Earthworm Dissection
The following is a
classification of a species in the earthworm family Lumbricidae. This common
species is Lumbricus terrestris also known as the night crawler
or dew worm.
|
Phylum
- |
Annelida |
Objectives:
• Describe the
appearance of various organs found in the earthworm.
• Name the organs that make up various systems of the earthworm.
Materials:
Safety goggles,
dissecting pins, gloves, forceps, lab apron, scissors, paper towel, scalpel,
water, dissecting probe, preserved earthworm, hand lens, dissection tray.
Purpose:
In this lab, you will
dissect an earthworm in order to observe the external and internal structures
of earthworm anatomy.
Background:
Among the most
familiar invertebrate animals are the earthworms, members of the phylum Annelida.
The word annelida means "ringed" and refers to a series of rings or
segments that make up the bodies of the members of this phylum. Internally, septa,
or dividing walls, are located between the segments. There may be more than 100
segments in an adult worm. The clitellum is a swelling of the body found
in sexually mature worms and is active 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. 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. The pumping organs of the circulatory system are five aortic
arches. Circulatory fluids travel from the arches through the ventral
blood vessel to capillary beds in the body. The fluids then collect in the dorsal
blood vessel and reenter the aortic arches. The earthworm takes in a
mixture of soil and organic matter through its mouth, which is the beginning of
the digestive tract. 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. The
mixture that the earthworm ingests is ground up in the gizzard. In the intestine,
which extends over two-thirds of the body length, digestion and absorption take
place. Soil particles and undigested organic matter pass out of the worm
through the rectum and anus. The nervous system consists of the ventral
nerve cord, which travels the length of the worm on the ventral side, and a
series of ganglia, which are masses of tissue containing many nerve
cells. The nerve collar surrounds the pharynx and consists of ganglia
above and below the pharynx. Nervous impulses are responsible for movement and
responses to stimuli. Each segment contains an enlargement, or ganglion, along
the ventral nerve cord. Excretory functions are carried on by nephridia, which
are found in pairs in each body segment. They appear as tiny white fibers on
the dorsal body wall. The earthworm has no gills or lungs. Gases are exchanged
between the circulatory system and the environment through the moist skin.
Procedure:
1. Put on
safety goggles, gloves, and a lab apron.
2. Place
earthworm in the dissecting tray & rinse off the excess preservative.
Identify the dorsal side, which is the worm’s rounded top, and the ventral
side, which is its flattened bottom. Turn the worm ventral side up, as
shown in the diagram below.
3. Use
a hand lens as you observe all parts of the worm, externally and internally.
Find the anterior end by locating the prostomium, which is a
fleshy lobe that extends over the mouth. The other end of the worm’s body is
the posterior end, where the anus is located.
4. Locate
the clitellum, which extends from segment 33 to segment 37. Look for the
worm’s setae, which are the minute bristle-like spines located on every
segment except the first and last one.
5. Refer
again to the diagram of the ventral view of the worm to locate and identify the
external parts of its reproductive system. Find the pair of sperm grooves
that extend from the clitellum to about segment 15, where one pair of male
genital pores is located. Look also for one pair of female genital pores
on segment 14. There is another pair of male genital pores on about segment 26.
Try to find the two pairs of openings of the seminal receptacles on segment
10. Note: These openings are not easy to see.
6. Turn
the worm dorsal side up. Using a scalpel and scissors, make a shallow incision
in the dorsal side of the clitellum at segment 33. CAUTION: Scalpels and
scissors are very sharp. Report any cuts to your teacher. Using the
forceps and scalpel, spread the incision open, little by little. Separate each septum
from the central tube using a dissecting needle, and pin down each loosened bit
of skin. Continue the incision forward to segment 1.
7. Use
the diagram below to locate and identify the five pairs of aortic arches,
or hearts. Then find the dorsal blood vessel. Look for smaller blood
vessels that branch from the dorsal blood vessel.
8. Locate
the digestive tract, which lies below the dorsal blood vessel. Refer to the
diagram above to locate the pharynx, esophagus, crop, gizzard, and intestine.
9. To
find organs of the nervous system, push aside the digestive and circulatory
system organs. Use the diagram below to locate the ventral nerve cord.
Trace the nerve cord forward to the nerve collar, which circles the pharynx.
Find one pair of ganglia under the pharynx and another pair of ganglia
above the pharynx. The ganglia above the pharynx serve as the brain of the
earthworm.
10. The
worm’s excretory organs are tiny nephridia. There are two in every
segment. Use the preceding diagram to locate some nephridia.
11. Use
the diagram below to locate and identify a pair of ovaries in segment
13. Look for two pairs of tiny testes in segments 10 and 11. To find these
organs, you will again have to push aside some parts already dissected.
12. Dispose
of your materials according to the directions from your teacher.
13. Clean up your work area and wash your
hands before leaving the lab.
Worksheet
1. What is the name of the pumping organs of an earthworm?
2. Trace the parts of the digestive tract through which food passes.
3. Which parts of the earthworm serve as its brain? How are these
parts connected to the rest of the body?
4. Which of the parts of the worm’s body that you saw are included in the excretory system?
5. How can you find out whether an earthworm eats soil?
6. Among the earthworm’s structural
adaptations are its setae. How do you think the earthworm’s setae make it well adapted to its habitat?
7. How is the earthworm’s digestive
system adapted for extracting relatively small amounts of food from large
amounts of ingested soil?
8. Your dissection of the earthworm did not go beyond segment 32.What will
you observe if you dissect the remainder of the worm to its posterior end?
9. On a separate piece of paper, draw and label the parts of the earthworm
you observed, and color code the systems. Use green for the reproductive
system, yellow for the digestive system, blue for the excretory system, and red
for the nervous system.
10. During mating, two earthworms exchange sperm. Fertilization is external,
and cocoons are produced from which the young eventually emerge. Refer again to
steps 5 and 11, where you located the earthworm’s reproductive organs. Use a reference to identify
the role of each organ in the reproductive process of the earthworm. On a
separate paper, summarize your findings.
FAQ’s
Q. How long do dew worms live? A. Dew worms can live for approximately
six and a half years.
Q. How many young are
produced per year? A. It is estimated that sexually mature
dew worms (about one year old) produce about two cocoons per year with 1-2
young each (more research under field and laboratory conditions required).
Q. Do earthworms have eyes? A. They do not have eyes but they do possess light- and
touch-sensitive organs (receptor cells) to distinguish differences in light
intensity and to feel vibrations in the ground.
Q. How do earthworms breathe? A. Earthworms respire through their skin, and therefore require
humid conditions to prevent drying out. They coat themselves in mucus to enable
the passage of dissolved oxygen into their bloodstream.
Q. Can earthworms smell?A. Worms have specialized chemoreceptors
or sense organs ("taste receptors") which react to chemical stimuli.
These sense organs are located on the anterior part of the worm.
Q. What do earthworms eat and how much can
they eat in one day? A. Earthworms derive their nutrition from
many forms of organic matter in soil, things like decaying roots and leaves,
and living organisms such as nematodes, protozoans, rotifers, bacteria, fungi.
They will also feed on the decomposing remains of other animals. They can
consume, in just one day, up to one third of their own body weight.
Q. Can earthworms freeze?
A. Like all
invertebrates their body processes or metabolism slow down with falling
temperatures. They will hibernate at near freezing temperature. If frozen they
will die. They react to advancing colder winter weather by burrowing deep (up
to two meters) in the soil to avoid the extreme cold.
Q. What is the "bump" in the middle of the
earthworm? A. The bump is the clitellum, the saddle
shaped swollen area 1/3 of the way back containing the gland cells which
secrete a slimy material (mucus) to form the cocoon which will hold the worm
embryos.
Q. How can you determine if an earthworm is sexually
mature? A. If the worm has a clitellum, it is
sexually mature.
Q. Can earthworms lose their clitellum? A. The answer is yes! During periods of drought, when soils dry
up, some species of earthworms do in fact temporarily lose all secondary sexual
characters such as the clitellum. When conditions become favorable, it comes
back. The clitellum can also disappear at the onset of old age or senescence.
Q. How do earthworms mate?
A. Earthworms are hermaphroditic meaning
each worm has organs of both sexes. The male gonopores are usually within the
first 12-15 segments, and the female gonopores are further back, close to the
clitellum (the swollen area in adult worms). One worm has to find another worm
and they mate juxtaposing opposite gonadal openings exchanging packets of
sperm, called spermatophores. Some species also appear to be either
parthenogenetic (females producing all females, "virgin birth") or
may be able to self-fertilize.
Q. How are cocoons
produced? A. The clitellum produces a mucous sheath
and nutritive material, and as the sheath slides forward, it picks up ova from
the earthworm's ovaries then packets of sperm that had been transferred to the
worm from another worm during mating. As the sheath slides off the worm’s head,
the ends are sealed to form the cocoon. Initially, the cocoon is quite soft but
soon after it is deposited in the soil it becomes slightly amber in color,
leather-like and very resistant to drying and damage. The ova within each
cocoon are fertilized, and the resulting embryos grow inside the sealed unit,
much like a chick developing inside an egg. When the embryos have consumed all
the nutritive material, they completely fill the lemon shaped cocoon and are
ready to hatch out one end.
Q. How long does it take worms to hatch? A. Young worms hatch from their cocoons in three weeks to five
months as the gestation period varies for different species of worms.
Conditions like temperature and soil moisture factor in here...if conditions
are not great then hatching is delayed.
Q. How many young worms are produced per year? A. Earthworms can produce between 3 and 80 cocoons per year
depending on the species. The deeper-dwelling species don't have to produce as
many cocoons because they are protected much better from predation than surface
dwelling species which tend to produce many more cocoons. The number of
fertilized ova or eggs within each cocoon ranges from one to twenty. This
depends on the species and also factors such as nutrition of the adults laying
them and environmental conditions with soil moisture being most important.
Usually, though, only few to several young worms will ever successfully emerge
from each cocoon.
Q. How long does it take earthworms to mature? A. Worms mature in 10 - 55 weeks depending on the species.
Q. Can different species of worms mate creating a hybrid
worm? A. No, this does not usually occur;
hybrids can usually only occur between very closely related species and their
offspring would likely be infertile.
Q. How long do earthworms live? A. Earthworm longevity is species dependent. Various
specialists report that certain species have the potential to live 4-8 years.
In protected culture conditions (no predators, ideal conditions) individuals of
Allolobophora longa have been kept up to 10 1/4 years, Eisenia foetida for 4½
years and Lumbricus terrestris for 6 years.Worms continue to grow once they
reach sexual maturity but once at this stage there is a much slower increase in
weight until the disappearance of the clitellum indicates the onset of old age
or senescence. During this period there is a slow decline in weight until the
death of the worm.
Q. How do earthworms move?
A. Earthworms
have bristles or setae in groups around or under their body. The bristles,
paired in groups on each segment, can be moved in and out to grip the ground or
the walls of a burrow. Worms travel through underground tunnels or move about
on the soil surface by using their bristles as anchors pushing themselves
forward or backward using strong stretching and contracting muscles.
Q. What characteristics are used to identify
earthworms? A. The external body characters used in
identifying different species of earthworms are: the segmental position of the
clitellum on the body, body length, body shape (cylindrical or flattened),
number of body segments, type and position of body bristles or setae, the
description of the tongue-like lobe, the prostomium, projecting forward above
the mouth, type of peristomium or first body segment, external position and
morphology of genital apertures or opening and type of glandular swellings on
the clitellum. The shape and the relationship of various internal organs are
also used to identify some species of worms.
Q. What enemies do earthworms have? A. Snakes, birds, moles, toads and even foxes are known to eat
earthworms. Beetles, centipedes, leeches, slugs and flatworms also feed on
earthworms. Some types of mites parasitize earthworm cocoons and the cluster
fly (Pollenia rudis) parasitizes worms of the species Eisenia rosea.
Q. Can earthworms regenerate themselves? A. Yes, but only the front or head end of the earthworm will
survive and the amputated tail portion will die. This remaining front portion
must also be long enough to contain the clitellum and at least 10 segments
behind the clitellum. This makes up about half the length of the worm. The new
posterior segments grown will be slightly smaller in diameter than the original
segments and sometimes a bit lighter in color.
Q. How can you distinguish the head of an earthworm from the
tail? A. The head of the worm is always located
on the end of the worm closest to the clitellum and has some differentiated
structures if you can view with magnification. Even though worms can move both
frontward and backward they tend to travel forward more. Place a worm on a
rough piece of paper and observe which direction it travels. They usually
extend their "head" first when crawling.
Q. How do earthworms obtain their food? A. Earthworms possess very strong mouth muscles - they do not
have teeth. Dew worms or nightcrawlers often surface at night to pull fallen
leaves down into their burrow. When the leaf decomposes or softens a little
they pull small bits off at a time to munch on. They also "swallow"
soil as they burrow and extract nutrients from it.
Q. How big do earthworms get? A. Size depends on the species of worm, it’s age, diet and
environmental conditions like moisture, temperature and soil conditions.
Lumbricus terrestris (Nightcrawler, Dew worm) is one of North America’s largest
and ranges in size from 9-30 cm with a diameter of 6-10 mm. The largest L.
terrestris we've collected was close to 30 cm long (stretched out), weighed
11.2 g and was collected in a no-till, soybean field in Ontario up near
Georgian Bay, Ontario. The largest tropical species (Glossoscolex and Megascolides)
are up to 120 cm long and the largest in the world are some Australian forms
which may reach 300 cm in length. Bimastos parvus (American bark worm) is quite
small at less than 2 cm long.