the other throughout the worm. The longitudinal muscles run along the length of the body, whereas the circular muscles are arranged in rings around the anteroposterior axis of each segment. The function of both muscle types is to aid the worm’s movement as it inches forward. First, the worm contracts the longitudinal muscles while the circular muscles remain steady as an anchor. Then the circular muscles contract while the longitudinal muscles are the steady anchor. Setae provide traction as the worm moves.

Annelids and nematodes are two similar phyla. Nonetheless, how they move during locomotion is very different. Nematodes have longitudinal muscles like annelids, but no circular muscles.

Reproductive System

Each worm produces both sperm and eggs, but they do not self-fertilize. The sperm of one earthworm fertilizes the eggs of another earthworm. The testes are small and funnel-shaped. They are located on top of seminal vesicles that store the semen. An earthworm’s seminal vesicles are the only reproductive organs that are large enough to be clearly visible using a compound light microscope.

Once fertilization occurs, the clitellum excretes mucus to form a cocoon that protects the fertilized eggs. The fertilized eggs are deposited below the surface of the ground until they are ready to hatch, two or three weeks later.

About This Lab

In this lab, you will explore the anatomical structure and function of an earthworm using the virtual microscope. You will view slides of anatomical samples of an earthworm using the microscope. You will also label anatomical features of and explore the anatomical orientation of an earthworm.

Experiments

Open the simulation by clicking on the virtual lab icon below. The simulation will launch in a new window.

You may need to move or resize the window in order to view both the Procedure and the simulation at the same time.

Follow the instructions in the Procedure to complete each part of the simulation. When instructed to record your observations, record data, or complete calculations, record them for your own records in order to use them later to complete the post-lab assignment.

Procedures

This lab requires you to use the virtual microscope. Click here for a short video about how to use the features and functions of the virtual microscope.

Topic 1: External Anatomy

1. View the earthworm image below.

1. Find and identify the following structures:
   • Anterior end
   • Posterior end
   • Dorsal side
   • Clitellum

1. Note that the body is composed of ring-like segments. Make a rough approximation of the number of segments and record your estimate to reference later.

Topic 2: Locomotion

1. View the Earthworm Locomotion video to watch how the earthworm moves.

1. In your notes, describe how the earthworm’s paired muscles function to propel the earthworm forward. Make sure to include the action of all the segments involved in one “step” forward.
2. Replay “Earthworm Locomotion” and watch the earthworm’s mouth. What does the earthworm seem to be doing with the prostomium during each “step” it takes? Record your observations to reference later.
3. Observe nematode locomotion in this link. In your notes, describe the sequence of muscle contractions and relaxations. How does nematode locomotion compare to earthworm locomotion? Record your observations.

Topic 3: Internal Anatomy in a Cross Section

Part 1: Internal Anatomy in a Cross Section

1. Take a microscope from the Instruments shelf and place it onto the workbench.

1. Take the earthworm cross section slide from the Containers shelf and place it on the microscope stage.
   If you want to take a closer look at what you are working on, use the navigation tools in the lower right corner of the lab Zoom In (+) and Zoom Out (-). Use the pan buttons to move around the screen. You can use these navigation tools anytime.

1. View the diagram of an earthworm in cross section below.

1. Referring to the diagram of an earthworm cross section and the information, including diagrams, in the Background, label the following parts of the organism on the microscope slide images:
   Musculoskeletal System
   • Cuticle
   • Epidermis
   • Circular muscle
   • Longitudinal muscle
   • Coelom
   • Peritoneum

Circulatory System

• Dorsal blood vessel
• Ventral blood vessel

Digestive System

• Intestine
• Typhlosole

Excretory System

• Nephridium
• Nephridiopore

Nervous System

• Ventral nerve cord

You may need to make multiple slide images. Be sure to save screenshots of all your annotated images in order to upload and submit them later.

1. When you are finished, drag the slide back to the Containers shelf.

Part 2: The Setae

1. Take the earthworm seta slide from the Containers shelf and place it onto the microscope stage.
2. Zoom in until you can see a hair-like seta protruding from the bottom of the earthworm segment.
3. Label the seta and save a screenshot.
4. When you are finished, drag the slide back to the Containers shelf.

Topic 4: Internal Anatomy in a Longitudinal Section

Part 1: Longitudinal Anterior Section #1

1. Take the earthworm anterior longitudinal section slide from the Containers shelf and place it onto the microscope stage.

The section of the earthworm included on the slide is shown below.

View the labeled diagram of the anterior section of an earthworm in longitudinal section.

Label the following parts of the organism on the microscope slide image using the diagram above and the information, including diagrams, in the Background as reference:
Digestive System

• Prostomium
• Mouth
• Pharynx
• Esophagus

Nervous System

• Cerebral ganglia or “brain”
• Ventral nerve cord

1. Save a screenshot of the annotated image(s) in order to upload and submit it later.
2. When you are finished, drag the slide back to the Containers shelf.

Part 2: Longitudinal Anterior Section #2

1. Take the earthworm circulatory and reproductive systems longitudinal section slide from the Containers shelf and place it onto the microscope stage.

The section of the earthworm included on the slide is shown below.

1. Label the following parts of the organism on the microscope slide image using the diagram in Part 1 above and the information, including diagrams, in the Background as reference:
   Circulatory System
   • Dorsal blood vessel
   • Aortic arches
   • Pharynx
   • Esophagus

Nervous System

• Ventral nerve cord

Reproductive System

• Seminal vesicles

1. Save a screenshot of the annotated image(s).
2. When you are finished, drag the slide back to the Containers shelf.

Part 3: Longitudinal Anterior Section #3

1. Take the earthworm digestive system longitudinal section slide from the Containers shelf and place it onto the microscope stage.

The section of the earthworm included on the slide is shown below.

1. Label the following parts of the organism on the microscope slide image using the diagram in Part 1 above and the information, including diagrams, in the Background as reference:
   Digestive System
   • Esophagus
   • Crop cavity
   • Crop muscles
   • Gizzard cavity
   • Gizzard muscles
2. Save a screenshot of the annotated image(s).

Part 4: Septa

1. Clear all annotations from the earthworm digestive system longitudinal section slide.
2. Identify three septa and use the annotation tools to label them.
3. Be sure to save a screenshot of the annotated image.
4. Clear the bench of all slides and instruments by dragging them back to the shelves, then return to your course page to complete any assignment for this lab.