Dissection: The Earthworm

Lab Exercise23bDissection: The EarthwormObjectivesIntroduction- To learn some of anatomical structures of the earthworm.- To be able to make contrasts and comparisons ofstructures between different animal phyla as additionalorganisms are observed.- To deduce the adaptive significance of differences in thestructures of animal phyla as additional organisms arestudied.The following exercise allows you to examine some of themorphological and anatomical structures of a memberof the phylum Annelida. This phylum includes more than9,000 species. Members of this phylum are characterized by their segmented bodies, a well developed coelom,closed circulatory system and a well developed centralnervous system with the sensory organs concentrated atthe anterior end of the animal.The phylum is divided into three classes; the Polychaeta(the polychaete worms), the Oligochaeta (which includesthe earthworms), and Hirudinea (the leeches). Here youwill examine the external features and anatomy of theearthworm, genus Lumbricus. To begin this exercise, go tothe Diversity section of the BiologyOne DVD. Select Dissections and then, after the introduction screen, select theEarthworm from the list of organisms.In the dissection exercises, you will be asked to examinethe organisms and learn something of their individualanatomy. Equally important is a comparison of the anatomical structures of between organisms, noting how theyare similar, how they differ, and how their differences maybe adaptive to the different life styles of these organisms.Ramp. Copyright 2012 by F.one Design. All rights reserved.23b 1

Activity 23b.1External FeaturesActivity 23b.2Internal AnatomyBefore examining the internal anatomy of the organismyou should examine its external morphology. From thisyou can observe a number of features that show adaptations to the way the organism lives and clues to its internalstructures.To dissect the earthworm, you would lay it out in a dissecting tray, dorsal side up, and hold it in place by pinningdown its prostomium at the anterior end and also near itsposterior end. You would make a shallow incision behindthe clitellum just to one side of the mid-dorsal line. Cut justdeep enough to cut through the skin but avoid cutting intothe internal organs. Then, insert one tip of a pair of scissors into the incision, lift the skin away from the internalorgans and carefully cut toward the anterior end of theworm. When you have completed this dissection, revealthe internal organs by pinning the skin back on either sideof the worm. Click on the forward arrow in the lower rightto complete this dissection of the earthworm.Examine the external features of the earthworm. Note thesegmented body of the earthworm. The anterior end canbe recognized by noting the location of the clitellum. Thisis a lighter colored, swollen region that covers severalsegments near the anterior end of the worm. During reproduction, the clitellum slips off the anterior end of the wormand forms a cocoon for the development of fertilized eggs.You may also be able to find the prostomium, a projectionthat overhangs the mouth. The anus is located at the lastsegment of the posterior end.If you were to rub your finger along the sides of the wormyou would feel short bristles. Except for the first and last,each segment has a two pair of bristles or setae extending from its side, pointing toward the worm’s back. Therows of these bristles are located on the ventral side of theworm. What function could they serve?If you could closely examine the segments you may alsosee pores in the skin of the worm. Each segment has apair of excretory pores that remove metabolic wastes fromthe worm. In addition, in segments 14 and 15 are pairsof pores for the male and female reproductive systemrespectively (the location of these pores varies some indifferent species). The female pores are relatively small,located on segment 14. The male pores located on segment 15 are surrounded by swollen ‘lips’.After studying the external features of the earthworm,label the illustration located in the Results Section.Ramp. Copyright 2012 by F.one Design. All rights reserved.As you examine the internal organs of the earthworm, youshould note the lack of organs for respiration. The earthworm relies on diffusion across the skin for gas exchange.Given the relative size and diameter of these organisms,how effective do you think this method of respiration is inthe earthworm relative to the other organisms you have/will study? Do you see any mechanism that increases theefficiency of gas exchange in the earthworm? What is it?The earthworm has a closed circulatory system. Thismeans that the blood is contained within blood vessels.There is a large dorsal blood vessel and a large ventralblood vessel that run the length of the worm’s body. Ineach segment a pair of vessels extends laterally fromthese main blood vessels. In segments 7 through 11, thelateral extending blood vessels have become enlargedand are much more muscular. These are the 5 pairs of‘hearts’ that pump the blood through the worm’s circulatory system.The earthworm’s digestive system forms a tube extendingthe entire length of the body from the mouth located in thefirst body segment to the anus located in the body’s lastsegment. Near the anterior end this digestive tube has regions that have specialized to perform different functions.The first part of the digestive system past the mouth is thesmall buccal cavity. This leads to an enlarged cavity, thepharynx located in segments 3 through 5. The pharynx isattached to the body wall by muscles that create a sucking23b 2

action when they contract. From the pharynx, food passesthrough the esophagus and empties into the thin-walledcrop. Here food is stored until it is ready to pass into themuscular gizzard that grinds the food into small particles.Around segment 19 the gizzard joins with the intestinethat runs the rest of the length of the worm’s body until itreaches the anus at the terminal segment. In the intestine, the chemical digestion and absorption of food takesplace. A fold on the dorsal side of the intestine called thetyphlosole increases the inner surface area of the intestine improving the efficiency of food absorption. What isanother way an earthworm can increase the intestine’ssurface area so that it can absorb more nutrients from thematerial passing through its digestive system?Located in segment three are two large ganglia that wraparound the digestive tract just anterior to the pharynx.These make up the brain of the earthworm. A ventralnerve cord runs the length of the organism.Ramp. Copyright 2012 by F.one Design. All rights reserved.Earthworms are monecious. Each individual producesboth sperm and eggs although the eggs must be fertilized by sperm from a different worm. The two ovaries arelocated in segment 12. When the eggs are produced theytravel through an oviduct that exits the body in segment14. Two pairs of testes are located in segments 10 and 11.After being produced in the testes, the sperm mature andare stored in the large seminal vesicles. These are locatedin segments 9 through 13. When copulation occurs, thesperm travel through a tube (the vas deferens) that existthe body at segment 15. During copulation, the spermare stored by the receiving worm in two pairs of sacs, theseminal receptacles. These are located in segments 9and 10. When the worm’s eggs are mature and it is readyto release these, the clitellum slips forward, first receivingeggs as it passes the female pores and then sperm as itpasses the pores to the seminal receptacles.Study the anatomy of the earthworm and label the illustration in the Results Section.23b 3

Lab Exercise23bNameResults SectionActivity 23b.1External FeaturesLabel the illustration below3.2.prostomiumfemale poreopenings to seminalreceptacles1.Ramp. Copyright 2012 by F.one Design. All rights reserved.23b 44.

Activity 23b.2Internal AnatomyLabel the illustration below5.1.6.7.2.8.3.(the black line)4.Ramp. Copyright 2012 by F.one Design. All rights reserved.23b 5

seminal receptacles. These are located in segments 9 and 10. When the worm’s eggs are mature and it is ready to release these, the clitellum slips forward, first receiving eggs as it passes the female pores and then sperm as it passes the pores to the seminal receptacles. Study the anatomy