Unit 5 Notes #6 - Phylum Annelida:
Segmented Worms
A) Body Plan:
Similarities To Phylum Aschelminthes
(Nematoda):
1. Bilateral symmetry
2. Three layers (Ectoderm, endoderm, mesoderm)
3. Complete digestive system (mouth to anus).
Differences:
-Contains a
“Coelom”,
this is considered to be a “True Cavity”. Unlike the “Pseudocoelom”, this cavity is not just fluid-filled, but it is also lined with mesoderm.

This provides several advantages:
1) This coelom separates the body wall from the gut. Thus food does not interfere with body movements.
2) The fluid in the coelom may circulate many substances, including wastes, food and dissolved gases in the body.
3) The coelom provides more space where true systems and their organs can operate.
(Ex, stomach, heart, lungs etc.)
-Annelida are the first to exhibit
SEGMENTATION: “The repetition of body units that contain similar structures. Most segments contain duplicate structures for locomotion, excretion, muscle and nervous tissue.
-This segmentation is visible on the outside of these worms.

B) Annelida Diversity:
1. Class Polychaeta : These are mainly marine worms that live in sand or mud of shallow coastal areas.
They usually have pairs of paddlelike appendages that are tipped with numerous bristles.

2. Class Oligochaeta : Most are terrestrial but some are freshwater, found burrowed into soil and mud.
-They do not have any leg-like appendages but do have four pairs of bristle per segment.
Example : Earthworm

3. Class Hirudinea: These are the leeches.
-Most live in freshwater as external parasites.
-They have a sucker at each end of their body, they secrete an anticoagulant and an anesthetic to facilitate “blood sucking”

THE EARTHWORM

A) Digestive System of the Earthworm:
-Adapted for ingesting leaf fragments, seeds, bodies of small animals and protists, as well as rich organic soil.
-Soil and organic matter are sucked into the mouth by the muscular pharynx .
-Muscular action pushes the food through the narrow esophagus and into the crop for temporary storage.
-The crop directs food matter into the muscular gizzard.
-The gizzard, with help of sand particles, grinds the food into smaller pieces.
-At this point the food is pushed into the
intestine, which extends from the gizzard to the anus .
-Gland cells of the intestinal wall, contain enzymes, which digest proteins, fats, and carbohydrates, including cellulose.
-The digested nutrients are absorbed by the blood, while waste passes out through the anus.
-The muscular action that pushes the food along is called peristalsis. It results from the

alternate contraction and relaxation of certain muscles to create a wave motion along the tube. Check out this link: http://images.google.ca/imgres?imgurl=http://www.ncsu.edu/scivis/lessons/earthworm/Image5.gif&i mgrefurl=http://www.ncsu.edu/scivis/lessons/earthworm/Overview.html&h=352&w=500&sz=19&tb nid=oki0wkxCTUoJ:&tbnh=89&tbnw=126&start=2&prev=/images%3Fq%3DEarthworm%2BDige stive%2BSystem%26hl%3Den%26lr%3D%26sa%3DG

B) Circulatory System of Earthworm:
- Need a closed circulatory system, as animals get larger, simple diffusion of food, oxygen and wastes can no longer meet the needs of all body parts.
-Special circulatory tissue called blood is directed through a continuous system of muscular tubes called blood vessels. Blood is red due to Hemoglobin.
-Blood flows through the dorsal (top/back) vessel. From here, five pairs of beating aortic arches (hearts) direct the flow to the ventral
(bottom/belly) vessel.
-In each segment, the ventral vessel branches to send blood out to the body wall.

-Other branches take blood to the intestine where the blood absorbs nutrients.
-Capillaries return the blood to the dorsal vessel.
C) Respiration :
-This occurs by diffusion of oxygen and carbon dioxide through the moist skin.
-Kept moist by mucous glands in the skin and by the cuticle which prevents desiccation
(drying out)

D) Nervous System :
-Small brain is located above the pharynx, the brain is attached to the ventral nerve cord, which runs the length of the worm.
-In each segment there are sensory and motor nerves that branch off of the ventral nerve cord.
-These sensory receptors are sensitive to light, touch, and a number of chemicals.

E) Reproduction in Earthworms:
- Earthworms are monoecious
(hermaphroditic).
The male system:
1. Two pairs of small testes to produce sperms (segments 10 and 11)
2. Two large seminal vesicles store the sperm.
3. The openings of two sperm carrying tubes called vasa deferentia are located on the ventral side of segment 15.
The female system:
1. A pair of small ovaries in segment 13
2. Two pairs of seminal receptacles in segments 9 and 10, which store sperm received from another worm during mating.
- During the process of mating two worms line up beside each other pointing in opposite directions.
- The clitellum secretes a mucous sheath around the two worms, this sheath will ensure they stay together.
- Later each worm secretes a cocoon around its clitellum, and eggs from the oviducts are passed into the cocoon.

- The worm backs out of the cocoon; as the cocoon passes the openings of the seminal receptacles, sperm stored from the mating is released into the cocoon.
- Fertilization now takes place in the cocoon.
- When the worm leaves the coccoon, the ends of the coccoon close over.
- The young worm emerges from the cocoon in 2-3 weeks, it appears similar to the adult stage.
- Worms do not develop a clitellum until they are sexually mature.

OVERALL : EARTHWORM ANATOMY