Phylum: Annelida - the segmented worms

 

Annelid is the common name for about 9000 species of wormlike invertebrate animals with distinct segmentation.

 

General characteristics of the phylum: 

 

A complete digestive tract with mouth and anus. The gut is a straight tube.

 

Bilateral symmetry.

 

The body is made up of several similar units, or segments, separated externally by furrows and internally by septa (partitions).

 

The segments generally bear setae (bristles) that are used in locomotion.

 

The coelom (fluid‑filled body cavity) provides a kind of structural support, so that the body is flexible.

 

The nervous system fairly simple, with simple sense organs.

 

Some are dioecious (bristle worms), but most are hermaphroditic (earthworm and leeches) with internal cross-fertilization.

 

Some produce cocoons (earthworm), but some care for their young (leeches).

 

The three major classes of the annelid phylum are: 

 

Polychaeta - the bristle worms (about 5300 species), which are mainly marine and often luminescent; 

 

Oligochaeta - (about 3100 species), which are mostly freshwater or inhabit the soil, such as the earthworm; and

 

Hirudinea - the leeches (about 300 species), which are mainly freshwater but may also be marine or terrestrial.

 

Class Polychaeta - the bristle worms

 

Bristle worms, or Polychaeta, (many bristles) are marine organisms related to the common earthworm.

 

They have segmented bodies and very distinct bristles that are implanted in each segment.

 

The bristles are implanted in the body. With the use of muscles the animal can move the bristles backward and forward.

 

In some species the bristles can grow to considerable size.

 

This could serve as a defense structure against predators.

Two reddish eye spots are present to detect light and dark.

 

Behind the mouth lies a pharynx which is a sucking device. 

 

Bristle worms have a variety of ways of reproduction.

 

Asexual reproduction does occur.

 

In some species a rather peculiar way of sexual reproduction exists.

 

At the tail end the segments slowly change into new organisms.

 

These sexual organisms bud off to form special sexual versions of the organism.

 

These have larger eyes and swim to more open water to find a partner to mate.

 

Then the eggs are fertilized.

 

The first larval stage of a Polychaete is a so‑called Trochophore larva.

 

It requires a real specialists to be able to identify to what species it belongs to since many species have almost identical trochophore larvae.

 

The larva lives free swimming in the plankton.

 

It has a large flat head with cilia.

 

With this it can suck in small food particles.

 

After they become adults they start their lives on the bottom of the sea.

 

Some will live buried in the mud.

 

Many of them build special tubes for protection.

 

Others move around searching for prey.

 

The tube worms are a remarkable group of bristle worms:

 

Researchers at Pennsylvania State University say they have determined that some giant worms that live 1,700 feet below the surface of the Gulf of Mexico range in age from 170 to 250 years old.

 

The scientists don=t know for sure why the immense worms live so long.

 

It=s an age record for invertebrates, although some tortoises live longer.

 

The tube worms [known by the scientific name Lamellibrachia] do not eat. 

Over years, they grow to lengths of 10 feet or more.

 

That=s not a record for worms. That distinction goes to a tapeworm that lives in the intestines of whales.

 

The tube worm is a fairly new discovery; scientists learned about them in the 1980s.

 

They live in clusters of millions, covering acres of ocean floor.

 

Each worm is protected by a thin, flexible, shell‑like tube.

 

Age estimate based on growth rates:

 

Finding out the age of a giant sea worm is a bit more complex than counting the rings of a tree.

 

Researchers rode a submarine to the bottom of the gulf, where they used robotic arms to mark the ends of the tubes that the worms live inside.

 

Three months later, they returned and measured how much the worms had grown.

 

They kept on returning and measuring every few months for four years.

 

Once they had that data, they were able to calculate how long it would have taken for the worms to grow to their existing lengths to determine the ages of the worms.

 

So what=s the secret to this longevity? Scientists have some guesses.

 

Some cousins of these worms that live near deep‑sea hydrothermal vents that make the water boil live only a few years, or a few decades.

 

One hypothesis is that the Lamellibrachia worms live in an environment where they=re less likely to get bruised, broken, or run out of the energy they absorb from the chemicals.

 

A quarter of a mile below the surface of the Gulf of Mexico is about as stress‑free a habitat as you can get.

 

How do they manage to live?

 

Tube worms appear like thick‑stemmed tulips up to 3 meters long with bright red plumes.

 

They have no mouth or gut.

 

Instead, their red plume rimmed with blood vessels reaches into the  water.

 

Oxygen, carbon dioxide and hydrogen sulfide bind to carrier molecules in the blood.

 

As the heart pumps, these three compounds are delivered to a very specialized tissue which contains densely packed bacteria.

 

In return for a stable environment in which to live, the bacteria provide the tube worm with all the organic compounds required for growth.

 

The association of two different living entities is called symbiosis.

 

In this instance, it can be said that the tube worms provide housing, and the bacteria pay rent.

 

There are other Polychaetes that are fierce and who will not hesitate to eat members of their own species. 

 

The 'jaws' of the Rag worm (Nereis ) can be protruded to snatch prey.

 

It has many appendages  around the mouth opening.

 

Class Oligochaeta:

 

Earthworm is the name given to more than 1000 species of worms in a class of the annelid phylum.

 

The earthworm has a cylindrically shaped, segmented body that tapers off at both ends.

 

Minute bristles called setae project from the body.

 

Although a difference in shading exists between the upper and under surfaces and between different parts of the body, earthworms in general are uniform in color, usually pale red, but varying from dull pink to brown.

 

Many species grow to a length of only a few centimeters, but some tropical species grow much longer (see below).

 

Earthworms play an important role in soil ecology. By being continually loosened, stirred up, and aerated by the action of earthworms, soil is made more fertile.

 

If one took all the earthworms under a typical football field and lined them up, they would extend 94 miles!

 

Earthworms also form a source of food for many animals, and constitute the principal food of moles and shrews.

 

Worms have nutritional value as they are high in protein and low in fat. (Discuss urban legend about McDonald=s.)

 

 

Behavior:

 

Earthworms must live in moist soil containing organic matter.

 

They usually live in the upper layers of the soil, but in winter they penetrate more deeply to escape frost.

 

During unusually hot weather they also penetrate downward to avoid dehydration.

 

Earthworms shun daylight but frequently come to the surface of the soil at night to feed and to throw off their castings.

 

In the daytime they appear upon the surface of the soil only under unusual conditions, such as the flooding of their burrows by excessive rainfall.

 

Earthworms are capable of burrowing with considerable speed, especially in loose soil; the bristles along the sides of the body are of great assistance in their movements.

 

In burrowing, they swallow large quantities of earth that often contain considerable amounts of vegetable remains.

 

They are able to digest the nutritive matter of the soil, depositing or casting out the remains on the surface of the earth or in their burrows.

 

Structure and life cycle: 

 

The muscular system of the earthworm consists of:

 

An outer series of circular muscle fibers that girdle the body and elongate it.   

 

An inner series of longitudinal muscle fibers employed in shortening the body.

 

The circulatory system consists of:

 

A prominent dorsal blood vessel and at least four ventral blood vessels, running longitudinally in the body. 

 

The dorsal vessel is provided with valves and is the true heart.

 

Most of the pumping of blood, however, is performed by general muscular movements.

 

Transverse vessels connect the dorsal and ventral vessels.

 

 

 

 

The central nervous system consists of:

 

A pair of suprapharyngeal ganglia, often called the Abrain@ and

 

A ventral cord that lies beneath the alimentary canal with ganglia in every segment.

 

Earthworms have no sense organs other than those of touch.

 

The digestive system consists of:

 

A muscular pharynx, a slender esophagus, a thin‑walled crop or food receptacle, a muscular gizzard used for grinding ingested earth, and a long, straight intestine (with typhlosole to increase surface area).

 

The reproductive system: 

 

How a worm reproduces is extremely interesting.

 

Each worm is hermaphroditic, meaning it is both male and female, and has both ovaries & testes.

 

Each worm also has two sperm receptacles.

 

However, a worm cannot simply fertilize itself. There must be two worms to make more little worms.

 

Two worms get against each other, with their heads pointing in different directions.

 

Then, they secrete a mucus, which keeps the two worms together, and covers them from their 9th segment to the rear of their clitellum.

 

The sperm is carried and released in grooves in the skin of the earthworm.

 

After the sperm has been passed between the worms, they separate.

 

Afterwards, the clitellum again secretes a mucus, but this time it is not for keeping two worms together.

 

This "mucus ring" slides forward, toward the head of the worm.

 

As it slides forward, it takes several eggs from the oviducts and sperm from the receptacles.

 

Eventually, it slides over the head of the worm, and comes off.

 

Then, both ends close, forming a capsule.

 

The fertilization then takes place inside the capsule.

The eggs, containing considerable yolk, are buried in the earth in the capsules.

 

In two or three weeks, one or two worms will emerge from the capsule.

 

However, worms may form new egg capsules very few days after mating.

 

The capsules protect the young until they hatch as small, fully developed worms.

 

Some species live for ten years or longer.

 

Benefits of earthworms:

 

As earthworms tunnel through the soil, they ingest the soil and digest any organic matter in it.

 

Organic matter is dragged into their burrows and is broken down.

 

Although they are the most numerous in the top 6 inches, they also work in the subsoil, bringing mineral rich soil from below to the surface.

 

This adds to the supply of nutrients available to the plants.

 

Research shows that in 100 sq ft of garden soil, earthworms may bring from 4‑8 lbs of dirt to the soil surface each year.

 

Besides incorporating organic matter to soil, earthworms are good manufacturers of fertilizer.

 

Castings have a nutrient level and organic matter level much higher than that of the surrounding soil.

 

Each day they produce nitrogen, phosphorous, potassium and many micro nutrients in a form that all plants can use.

 

For example, a 200 sq ft garden with a low worm population of only 5 worms/cubic foot will be provided with over 35 lbs (about 1/3 lb per worm) of top‑grade fertilizer by the worms, each garden year.

 

Not only do they produce this fertilizer but spread it thoroughly within the top 12 inches of soil.

 

They may also incorporate it as far down as 6 ft.

 

A soil that is well managed, rich in humus may easily support 25 worms per cubic foot, which translates into at least 175 lbs of fertilizer per year for the same 200 sq ft garden.

 

This means that a garden or lawn can be supplied with far more fertilizer and that of superior quality than a dry or granular fast‑acting chemical fertilizer of 10‑20 lbs.

 

In fact, these fertilizers may even repel the earthworms that are present. As the fertilizers become soluble, they may leach down into the soil and force the earthworms to seek refuge elsewhere.

 

Worms make other contributions, such as adding calcium carbonate, a compound which helps moderate soil pH.

 

Over time earthworms can help change acid or alkaline soils toward a more neutral pH.

 

Earthworm tunnels help to aerate and loosen the soil. This allows more oxygen in, which not only helps the plant directly, but also improves conditions for certain beneficial soil bacteria.

 

The tunneling of the earthworms provide an access to deeper soil levels for the numerous smaller organisms that contribute to the health of the soil.

 

Do giant earthworms actually exist?

 

A long species of earthworm is the Megascolides australis, found in Australia in 1868.

 

An average specimen measures 4= in length, 2= when conracted, and 7= when naturally extended.

 

The longest of such species was found in 1930, which measured 7= 2@ in length, and over 13= when naturally extended.

 

The eggs of this worm measure 2@ to 3@ in length, and 3/4@ in diameter.

 

In November, 1967, a specimen of the African giant earthworm Microchaetus rappi measuring 11= in length and 21= when naturally extended was found on the road between Alice and King William=s Town, South Africa.

 

The average length of this species, however, is 3= 6@ in length, and 6= to 7= when naturally extended.

 

This was a very unusual specimen!

 

Lumbricus terrestris is the worm to be studied in lab.  How does it differ from Afishing worms?@

 

It is called ANightcrawler@

 

About a foot long.

Dark anterior; pale, flattenable posterior

No dormancy

Permanent burrows are up to 2.5 m deep

Lives 862‑887 days or up to 6 years

Matures in 350 days

Obligate sexual reproduction

38 cocoons per year per worm

Colonies spread about 3‑5 m per year

 

 

Class Hirudinea:  - leeches

 

Introduction:  

 

Leech is the common name for carnivorous or bloodsucking worm once widely used by physicians and barbers for bloodletting and still used for this purpose in some regions of the world.

 

In modern medicine, bloodletting is no longer practiced, but leeches continue to be used to relieve blood congestion in certain delicate operations, where such use is less likely to cause infection than other techniques.

 

Leeches are widespread in marine waters and are found in fresh water and on land in temperate and tropical regions.

 

Physical features: 

 

The animals are flattened ringed worms, measuring from 0.2 to 18 in. in length.  

 

They are equipped with a sucking disk at both the anterior and posterior ends.

 

Typically, the body is made up of 33 segments. 

 

On the anterior end of the worm are several eyes.

 

In some leeches, the mouth contains three toothed plates with which the animal pierces the skin of its prey.

 

Blood ingested by leeches is mixed with salivary juices containing an anticoagulant substance known as hirudin (which can be extracted and has been used in medicine to prevent blood clotting).

 

The blood passes into a dilated, branched stomach, or crop, where it is stored for several months before being completely digested.

 

A leech consumes about three times its weight in one feeding and then subsists for months on the stored food.

 

Leeches are hermaphroditic, each specimen containing several pairs of testes and one pair of ovaries.

 

Typical species lay their eggs in mucous cases known as cocoons; upon hatching, the young of some forms attach themselves to the underside of an adult and are carried about with it until they can live independently.

 

Aquatic leeches can swim; both aquatic and terrestrial species move over solid surfaces by muscular expansion and contraction; some also move by attaching themselves with one sucker and then somersaulting onto the other sucker.

 

Medicinal use of leeches:

 

Scientific interest in leeches date back to ancient India.

 

However, the first Western citation is credited to the Greek, Nicander of Colophon (c 130 BC).

 

These early references were accounts of the use of leeches for bloodletting.

 

This therapeutic use of leeches, the medicinal leech in particular, reached a height between 1825 and 1840.

 

A more contemporary use of leeches was discovered in 1957.

 

The leech secretion hirudin was isolated and subsequently its anticoagulant properties with respect to understanding of blood clotting mechanisms were examined.

 

Today medicinal leeches are used as tools in tissue grafts and reattachment surgery.

 

Not only do they secrete anitcoagulants to prevent blood clots and relieve pressure due to pooling blood.

 

It appears that leech saliva has other therapeutic properties.

 

Leech saliva helps reestablish blood flow to reattached body parts by means of a vasodilator,

 

Provides a numbing anesthetic, and

 

Lessens the risk of infection due to an antibiotic property.