Bio 11 - Earthworm

Annelida

The phylum of the earthworm

Segmented body (ringed) 

Presence of a coelom (a fluid filled cavity allowing for complex organs present)

3 Germ Layers Present

Complete Digestive Tract 

Bilateral symmetry

Characteristic of Phylum Annelida

Anterior End

The more pointed, darker end of the earthworm.

Posterior end

The end of the worm which is lighter in color and less pointed.

Dorsal surface

The darker, rounder surface (clitellum is clearly visible from this surface)

Ventral surfacre

The lighter, flatter surface (the male pore and setae are on this surface)

Cuticle

Outer moist layer.

Prevents the worm from drying out and losing its abilities to exchange gases.

Setae

Tiny bristles found on the side of the worm.

Serve as anchors for the worm to move through the soil.

Clitellum

Collar like swelling found near the anterior end.

Used in reproduction to secrete a slime ring which will eventually form a cocoon.  

Anus

Opening at the posterior end.

Allows undigested material to be eliminated from the worm.

Male Pore

Opening found in segment 15.

Allows sperm to be released during mating.

Prostomium

First segment, pointed end.

Covers mouth and helps worm burrow through the soil

Mouth

Opening at anterior end.

Allows food to enter the earthworm.

Pharynx

Muscle found just behind the mouth.

Contracts to draw food into the mouth.

Esophagus

Short tube lying under the aortic arches.

Transports food from mouth to the crop

Crop

Soft organ, usually gray in color.

Stores food.

Gizzard

Large muscular organ.

Grinds food with the aid of sand.

Intestine

Long thin organ.

Where food is broken down by enzymes and later absorbed.

Aortic Arches

5 pairs of swollen vessels lying over the esophagus.

Contract and relax to keep blood flowing through the worm.

Dorsal Blood Vessel

Runs down the top of the worm.

Carries blood toward the anterior end.

Ventral Blood Vessel

Runs down the bottom of the worm.

Transports blood toward the posterior end.

Seminal Vesicles

Pairs of large yellow structures found near the aortic arches.

Stores sperm until released during reproduction.

Seminal Receptacles

Small cream colored structures found near the seminal vesicles.

Structures which store sperm that enters the worm from another worm.

Brain

Two very tiny lobes at the anterior end above the pharynx.

Sends and receives messages.

Ventral Nerve Chord

Long thin cord which runs down the ventral surface of the worm.

transmits messages to and from the brain.

Nephridia

Paired coiled tubules found on nearly all segments .

Excrete nitrogenous wastes from the earthworm's body (similar to the kidneys in vertebrates.)

Physical Digestion

Digestion which requires the use of muscles or teeth which results in a change in the size and shape of the food. This helps to increase the surface area of the food so that chemicals are better able to act on the food molecules.

Chemical Digestion

Digestion which requires the use of acids and/or enzymes which results in a change in the chemical complexity of the food. The food is reduced to a simpler state that can be absorbed by the organism. An organism relies on this form of digestion in order to get the nutrients it requires.

Respiration

The outside of the earthworm, called the cuticle, helps to keep the earthworm moist. The moist mucus covering of the earthworm helps to ensure that gas exchange can occur.  Oxygen will diffuse into the earthworm across the moist outer layer and carbon dioxide will diffuse out.

Reproduction

After the worms part, the clitellum of each worm secretes a slime ring.  First the eggs moves out of the worm’s body through the female pore and into the slime ring.

The stored sperm are released into the slime ring.  Then fertilization takes place as the egg and sperm nuclei unite.  The slime ring becomes the cocoon for the young worms to develop in.