The Annelids and Arthropods Laboratory - Support Center

The Annelids and Arthropods
Laboratory

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Station 1A

The Annelids – Phylum Annelida

The annelids, or segmented worms, occupy marine, aquatic and aquatic habitats. Annelids are
coelomate protostomes and typically, are elongated ‘worm-like’ animals. By far, the most
distinguishing characteristic of annelids is metamerism, the serial repetition of body parts which
gives them a segmented appearance.

Metamerism is an important evolutionary landmark due to the significance of increasing
specialization of body parts. Each segment, called a metamere, can become specialized to
accomplish a specific task. Metamerism is not confined to annelids; arthropods and chordates are
also segmented.

Segmentation is seen not only in external morphology; internal segmentation of body organs is
also quite evident. The coelomic cavity is divided by septa (sing –septum). This allows the
hydrostatic pressures to be confined to individual compartments. Altering these hydrostatic
pressures in conjunction with the muscular contractions of individual metameres allows for more
coordinated movements. Adaptations like these allow annelids to be efficient swimmers, creepers
and burrowers.

Annelids have a complete digestive system that exhibits a great deal of regional specialization
and increased muscular support. A closed circulatory system and one or more pumping hearts
deliver blood to body tissues. Most exhibit pronounced cephalization and have a central nervous
system. Metabolic waste removal is accomplished by metanephridia located in each segment.
Gas exchange can occur through the epidermis or using specialized respiratory organs.

Examples of Annelids

The Leech

Leeches are common aquatic and marine ectoparasites. A leech’s segmented body undulates as it
swims in search of a suitable host. Once located, the leech uses its anterior oral sucker to firmly
attach to the host’s body. The host’s integument is perforated and the leech introduces an
anticoagulant called hirudin into the open wound.

Blood and body fluids flow from the host into the leech’s digestive tract. The stomach of a leech
forms an expansive crop. As the crop fills with blood, the leech expands like an accordion. Once
filled, the leech releases and swims to a suitable location where it will attach to the substrate
using its posterior caudal sucker to digest its meal.

Leeches have been used medically for centuries and are still an important pharmaceutical source
of the anticoagulant hirudin.

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Examine the leech under the microscope and sketch your observation in the space below.

Leech
Polychaete worm – The Clamworm, Nereis
The segmented polychaetes (“many bristles”) exhibit a higher degree of cephalization than
leeches. The clamworm, Nereis, is a marine polychaete which can be found in burrows in soft
mud, sand and debris. Clamworms feed on just about anything from detritus to small animals
using their pincher-like jaws to rip bits of flesh. Each body segment has a pair of lateral
parapodia (sing – parapodium) covered with many stiff bristles called seta. These parapodia help
the clamworm move through the substrate and provide additional surface area for respiration.
Examine the preserved specimens, the parapodium slide and label figure below.

Clamworm- Label metamere, jaws, parapodium, seta
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Review Questions
1. Describe metamerism.
2. Why is metamerism considered to be an evolutionary landmark?
3. List three ways that a segmented worm differs from a roundworm.
4. Name the internal structure which forms the partition between body segments.
5. Describe the feeding habits of a leech. How does hirudin aid in this process?

6. Name an organism possessing parapodia. What are these structures used for?
7. Would you expect a marine clamworm to escape predators by burrowing or swimming?
Why?

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Station 1B

Oligochaetes – The Earthworm, Lumbricus

Oligochaete (“few bristles”) worms have fewer setae associated with each segment. The
common earthworm, Lumbricus, prefers moist soil rich with organic matter on which it feeds.
Using muscular contractions, hydrostatic pressures and pairs of ventral seta for traction,
earthworms burrow into the soil and perform an important ecological service by aerating the soil.
Mucus produced by the glandular epidermis keeps the earthworm moist and allow for gas
exchange.

Earthworms are monoecious but must mate for cross-fertilization. Special reproductive segments
form the clitellum, a broad region located in the anterior one-third of the body. The clitellum
secretes thick mucus which will allow for the exchange of sperm and will ultimately encase the
fertilized eggs in a cocoon.

Sperm are produced in the testes and mature in the sac-like seminal vesicles which surround
each testis. The sperm are received during copulation by the balloon-like seminal receptacles of
another earthworm. Eggs from the ovaries are routed through the oviduct and out of the body.
Sperm from the seminal receptacles is released and fertilizes the eggs externally in the thick
mucus cocoon.

The mouth is located on the first segment and leads to the muscular pharynx. As food is
swallowed, muscular contractions of the digestive tract move the food to the esophagus and
stomach. The stomach of an earthworm is divided into two distinct regions; an expandable crop
for storage and a muscular gizzard. After the food is masticated by the grinding actions of the
gizzard, it is passed to the elongated intestine for digestion and absorption. Waste is eliminated
through the anus located on the terminal body segment.

The central nervous system of the earthworm includes a ventral nerve cord and a pair of
cerebral ganglia (the brain) located on the anterior, dorsal aspect of the pharynx. The closed
circulatory system includes a dorsal blood vessel, a ventral blood vessel, five muscular aortic
arches (the hearts) which pump the blood. Metabolic wastes are collected in the pairs of tubular
metanephridia located in each body segment and are excreted.

Observe the preserved earthworm specimens as well as the model and use the laminated sheets to
label the figures below.

Earthworm external anatomy – label mouth, body segment, seta, clitellum, anus 4

Earthworm cross-section – Label seta, coelom, dorsal blood vessel, ventral
blood vessel, epidermis, muscle, lumen of intestine

Earthworm internal anatomy, dorsal view – Label cerebral ganglia, pharynx, aortic arches, esophagus, crop, gizzard,
intestine, mouth, anus, dorsal blood vessel, testis, seminal vesicles, ovary, seminal receptacle, metanephridium,
clitellum
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Review Questions
1. A traveling salesman tells you he has a sure fire fishing bait that never fails to catch fish; he
will sell them to you for $10 a dozen. He states that he raises special earthworms and he carefully
selects the aggressive male earthworms to sell as this sure-fire bait. Why should you be skeptical
of this entrepreneur?

2. List the structures of the earthworm digestive tract starting with the most anterior.
3. Describe the difference between the crop and the gizzard.

4. Earthworms have a reduced degree of cephalization. How can you determine the head region
of an earthworm without opening the body cavity?

5. When earthworms mate, where does fertilization of the eggs occur?

6. In addition to providing moisture for gas exchange, what other function might you suggest for
the mucus secreted by the epidermis of an earthworm?

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Station 2

The Arthropods – Phylum Arthropoda

The arthropods (joint foot) represent the greatest diversity of species in the animal world with
well over a million different species. Arthropods can be found occupying every habitat on earth.

Arthropods are segmented (metamerism) and arising from these body segments are jointed
appendages. Unlike the annelids, whose external metamerism shows little true regional
specialization of appendages, the arthropods body segments are clearly fused into discrete
functional units called tagma (i.e. head, thorax and abdomen of an insect). With this tremendous
regional specialization, various appendages evolved to serve a great variety of functions including
sensory reception, equilibrium, locomotion feeding and reproduction.

Arthropods are covered by a hardened exoskeleton made of chitin. This non-cellular
exoskeleton is secreted by the epidermis and must be periodically shed by molting (ecdysis) as
the arthropod grows. The exoskeleton not only provides mechanical protection and support but
provides a rigid attachment for powerful muscles.

Arthropods are coelomate protostomes and have pronounced cephalization, a well-developed
central nervous system, complete digestive system and an open circulatory system. Respiratory
structures vary by group and include tracheal tubes, gills and book lungs. Most species are
dioecious, some are sexually dimorphic while others have complex social caste systems.

Arthropods may be classified into several lineages based upon the tagmata present, structure of
the appendages (as biramous with a fork at the distal end or uniramous with a single claw at the
distal end) and the number of antennae present.

Examples of Arthropods

The trilobites are thought to have been the
first arthropods. Trilobites were all marine
organisms and are well represented in the
fossil record. All trilobites were extinct by
the end of the Permian some 250 million
years ago.

Observe the trilobite fossils and compare to
the illustration.

Generalized trilobite

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The Chelicerates – horseshoe crabs, spiders, ticks and scorpions
Chelicerate arthropods have a body which is divided into two tagmata; the cephalothorax and
abdomen. The first pair of appendages is the chelicerae, which are feeding appendages adapted
for seizing and tearing food. The second pair of appendages is the pedipalps which are also used
for feeding. Four pairs of walking legs are used for locomotion. Chelicerates have no antennae.

Horseshoe crabs are marine bottom-dwellers found along the Atlantic coastline of North
America. They have a large protective dorsal carapace which covers the cephalothorax. A
pair of compound eyes allows for keen vision. The pincer-like chelicerae are used to catch
and manipulate food. The flap-like book gills are ventrally located and are for gas exchange.
The flapping motions of the book gills are also used as the horseshoe crab swims. A hard,
stiff telson or tail spine extends from the abdomen. This telson is used to anchor the animal
when burrowing or to right itself if turned over.

Horseshoe crab – Dorsal view, right; ventral, left. Label chelicerae, pedipalps, walking legs, telson,
compound eye, cephalothorax, abdomen, book gills

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Spiders are found in many different habitats. Spiders are well known for their predatory
behaviors associated with the construction of webs. Spinneret appendages located on the
abdomen control the delivery of silk. Many spiders construct huge webs; the common garden
spider may construct a web that is four-five feet in diameter. Not all spiders spin webs for prey
capture, but all spiders use silk from the silk glands to control prey or to enclose their eggs in
cases.

The chelicerae of spiders are modified into fangs and deliver paralyzing poisons to prey once
captured. Elongated pedipalps are used in food manipulation while the four pair of walking legs
are used for locomotion or to grapple with prey.

Black widow spiders are well known for their toxic venom and are some of the most easily
recognized spiders with the hourglass figure on the ventral side of the abdomen being a
prominent field identification marking.

Garden spider, left; Black widow, right.

Ticks are parasitic chelicerates. The chelicerae and pedipalps are used for burrowing and
attachment to a host’s integument. The deer tick (Ixodes) is known to be the vector of Lyme’s
disease, which is caused by a spirochete bacterium. The arthritic-like symptoms of Lyme’s
disease can cause major discomfort and if left untreated, can cause permanent damage to joints or
even death.

View the slide of the deer tick’s head and sketch you observation in the space below.

Deer tick

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Scorpions are predatory chelicerates that are common in arid to semi-arid habitats. The
chelicerae are modified to process food and their pedipalps have evolved into large pinchers used
to catch prey. Scorpions also have a modified telson, a stinger, used to deliver a venomous toxin
to subdue and begin digestion of their prey. This stinger may also be used in self-defense to
prevent unwanted advances from predators looking to make a meal of the scorpion.

Scorpion, dorsal view

The crustaceans – crabs, shrimp, crayfish and barnacles
The crustaceans are gill-breathing arthropods with biramous appendages. The body is divided
into the cephalothorax and the abdomen. Mandibles are used to masticate food and several pair
of maxillae located on the head are used in food handling. A large sheet of the exoskeleton
known as the carapace covers and protects the cephalothorax. Crustaceans use two pairs of
antennae for chemoreception, tactile sense and equilibrium. Most crustaceans are aquatic (i.e.
crayfish) or marine (i.e. lobster and blue crabs) although there are a few terrestrial examples (i.e.
sow bugs or pill bugs).

Crustaceans – goose barnacle, left; crayfish, right

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Review Questions
1. Name the earliest known arthropods.
2. What does the term ‘tagma’ mean?
3. Describe the difference between a biramous and a uniramous appendage.
4. Why must an arthropod undergo ecdysis?
5. Describe the use of a telson by a horseshoe crab.
6. Name the structures a horseshoe crab uses for respiration.
7. Spiders, horseshoe crabs and ticks belong to what sub-group of arthropods?
8. Name the appendages used by a spider to spin its silken web.
9. Name the vector of Lyme’s disease.

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Station 3

The myriapods – centipedes and millipedes

Myriapods typically have two tagmata; the head and the abdomen. A single pair of antennae
originates from the head and is used to provide a variety of sensory information. Each segment of
the abdomen bears uniramous jointed legs. Most myriapods are terrestrial and use tracheal tubes
for respiration. Spiracles, external openings to the tracheal tubes, are located along the
abdominal segments.

Centipedes are active terrestrial predators. They are often found under rocks, logs, and the bark
of trees. They are extremely agile and hunt other arthropods, worms and even small mammals.
The centipede’s body is dorso-ventrally flattened and single pair of walking legs arises from each
abdominal segment. Fangs are located near the mouth and deliver venom into their prey.

Millipedes are primarily herbivores or detritivores and often occupy similar habitats as
centipedes. Millipedes are more rounded than centipedes and often roll into a protective ball
when disturbed. Each abdominal segment bears two pairs of walking legs and these walking legs
move in rhythmic waves to move the millipede from place to place. This type of locomotion is
much slower and less agile as compared to a centipede.

Myriapods- millipede, left; centipede right

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Hexapods – The Insects

Insects are by far the most diverse group of animals, much of the diversity seen in arthropods is
due to insect diversity. Well over a million species of insects have been described and new
species are discovered all the time. Insects occupy many terrestrial and aquatic habitats. Insects
have also had profound effects on human society as many forms may be vectors of deadly
diseases, are annoying pests or beneficial pollinators of plants.

The body of an insect is divided into three tagmata; the head, thorax and the abdomen. The
appendages which arise from these tagma are uniramous. A single pair of sensory antennae
arises from the head and compound eyes allow for vision. The appendages around the mouth are
extremely specialized in the various groups of insects. Some, as in the butterfly, have mouth
appendages modified into hollow tubes for feeding on nectar deep in flower whorls. Others, like
grasshoppers and beetles have chewing mouth parts modified for tearing plant material or flesh.

Three pairs of walking legs (hexapod = ‘six foot’) and two pairs of wings arise from the thorax.
Again, a tremendous degree of specialization in these appendages is seen in the insect groups.
Grasshoppers have large hind legs for jumping and powerful wings for short, quick flights. The
ectoparasitic lice have lost their wings through the evolutionary process and their walking legs are
adapted to clinging to the hair or feathers of their host. Spiracle openings are also located on the
thorax and may be found laterally on the abdominal segments as well. The abdominal segments
may house the genitalia, external appendages used during copulation. Insects are dioecious and
most are sexually dimorphic.

Insect life cycles follow two basic patterns: incomplete metamorphism and complete
metamorphisms (metamorphic = many forms). Incomplete metamorphism is seen in insects
such as grasshoppers where the egg hatches into larvae which resembles the parent but is much
smaller, may lack fully developed wings and is sexually immature. Gradually, the larvae grows
(and molts) and increases size until adulthood. In complete metamorphism, such as in a
butterfly, the change is much more dramatic between the stages. The egg hatches into larvae,
which oftentimes does not resemble the adults at all. After several larval stages, a pupal stage
occurs where the final changes are made in a cocoon or case.

In some insects, there is a well-defined social structure. Bees, ants, and termites are all known to
form large colonies which consist of reproductive individuals (queens and drones) and workers.
The workers do the majority of the construction of the hive or mound as well as the collection of
food and provide protection.

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Insect Diversity - Common Insects

(Illustrations not to scale)

Grasshopper – Label head, thorax, abdomen, antennae, spiracles, wings, walking legs, compound eye

Gulf Coast Fritillary life cycle – label larva (caterpillar), pupa in cocoon, feeding adult

June beetle Worker ant Crab louse
Fruit fly 14

Review Questions
1. Name the two tagmata of a centipede.
2. What are spiracles?
3. Name a predatory myriapod.
4. How many walking legs are there on a millipede with 50 abdominal segments?
5. Which tagma do the wings arise from in a hexapod?
.
6. Name a social insect species.

7. How is the body of a crab louse adapted as a mammalian ectoparasite?

8. Name an insect that exhibits incomplete metamorphism.

9. Name an insect which exhibits complete metamorphism.
10. Chose one of the insect specimens you have looked at today and describe below how the
locomotive appendages and feeding appendages are adapted for a particular lifestyle.
.

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Class Activity

Crayfish Anatomy and Specialization of Appendages

You will need a dissection pan, two dissection probes (needles), a pair of forceps and scissors to complete the exercise.
Obtain a two crayfish (male and female) from your instructor and place in the dissection pan.
Using the laminated sheets and information below, identify the external structures of your crayfish and label the illustrations.

The crayfish’s body is divided into the cephalothorax and the abdomen. Like most crustaceans,
the dorsal aspect of the cephalothorax is covered by a hardened sheet of exoskeleton called the
carapace. You may notice the transverse cervical groove which marks the location where the
head and thorax are fused together. If you carefully lift the posterior end of the carapace, you can
see the feather-like gills which are attached to the larger appendages.

The anterior portion of the carapace forms the pointed rostrum which covers and protects the
eyes and the base of the antennae. There are two pairs of antennae; a smaller forked pair of
antennules and a larger pair of antennae. Both provide sensory information such as taste, smell
and equilibrium.

Surrounding the mouth are a pair of serrated mandibles used for mastication of food. Posterior
to the mandibles are two pairs of maxillae, small flat appendages used in food handling and water
circulation under the carapace and across the gills. Posterior to the maxillae are three pairs of
maxillipeds which assist with food handling and taste.

The crayfish has five pairs of walking legs. The first pair, the chelipeds, are enlarged to form
pinchers used in prey capture and defense. The remaining four pairs of walking legs are used in
locomotion. The three pair of maxillipeds and the five pair of walking legs each have a gill
associated with their bases under the carapace.

The abdominal segments have appendages in the ventral sides. Each segment has a pair of
swimmerets which are used in locomotion and reproduction. In male crayfish, the first two pairs
(anterior) are enlarged and are known as copulatory swimmerets. They are used to transfer
sperm to the seminal receptacle of the female crayfish during copulation. After fertilization and
egg production, the female uses her swimmerets to carry the eggs and young crayfish once they
emerge.

The anus opens on the ventral side of the telson and is flanked laterally by a pair of uropods.
The uropods and telson form a fan which is used to produce rapid backwards locomotion as the
powerful abdominal segments contract.

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