Earthworm DissectionLab

Learning Targets:• Identify the various organs/structures found in the common earthworm.

• Compare a worm’s physical makeup to your own human layout.

Tahoma Jr. High8th Grade ScienceMaple Valley, WA

Procedure:In this lab, you will dissect an earthworm in order to observe the external and internal anatomy (parts) of earthworm anatomy (as well as some physiology, the “function” of the parts).

Materials List:Dissection tray with wax bottom (do NOT damage the wax any more than necessary by just putting a few pins in it), dissecting pins, forceps (tweezers), scissors, paper towel, scalpel (be VERY careful with the scalpel – DO NOT PUSH DOWN HARD ON THE SCALPEL! – only use gentle slicing motions), dissecting probe, preserved earthworm.

Background: Among the most familiar invertebrate (no backbone) animals are earthworms. Members of the phylum Annelida (the word annelida means "ringed" and refers to the ring-like segments that

make up the bodies of the members of this phylum). There may be >100 segments in an adult worm.

Setae (little bristle hairs) help the worm move and also “hold on” when predators try to pull them out of the ground.

The wide band (clittelum) #1 in the picture on left, shows which end is the head.

There are many species of earthworms worldwide. Some are only a few mm in

length and the largest may be 4 meters (~12 feet) long and are found in Australia.

Reproduction: The clitellum is a swelling of the body found in sexually mature worms and helps in the formation of an egg

capsule, or cocoon. Eggs are produced in the ovaries and pass out of the body through female genital pores. Sperm are produced

in the testes and pass out through tiny male genital pores.

Earthworms are hermaphrodites (both male and female) but they cannot fertilize their own eggs.

During mating, sperm from one worm travel along the sperm grooves to the seminal receptacles of another worm. Fertilization of the eggs takes place

outside the body as the cocoon moves forward over the body, picking up the eggs of one worm and the

sperm of its mate.

Worm eggs compared to a pin head

Now that you’ve looked at the outside a bit, let’s look at the inside. Use your scalpel or scissors (scissors work surprisingly well if they have one sharp point and cut well) and make a long incision (cut)

from front to rear. Do so WITHOUT squishing/pushing hard or you’ll destroy the organs underneath. Pin the worm down as you go so

you can see inside easier.

(front)

(rear)

OVERALL LAYOUT:

outside, head (inside) and cross-section

Circulatory: Worm’s don’t have a true “heart”, instead having five small modified arteries called aortic arches.

Circulatory fluids travel from the arches through the ventral (bottom) blood vessel to capillaries (very

small blood vessels at the cells), then collect up in the dorsal (top) blood vessel to complete the circuit.

(front)

(rear)

Nervous: the nervous system consists of the ventral nerve cord, which travels the length of the worm on the ventral (bottom) side, and a series of ganglia, which are masses of many nerve cells. The nerve collar surrounds the pharynx and consists of ganglia above and below the

pharynx.(these will be very hard to see in a dissection)

Excretory system: (getting rid of wastes) are carried out by nephridia, which are found in pairs in each

body segment (also very hard to see).The earthworm has no gills or lungs – they breathe

(exchange gases) through the moist skin.

Digestive system: earthworms eat DIRT! It takes in a mixture of soil and organic matter through its mouth. The mixture enters the pharynx, which is located in segments 1–6. The esophagus, in segments 6–13, acts as a passageway between the pharynx and the

crop. The crop stores food temporarily.

(front)(rear)

It then moves tothe gizzard whichgrinds up thefood. Next, theintestine, (whichextends overtwo-thirds ofthe body length),does the digestionand absorption.Soil particles andundigested organic matter pass out of the worm through the rectum and anus.

Earthworm castings (poop) makes great fertilizer for plants

Not only are worms good for fertilizing the soil, but they also break it up allowing water to soak through more easily – and the tunnels also allow oxygen to get into the ground, which the roots need to be healthy. You often see farmers

“turning the soil” which does the same thing these little “earthmovers” do as part of their life of “eating dirt”.

Healthy ecosystems have lots of earthworms in the soil.(remember this for the Biosphere 3 unit at the end of the year!)

Try to compare the worm’s anatomy to your own human anatomy in the table on your paper.

What are three digestive system differences?What is different in the skeletons of each?How are the circulatory systems different?

A difference in how each do respiration/breathe is…?

When you are done, wrap ALL WORM PARTS IN PAPER TOWEL and dispose in the garbage can.

Rinse of all instruments, put a fresh piece of paper towel on the tray – then the equipment – and return

as you found it.

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