Earthworm Dissection. Classification of Earthworm. Kingdom Animalia : Multicellular eukaryote, heterotrophic.
Classification of Earthworm
Kingdom Animalia: Multicellular eukaryote, heterotrophic
Phylum Annelida: body made of many segments separated by a septum. Two body openings and a closed circulatory system. Ex. Earthworms, marine worms, leeches etc.
Class Oligochaeta: lack appendages and very few bristles. The can be terrestrial or found in fresh water
All living things must maintain a relatively stable internal environment to survive. The term used to describe this is "homeostasis." Living things carry out different essential functions in order to accomplish this task. Multicellular organisms have “systems” that work together to maintain this stable environment. For example when you run your body is placed under sever stress. Various systems will start to work overtime keeping your body alive. Your heart starts beating faster and your breathing increases. Your body is trying to deliver oxygen and take away carbon dioxide (respiratory waste) form working muscles etc. Your skin starts to sweat reducing the excess body heat created by the activity. Without your body systems working together your systems will fail and death would ensue. There are many more system accommodations that occur as you run or engage your body in different activities.
Multicellular organisms maintain homeostasis differently. Compare the differences between warmblooded and cold blooded organisms. To adjust body temperature insects or reptiles will move their body to a location that will ether warm or cool their body depending on their needs.
Below are a few examples of various systems found in multicellular organisms.
Single celled organisms do not have systems because they are only made of one cell. Thus each cell must carry out all of these functions without relying on tissues, organs and organ systems (ex. bacteria, yeast, and most protists).
Feeding(digestive), Earthworms have a fleshy projection on the anterior end called the prostomium. This structure covers the mouth. The mouth leads into a muscular pharynx that pumps food into a storage organ called the crop. Just posterior to the crop is a thick-walled, highly muscularized gizzard. Small rocks (grit) in the gizzard help to grind food before it is moves into the intestine where the nutrients from the food is absorbed. After absorption solid waste is eliminated through the anus. The solid waste of earthworms are called castings. Worm castings are highly sought after because of their concentrations of nitrogen, phosphorus, potassium, micronutrients and beneficial bacteria. Worm feces is a fantastic fertilizer and very expensive. Earthworms also help to till / mix the soil as they burrow through the ground. As a result they leave a network of tiny holes and spaces that help water and air get to plant roots. These spaces are critical niches for bacteria to colonize and decompose organic debris as well as fix nitrogen (nitrogen fixation).
The intestine extends from the posterior end of the gizzard and continues until it reaches the anus.
Respiration, (respiratory system)
Earthworms secrete a thin surface film of mucus to keep them moist so oxygen and carbon dioxide can diffuse through the body wall. Worms do not have lungs or gills like some organisms. Remember that living things accomplish each of the tasks necessary to maintain homeostasis differently. All living things MUST have a way to deal with these requirements. It is your quest to make sure you can describe and explain how different living things meet these requirements.
Circulation, (circulatory) Earthworms have five simple hearts located just anterior to the crop and posterior to the pharynx. These are very simple hearts / vessels that pump blood throughout the earthworm. Earthworm blood is red because it has hemoglobin. Remember that hemoglobin is a protein that binds oxygen. There are two relatively large blood vessels that run along the top (dorsal) and along the bottom (ventral). There are small vessels that connect the two. You will not be able to see these without a microscope. The earthworms circulatory system is considered to be a closed system because the blood circulates within a network of vessels. An open system has spaces or cavities filled with blood that surrounds body organs. The blood in these cavities is picked up by vessels and circulated throughout the organism. Don’t forget that earthworms have a closed circulatory system.
Excretion(excretory system) Solid waste passing though the intestine is eliminated through the posterior opening called the anus. Cellular waste is excreted in a different way. Remember all cells excrete cellular waste (nitrogenous). When waste products accumulate inside body tissues they become toxic and must be removed or the creature will die. To eliminate cellular waste metanephridia remove toxins keeping the cellular landscape free of excess garbage. The metanephridia are connected to openings called nephridiopores. The cellular waste is excreted from these openings.
The earthworm, as well as most annelids, have a well developed nervous system consisting of a brain and many nerve cords. You can see from this image that the ventral nerve cord is connected to the brain. See if you can see ganglia.
Movement(skeletal and muscular)
Earthworms alternately contract their longitudinal (shortens) and circular (elongates) muscles to move.
Before You Start The Dissection
The next slides provide you with some still images of the different parts of the earthworm. It is important that you use these still images along with my dissection video in your preparation.
We hope you will enjoy learning about the systems living things use to maintain homeostasis.
Materials Needed To Perform Your Dissection
Make sure you have a dissection guide on your desk. Look up on the white board and you will see a list of key anatomical structures you will be asked during your dissection quiz. You may refer to this list while being quizzed. You may not look at the dissection guide.
You or your partner will go get your dissection bag and carefully remove the earthworm. Secure your specimen bag and return the bag to the appropriate box. Double check and make sure both of you are wearing safety glasses.
I have a limited supply of latex gloves for those students who do not bring their own. Plan ahead and place your gloves in your biology binder / backpack. The biology schedule on the moodle gives you the dates for all dissections.
Place your earthworm on the dissection tray and go over the external anatomy.
You can tell this is dorsal because it is darker in color and the prostomium points downward. The dorsal blood vessel gives the dorsal surface its color.
Ventrally the earthworm is not as dark.
You can see my pointer is on one of the lines that separates the segments.
The probe is touching the Prostomium
The probe is in the Mouth
The setae are structures we cannot visualize on the earthworm you are dissecting. If you rub your fingers along the ventral surface it feels like sand paper. This sensation is a result of the little bristle like structures called setae. These bristles give them traction as they move through their tunnels and along the surface of the soil.
Once you have gone over the external anatomy you are ready to take a look at the internal anatomy. Double check and make sure you have the worm dorsal side facing up. Notice how the scalpel is placed against the outside of the dissection tray. When the scalpel is not in use make sure the sharp edge of the blade is facing the metal.
Place image of starting the proceedure
Use your forceps to hold the worm and gently cut into the upper body wall. Be careful not to damage the internal organs just under the body wall. You are cutting through the muscles used for movement etc.
Once you have cut through the body wall you can carefully begin to pin down one side.
Here we are pinning the body wall.
Continue to cut and pin as you go. Cut and pin just past the clitellum.
Angle your pins so you have access to the internal organs.
Continue to cut and pin until you have both sides pined at an angle.
On the subsequent slides we will be looking closely at the internal parts of the earthworm
Look very careful and you can see two small white dots. These dots are the cerebral ganglion or brain of the earthworm.
The probe is pointing to the crop.
The probe is under the esophagus
The probe is touching the dorsal blood vessel which runs
dorsally on top of the intestine.
Dorsal blood vessel
All fivehearts cannot be seen in this diagram because of tissue that is covering some of the arches. You can see at least two and the third you can see through the tissue.
ventral blood vessel
ventral nerve cord
Do not forget you must study both this PowerPoint and my worm dissection video before you come to class and complete the earthworm dissection.
Good luck on your dissection !
Mr. Glassford’s Image Gallery
Biology Text: Miller and Levine