ZOOLOGY – SEGMENTED WORMS (Phylum Annelida)

THINGS YOU NEED TO KNOW FROM THIS CHAPTER…

1. How many species belong in this phylum & where do most of them live?
2. What is metamerism? What are somites, metamers, annuli, and septa?
3. What are setae?
4. What are the prostomium and pygidium? Where do new segments grow?
5. What lines a coelom?
6. How do annelids move?
1. What happens when longitudinal muscles contract?
2. What happens when circular muscles contract?
3. What is peristalsis?
7. Know the following about class Polychaeta…
1. How many species?
2. Where do you find them?
3. What are parapodia?
4. What do sedentary polychaetes do for a living?
5. What do errant polychaetes do for a living?
6. What are nuchal organs and statocysts?
7. How do they reproduce?
8. What are some examples of polychaete animals?
8. What type of animals belong to Class Clitellata?
9. Know the following about Subclass Oligochaeta…
1. How many species?
2. Where do you find them?
3. Why are earthworms important?

i. Why do they move deep and shallow in the soil?

4. What are scavengers?
5. What do calciferous glands, crop, and gizzard do?
6. How do earthworms pump blood?

i. Do they have a closed circulatory system?

7. Do they have specialized sense organs (like eyes)?
8. How do earthworms reproduce?
9. Know the following about Subclass Hirudinea…
1. Are all leeches parasitic?
2. How do they reproduce?
3. Do they have setae?
4. Are leeches ever beneficial?

e. Can you see segments in leeches?

I. Compared and Contrasted to Phylum Mollusca.

A. They are like the molluscs in that they…

1. are eucoelomate animals.

2. have specialized sense organs.

3. have a closed circulatory system like the cephalopods.

4. are considered closely related to mollusks and primitive

arthropods by most biologists.

B. They are more complex than the molluscs in that they…

1. show primitive metamerism.

2. have fleshy parapodia (suggesting paired appendages of more

complex animals).

II. Characteristics of Phylum Annelida (the segmented worms)

A. Phylum Annelida includes about 15,000 species of segmented worms

(2/3 of which are ______________________________________ worms).

1. Class Polychaeta - mainly ____________________ and

usually ____________________.

2. Class Clitellata – the earthworms & their relatives & the

leeches.

a. Subclass Oligochaeta - mostly ____________________

or ____________________, some are parasitic, a few are

marine or live in brackish water.

b. Subclass Hirudinea - ____________________ have

developed both parasitic and predatory adaptations.

1) mainly in freshwater, a few are marine.

B. Annelids are true coelomates

C. The nervous system is more centralized and the circulatory system is

more complex than in other worms.

D. Segmentation

1. ____________________ would increase burrowing efficiency;

this would favor evolution of the nervous system.

2. Body segments are marked by circular grooves called

____________________.

3. Metamerism is the repetition of organs in segments called

metameres or ____________________.

4. Walls, called ____________________, separate segments.

E. Except for leeches, annelids have tiny chitinous bristles called

____________________.

1. Short setae anchor a segment in an earthworm so it prevents

slipping backward.

2. Long setae help aquatic worms swim.

III. Body Plan of Annelids

A. Body Wall

1. The anterior tip is the ____________________;

2. The terminal portion bearing the anus is the

____________________.

a. new metameres form just in front of the pygidium; thus

the newest segments are at the posterior.

3. Strong circular and longitudinal muscles underlie the body wall.

4. The surface is covered with an epidermis and a thin outer layer

of non-chitinous ____________________.

B. Coelom

1. The coelom develops embryonically as a split in

____________________ on each side of the gut

2. ____________________ (mesodermal epithelium) lines the body

wall and forms dorsal and ventral mesenteries.

a. Peritonea of adjacent segments meet to form the septa.

b. The gut and longitudinal blood vessels extend through the

septa.

3. Hydrostatic Skeleton

a. Except in leeches, the ____________________ is filled

with fluid and serves as a hydrostatic skeleton.

b. The fluid volume remains constant, therefore contraction

of longitudinal muscles causes the body to

____________________ and ____________________.

c. Contraction of ____________________ muscles causes

the body to narrow and lengthen.

d. Alternate waves of contraction, or

____________________, allow efficient burrowing.

e. Swimming annelids use undulatory movements.

IV. Classes

A. Class Polychaeta - the largest class of annelids with more than 10,000

species, mostly marine, varying from 1 mm to 3 meters long.

1. Polychaetes have paired appendages called

____________________ that are on most segments.

a. Parapodia help crawl, swim, and anchor the worm in a

tube.

b. Usually the parapodia are the chief

____________________ organ although the worm may also

possess gills.

2. They have no clitellum.

3. Polychaetes are an important part of marine food chains.

4. Sedentary polychaetes are mainly ____________________.

a. Sedentary polychaetes feed on suspended particles or

particles in sediment.

5. Errant polychaetes may be free-moving, burrow or crawl.

a. Errant polychaetes are ____________________ or

____________________.

6. Nervous System and Sense Organs

a. Dorsal cerebral ganglia connect to subpharyngeal ganglia

by a circumpharyngeal commissure.

b. A double ventral nerve cord runs the length of the worm

with ganglia in each metamere.

c. Sense organs include ….

1) eyes - vary from simple eyespots to well-

developed image-resolving eyes similar to mollusk

eyes.

2) ____________________ organs - ciliated sensory

pits that are probably chemoreceptive

3) ____________________ - Some burrowing and

tube-building polychaetes use statocysts to orient

their body

7. Reproduction and Development

a. In contrast to clitellates, polychaetes have no permanent

sex organs and are ____________________.

b. Gonads appear as simple temporary swellings of the

____________________.

c. Gametes are shed into the coelom and exit by gonoducts,

metanephridia or rupturing of the body.

d. Fertilization is external and the early larva is a

____________________.

8. ____________________ Worms: Nereis - errant polychaetes

that live in mucus-lined burrows near the low tide level or mark.

a. They wiggle out of hiding places at night to search for

food.

b. Clam worms feed on small animals, other worms and

larval forms.

9. ____________________ Worms

a. Their flattened bodies are covered with broad scales.

b. Some are large, all are ____________________ and

some are ____________________ (they live in the burrows

of other animals, to their benefit, without harming the other

animal).

10. ____________________ have hollow, brittle setae that contain

poisonous secretions; they feed on cnidarians.

Fireworm

Fanworm

11. ____________________ unfurl tentacular crowns to feed; food

is moved from radioles to the mouth by ciliary action.

12. The ____________________ worm lives in a U-shaped tube;

modified segments pump water through the tube.

B. Class Clitellata - the earthworms & their relatives & the leeches.

1. Subclass Oligochaeta - Over 3000 species occur in habitats from

soil to freshwater

a. Earthworms

1) Earthworms burrow in moist, rich soil; they emerge

at night.

2) In wet weather they stay near the surface; in dry

weather they burrow deep and become dormant.

3) Earthworms have an important role in churning the

soil, mixing materials and adding nutrients.

b. Form and Function of oligochaetes

1) In most earthworms, each segment bears four

pairs of chitinous ____________________

2) Each seta is a bristlelike rod set in a sac and

moved by tiny muscles.

3) Setae anchor segments during _______________.

c. Nutrition - Most are ____________________, feeding on

decayed organic matter, leaves, refuse, etc.

1) Food is moistened by the mouth and drawn in by a

sucking action of the muscular ________________.

2) Soil calcium produces a high blood calcium level;

________________________________________

along the esophagus keep down the calcium ion

concentration in the blood and are ion-regulatory

rather than digestive in function.

3) Food passes the ____________________ to be

stored in a thin-walled ____________________.

4) The muscular ____________________ grinds food

into small pieces.

5) Digestion and absorption occur in the __________.

d. Circulation and Respiration

1) Both coelomic fluid and blood carry food, wastes

and respiratory gases.

2) Blood circulates in a closed system with five main

trunks running lengthwise in the body.

3) The ___________________________ above the

alimentary canal has valves and functions as a true

____________________.

4) The dorsal vessel pumps blood anteriorly into

_______ pairs of ____________________________.

5) Earthworms have no special gaseous exchange

organs; the moist skin handles all exchanges.

e. Nervous System and Sensory Organs

1) Earthworms have both a central nervous system

and peripheral nerves.

2) A pair of cerebral ganglia connects around the

pharynx to the ganglia of the ventral nerve cord.

3) Fused ganglia in each somite contain both sensory

and motor fibers.

4) Neurosecretory cells in the brain and ganglia

secrete neurohormones to regulate reproduction,

secondary sex characteristics and _______________.

5) One or more giant axons are located in the ventral

nerve cord to increase the rate of conduction and

stimulate contractions of muscles in many segments.

6) Earthworms lack eyes but have many

photoreceptors in the epidermis.

7) Free nerve endings in the tegument are probably tactile.

f. General Behavior

1) Although they lack specialized sense organs, they

are sensitive to many stimuli.

2) They avoid ____________________ unless it is

very dim.

3) Chemical stimuli are important to find food.

4) Earthworms have limited learning ability; it is

mostly trial-and-error learning.

g. Reproduction and Development

1) Earthworms are ____________________.

2) In Lumbricus, reproductive systems are in somites

____________________.

3) Immature sperm from testes mature in __________

____________________and then pass into sperm

____________________.

4) Eggs are discharged by ovaries into the

____________________ cavity; ciliated funnels carry

them outside.

5) Two pairs of _____________________________

receive and store sperm during copulation.

6) Earthworms mate at night during warm, moist

weather.

7) They mate by aligning in different directions with

ventral surfaces together.

8) Mucus secreted by the ____________________

holds them together.

9) Sperm travel to the seminal receptacles of the

other worm along seminal grooves.

10) After mutual copulation, each worm secretes a

mucus tube and chitinous band to form a _________.

11) As the cocoon passes forward, eggs, albumin and

sperm pour into it.

12) Fertilization and embryogenesis takes place in the

cocoon; young worms emerge.

2. Subclass Hirudinea: ____________________ - most live in

freshwater but a few are marine or in moist terrestrial environments.

a. Most are flattened.

b. Some are carnivores on small invertebrates; others are

temporary or permanent parasites.

c. Leeches are ____________________ and have a

____________________, but only during the breeding

season.

d. The clitellum secretes a cocoon for reception of eggs.

e. They have lost setae and developed _________________

for attachment while sucking blood.

f. The gut is specialized for storage of large quantities of

blood.

g. Form and Function

1) Leeches lack distinct coelomic compartments and

septa have disappeared.

2) Most leeches use suckers to attach so they can

“inchworm” along the surface.

h. Nutrition

1) Although popularly considered parasites, many are

____________________.

2) Freshwater leeches have a proboscis for ingesting

small invertebrates as well as to suck blood.

3) Some terrestrial leeches feed on insect larvae,

earthworms and slugs.

4) Other terrestrial leeches climb trees or bushes to

reach warm-blooded vertebrates such as baby birds.

5) Most are fluid feeders that prefer tissue fluids and

blood pumped from open wounds.

6) Medicinal leeches were used when it was wrongly

believed disorders were caused by excess blood.

a) They are now being used to save severed

body parts.

i. Respiration and Excretion

1) Some fish leeches have gills; all other leeches

exchange gases across the skin.

j. Nervous and Sensory Systems

1) Leeches have two “brains”; the anterior fused

ganglia form a ring around the pharynx.

2) Seven pairs of fused ganglia are at the posterior.

3) The epidermis contains free sensory nerve endings

and photoreceptor cells.

4) Pigment-cup ocelli are present.

k. Reproduction

1) Leeches are hermaphroditic and cross-fertilize

during copulation.

2) Sperm are transferred by hypodermic

impregnation.

3) The clitellum secretes a cocoon to receive the

sperm and egg.

4) The cocoons are buried in mud or damp soil, and

development is similar to that of oligochaetes.

l. Circulation

1) The coelom has been reduced by invasion of

connective tissue and chloragogen tissue.

2) This forms a system of coelomic sinuses and

channels.

3) Some leeches have a typical oligochaete circulatory system; the coelomic system is auxiliary.

4) Some leeches lack blood vessels and the coelomic

sinuses are the only vascular system.