Name:

Phylum Nematoda: Roundworms

Learning Objectives: *Refer to Kingdom Animalia Unit Plan

Text page: 373 *This is required reading.

-Roundworms belong to the Phylum Nematoda, which means “thread” in Greek. This is appropriate as many of them look like fine sewing thread.

-Roundworms were the first animals to have a pseudocoelom and tube-within-a-tube digestive tract!

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Adults affected by River Blindness being led by children who are unaffected. Chad, 1972.

Onchocerca volvulus, a parasitic nematode causing River Blindness

Trichinella spiralis, a parasitic nematode acquired from eating undercooked meat

C. elegens, a free-living nematode

Nematoda Habitat

-Nematodes are found free living in soil and aquatic habitats and as parasites inside many plants and animals (humans included).

-There are 25,000 species of named nematodes and it is believed there could be as many as 500,000 species in total.

-Although you may never have seen one, nematodes are all around us: a) A single rotting apple may contain 90,000 roundworms; b) An acre of topsoil can contain billions of roundworms!

Nematoda Common Characteristics

-Remember, organisms after Platyhelminthes will share the innovations that they exhibit.

-This means that Nematodes will have bilateral symmetry, cephalization, organs and three germ layers.

-Some very important new innovations in this group include:

a) Coelom Development

-Nematodes have a fluid filled cavity between the endoderm and mesoderm called a pseudocoelom.

-They no longer have a gvc!

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-Like the gvc, the pseudocoelom provides several advantages:

-Fluid acts as a hydrostatic skeleton for movement

-Fluid distributes nutrients from digestive tract throughout the body.

b) Body Plan

-Nematodes have a tube-within-a-tube body plan instead of the sac-like body plan that is associated with a gvc.

c) Cuticle

-The cuticle is a tough, flexible non-living layer secreted by the epidermis that covers the outside of nematodes.

-What advantages do you think this layer might provide?

-Free living species: Prevents against dessication and mechanical abrasion

-Parasitic: Protection against host enzymes.

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-The cuticle also acts as a skeleton, providing a support and leverage point for movement.

-The cuticle must be molted periodically to allow the nematode to grow.

Nematoda Essential Functions

1. Circulation/Respiration/Excretion

-Like flatworms, roundworms can rely on simple diffusion to take care of their circulatory, respiratory and excretory needs. However, they do have a couple of adaptations:

a) Fluid in the pseudocoelom works to move nutrients around the body.

b) Metabolic wastes from cells are put into the fluid of the pseudocoelom and collected by excretory canals for excretion. This is similar to the protonephridia (flame cells) seen in flatworms.

2. Digestion

-What are some advantages of a complete digestive tract that has a separate mouth and anus?

-Food travels in one direction and is processed along the way.

-Allows specialization of sections of the digestive tract:

i) 1 st section digests food mechanically and chemically

ii) 2 nd section absorbs nutrients and disposes of wastes

3. Movement

-Muscles can work against both the cuticle and fluid in the pseudocoelom (hydrostatic skeleton)

-The muscles underneath the epidermis run longitudinally down the nematode body. This means they can only flex their bodies from side-to-side, making swimming nematodes appear to be thrashing around aimlessly (you will see this in your own nematode observations).

4. Reproduction

-In contrast to flatworms, nematodes have separate sexes.

Nematoda Species

1. Onchocerca volvulus

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a) Free Living or Parasitic: Parasitic, causing the disease River Blindness.

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b) Reservoir: Humans, in cysts.

c) Vector: Blackflies. A vector is a species that transfers the nematode between reservoirs.

d) Symptoms: i) Skin rashes, lesions and intense itching. These are caused by the host immune system attacking the dead microfilariae.

ii) Nodules with thousands of microfilariae living in them.

iii) Eye Lesions. These are caused by microfilariae migrating into the eyes. These can lead to blindness.

e) Treatment: Mectazin kills microfilariae, but not adult round worms.

f) Life Cycle Interruption: Spraying insecticides in rivers to kill blackfly larvae and distribution of Mectazin to all vulnerable individuals.

2. Caenorhabditis elegans

a) Free Living or Parasitic: Free living.

b) Reservoir: Moist soil.

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c) Vector: None. This species does not parasitize other organisms.

d) Ecological Role: Decomposer

e) Scientific Importance: A model organism for research into : i) Development of embryo cells; ii) Understanding how genes control animal development; iii) How

nervous systems function; iv) The mechanisms of aging.

-The life cycle diagram below illustrates how nematodes must go through periodic molts to shed their cuticle during growth.

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