The Phylum Annelida Amy Brown Science Stuff This

The Phylum Annelida © Amy Brown – Science Stuff

This phylum consists of segmented worms. These segmented worms may live in fresh water, salt water, or on land.

This phylum is divided into three classes: Class Polychaeta (Marine Worms) Class Hirudinea (Leeches) P h yl u m An n el i d a Class Oligochaeta (Earthworms)

The term “annelid” means “little rings”. n This name refers to the many body segments that compose the body of a segmented worm.

There about 15, 000 species of worms in this phylum. They all have: Bilateral Symmetr y A True Coelom Segmentatio n Three Germ Layers: Endoderm, Ectoderm and Mesoderm

Advantages of a True Coelom The true coelom and three germ layers allows annelids to have the most complex body structure of all the worms. All organ systems are present. The coelom is partitioned or divided into compartments. Different parts of the body can contract or expand independently of one another.

Setae and Parapodia Setae: external bristles on the body; used for movement Parapodia: fleshy appendages on each side of most body segments The number of setae and the presence or absence of parapodia is the basis for dividing the annelids into the three classes.

Class Oligochaeta ØGenerally live in the soil or fresh water. ØThese worms have no parapodia. ØThe word “oligochaeta” means “few bristles”. These worms have very few setae. ØThis class includes the earthworms.

External Structures of the Earthworm 1) Prostomium / Mouth 2) Setae 3) Clitellum 4) Cuticle 5) Anus

It is the first body segment in an earthworm. Prostomium It is in front of (but does not include) the mouth. It is a lip-like extension over the dorsal surface of the mouth.

Cli te llum Contains both male and female reproductive structures.

Setae q. In earthworms, there are 4 pairs of setae per segment. q. They are used for movement. They anchor the worm in the ground as the longitudinal and circular muscles contract and expand.

Cuticle The cuticle is a waxy, outer covering. It prevents water loss and enables gases to diffuse through the skin.

Movement of the Earthworm n Circular and longitudinal muscles line the interior body wall of the earthworm. n The setae are used to anchor the worm in the ground. n Circular muscles contract making the earthworm longer. n Longitudinal muscles contract making the earthworm shorter.

Internal Structures of the Earthworm 1) Prostomium 2) Pharynx 3) Aortic Arches 4) Esophagus 5) Crop 6) Gizzard 7) Seminal Receptacles 8) Seminal Vesicles 9) Sperm duct 10) Intestine 11) Clitellum 12) Setae

Feeding and Digestion 1. Earthworms ingest soil as they burrow through it. 2. Soil is sucked into the mouth by a muscular pharynx and then passes down the esophagus. 3. The soil and food enters the crop – a temporary storage area. 4. From the crop soil moves into the gizzard. The gizzard is thick and muscular. The gizzard grinds the soil, releasing and breaking up organic matter. 5. As the soil passes through the intestine, digested organic compounds and nutrients from the soil are absorbed into the bloodstream. 6. Undigested material is eliminated from the body through the anus.

Did you know that the earthworm’s digestive system is very helpful to us? a. Earthworms fertilize the soil by taking in decaying organic matter and eliminating the wastes. b. The burrows made by earthworms allow air to penetrate into the soil, bringing oxygen to plant roots and other living organisms. c. Earthworms loosen the soil, making it easier for plant roots to grow, and for water to seep in.

The earthworm has a closed circulatory system. Closed Circulatory System • The blood is contained within a system of blood vessels. • Found in all vertebrates as well as the annelids Open Circulatory System • The blood is not entirely contained within blood vessels. • Found in Arthropods and some Mollusks.

What is the purpose of the circulatory system? The purpose of the circulatory system is to transport oxygen, carbon dioxide, nutrients and wastes through the body of the worm.

The earthworm’s circulatory system consists of: Ventral Blood Vessel – takes blood to the posterior end of the body. Aortic Dorsal Blood Arches – Vessel – contract to returns blood send blood to the through the anterior end circulatory of the body. system.

Respiration and Excretion How are oxygen and carbon dioxide exchanged? Why must the skin stay moist? How does the skin stay moist? What are nephridia? O 2 and CO 2 diffuse directly through the skin. Just under the skin is a series of very small blood vessels. The gases diffuse through the skin and into and out of these blood vessels. The skin must remain moist for the diffusion of gases to occur. Gases cannot diffuse across a dry surface. The skin is covered by a thin cuticle as well as mucus to keep it moist. Nephridia are excretory tubules found just under the skin. They eliminate cellular wastes and excess water.

Nervous The nervous system consists of several ganglia System and a ventral nerve cord. Ganglia: A mass of nervous tissue that serves as a very primitive brain. The ganglia (anterior brain) can respond to simple stimuli such as light, touch, chemicals, moisture, temperature and vibrations. There are no specialized sense organs.

Reproduction Hermaphroditic: A single worm has both male and female reproductive organs. Sperm is produced in the testes and stored in the seminal vesicles.

u. Mating occurs when two worms fuse their bodies together at the clitellum. u. They exchange sperm; the exchanged sperm moves into the seminal receptacle where it is stored. u. The exchange of sperm occurs through the sperm duct. u. After several days, the clitellum secretes a tube made of mucus and a carbohydrate called chitin. u. Fertilization occurs in this tube. u. The eggs hatch in 2 – 3 weeks.

Class Polychaeta About 2/3 rds of all annelids are polychaetes. The term “polychaete” means “many bristles. This refers to the many setae that help polychaetes move.

Polychaetes differ from other worms in that they have antennae and specialized mouthparts.

Class Hirudinea This is the smallest class of annelids. It consists of only 300 different species of leeches. Leeches have no setae. Leeches are parasites. They have a sucker that is used to attach to the host body and suck blood. They secrete two substances: an anesthetic to prevent the host from feeling it, and another substance to prevent the blood from clotting.

Leeches are often used for medicinal purposes. Leeches are used to suck out blot clots from a wound, to promote circulation, and to relieve pressure in a joint or wound.

What are the advantages to a segmented body? Body segmentation makes it possible for different parts of the body to contract or expand independently of the other segments. Some organ systems can be duplicated in several different segments which provides insurance against injury.

How do polychaetes differ from earthworms? Polychaetes have more setae than earthworms. Most polychaetes live in marine environments. Most earthworms live in the soil. Polychaetes may have antennae and advanced mouthparts.

How are some leeches adapted to a parasitic way of life? Leeches have suckers to attach to the host to suck blood. Leeches secrete an anesthetic to prevent the host from feeling their presence. They secrete a second substance to prevent the blood from clotting.

How are the parasitic leeches different from the other parasitic worms such as the tapeworms and liver flukes? 1. Leeches are external parasites where the tapeworm and fluke are internal parasites. 2. The flukes must cycle between two hosts, a primary host and a secondary host. Leeches do not.

Why do the earthworms require a moist environment? The earthworm exchanges gases (carbon dioxide and oxygen) by diffusion across the skin surface. This exchange can take place only if the skin is moist.