CHAPTER FIVE

5.0.        PHYLUM NEMATODA

5.1.        General characteristics of Nematodes

1. Their bodies are unsegmented and cylindrical
2. They have a mouth and an anus
3. The sexes are separate e.g. Ascaris

 

diagram of male of female ascaris lumbricoides

5.2.        Mode of life of Nematodes

Most nematode are free living they are found everywhere in soil, hot springs etc. some however are parasitic affecting plants and other animal.

5.3.        Nutrition of Nematodes

All nematodes have a long tube-shaped digestive track with a mouth and anus at a respective terminal. They effectively digest food in their guts. Free living round worms (nematodes) are carnivorous; they catch and eat other small animals including algae, fungi and pieces of decay organic matter. Many nematodes that live in the soil attach to the root hair of green plants sucking out plant juices. They cause tremendous damage to food crops all over the world.

5.4.        Transport, Gaseous Exchange and Excretion of Nematodes

The nematodes carry out this process through their body walls (like Platyhelminthes) e.g. have no internal transport system and so rely on simple diffusion since e.g. have a large surface area to volume ratio.

5.5.        Coordination in Nematodes

Nematodes have a simple nervous system there is a group of nerve cells at the head region called ganglia. This however is not a brain. They also have several types of sense organs to detect the presence of chemical. Several nerves extend from the ganglia to the various part of the body transmitting sensory information. This nerves equally control movement through the muscles in the body cells.

5.6.        Reproduction in Nematodes

This worm reproduces sexually. Most of them have separate sexes but a few are hermaphrodite. Copulation occurs followed by fertilization in the body of the female. The parasitic nematodes usually have complex life cycle involving more than one host.

5.7.        Life Cycle of Ascaris

The Adult lives in the small intestine of humans. Other species closely related to ascaris also affect cattle, horses, pigs, chicken, dogs, goats etc. From the intestine, this adult reproduces eggs which leave the host body through feces. When food or water contaminated with such eggs is eaten by another host. From there, they enter the blood stream and are carried around the body. The tiny worms end up in the lungs. They break out into the tiny air passages, climb up into the throat and are then swallowed. Carried back into the intestine, they grow to maturity and the life cycle repeats itself.

LIFE CYCLE OF ASCARIS

5.8.        Some diseases Caused by Parasitic Nematodes.

These worms are responsible for some of the most painful and horrible diseases e.g.,

• Intestinal infections
• Trichinosis
• Elephantiasis
• Filaria
• Eye infection

We shall look at each of these diseases and how to identify them.

1. Intestinal Infection: one of the worms that cause this is the hook worm. It is very common in the tropics, affecting about ¼ of the world’s population. The adult lay eggs in the intestine which pass out in the host feces. The egg hatch outside the host body and develop in the soil. If they find an unprotected foot (leg) they use their sharp teeth to bore a hole through the skin into the blood stream. Like ascaris, the worms travel to the lungs up the throat and finally down the intestine here, they grow into adults, deep into the intestinal walls, sucking the host blood. The host experiences weakness, blood shortage and poor growth.

 

1. Trichinosis: This disease is caused by a round worm called Trichinella. The adult live and mate in the intestine of the host. The female digs into the intestinal walls and can lay up to 1500eggs. The eggs hatch into larvae which travel through the blood stream and infect the various organs of the body. In this organ they cause severe pains to the host. The larvae for cyst and become inactive for the life cycle to continue. Such infected flesh must be eaten hence the lost of trichinella are carnivorous e.g. rats and pigs. They eat infected flesh, the cyst is digested, releasing the larvae. This grow into adults in the intestines which mate and lay eggs for the life cycle to go around again. Humans occasionally get trichinosis when they eat raw or poorly cooked pork.

 

• Filaria and Elephantiasis: The filaria worms are thread-like, a lymph vessel of their host and blood vessels e.g. mammals like human and birds. They are transmitted from one primary host to another through biting insects e.g. mosquitoes. They cause a lot of itches on the skin. In severe infections, the number of the worms may block the passage of the fluid within the lymph vessels and feet. Consequently, the legs and feet swell enormously a disease called elephantiasis.

 

1. Eye infection e.g. (River blindness): The worms that cause this infection are called eye worms and are closely related to the filaria worms and closely related to the filaria worms. The are most common in Africa and in fact human and baboons. This worm lives in the skin moving from one part of the body to another. Occasionally they move across the surface of the eye (hence their name) the diseases river blindness is an eye infection. Among humans, it is spread by the bite of a tiny black fly which breaths around fast-moving streams. It affects millions of people in Africa, many of whom end up being blind. When this fly bites an infected person. It picks up the larvae of the worm upon biting the second host, it transmits the worm into the victim. This grow into threadlike adult and can live under the skin as long as 12years. This reproduce asexually giving millions of larvae which swarm through the skin and eyes. It is this offspring that cause diseases and not the original adult. The characteristics of the diseases include itches, decay of the skin, loss of its pigment and eventually blindness may follow.