Influence of Parasitic Life

If you compare the structure of a parasite, for instance, the Tapeworm, with a related but independent species, such as the Earthworm, you will notice that the Tapeworm's body is more rudimentary. Some of the vital organs, for example, those which arc used for digestion, are absent. On the other hand, others are very prominent (hooks, suckers, tough cuticle). These organs make it possible for the parasite to live at the expense of the host. The tremendous number of eggs passed out in the host's excreta gives the parasite remarkable powers of regeneration and assists in its survival.

Tasks

1. Write down whether the following statements are true or false. Give extensive answers.

1. Many of parasites lack organs or systems that are essential to free-living animals.

2. The Earth worm’s body is more rudimentary than the Tapeworm’s body.

3. Liver and Lung Flukes are not dangerous for their hosts.

4. The Tapeworm passes a very simple way of development.

5. All of the flatworms are bilaterally symmetrical.

6. There bodies are round and segmented.

7. Tapeworms get into the human intestine from the contaminated muscles of animals.

8. Parasitic worms produce just a few eggs.

2. Complete the sentences using the text and translate them into Russian.

1. Many parasitic worms living in the … cause great damage to … .

2. Their bodies are … and the … end is more sensitive than the … end.

3. Tapeworm infections lead to … .

4. Such organs as … are very prominent in parasitic worms.

5. Among the most dangerous parasitic worms are … .

6. The adult Liver Fluke lives in the … .

7. Lung Flukes resemble … .

3. Answer the following questions on the text in written form.

1. What animals are called parasites?

2. In what parts of their host’s body can parasitic worms live?

3. What parasitic worms are the most dangerous?

4. What is the life cycle of Liver Fluke?

5. In what way do tapeworms get into the alimentary canal of man?

6. What should we do to avoid tapeworm infection?

7. What are the characteristic features of parasitic worms in comparison with independent species?

8. What gives the parasite remarkable powers of regeneration?

4. Translate a passage into Russian in written form. Text 11 annelida

The name annelid means literally "ringed". The annelids include the ringed or segmented worms. This segmentation, which is characteristic of the annelids, is not found in either the flatworms or the roundworms. The annelids live in both fresh and salt water and occur on land where there is sufficient moisture. Most of the annelids are free-living but some are parasitic. The more common representatives of this phylum are the earthworm, the sandworm and the leech.

Earthworms. Habitat

Earthworms live in the soil. The holes leading into their cylindrical homes are often found in the well-manured, friable soil of vegetable and flower gardens and in compost and manure heaps, where they find plenty of food in the form of rotting organic matter. In such heaps it is easy to find Earthworms even in winter. Earthworms need oxygen; after heavy rain, when the air has been forced out of the soil by the water, masses of Earthworms burrow out to the surface. This is why they are also called rain-worms.

Oxygen enters their body through the skin. The act of respiration continues as long as the skin is wet. When it gets dry the worm dies.

Bilateral Symmetry

Since the Earthworm spends a major part of its life creeping on the ground, its ventral side is somewhat flattened, while the dorsal surface is more rounded. The dorsal surface is also darker. The worm's skin is thin and slimy, and covers two layers of muscular cells. The one nearer the surface is the circular muscle layer, which can be seen through the skin. Under it is the longitudinal muscle layer, which also consists of cells.

The body of the Earthworm is composed of a series of segments which look very much like rings. It is pointed at the anterior and posterior ends, which makes it convenient for the animal to move in both directions.

However, the anterior end is muscular and thicker. When burrowing the animal uses it as a driving wedge to separate the particles of soil.

Animals having different anterior and posterior ends and dorsal and ventral sides are said to have developed bilateral symmetry. The body of such animals as the Earthworm can be dissected into only two symmetrical, identical parts - left and right.

Ability to Orientate

The famous British scientist Charles Darwin discovered that Earthworms have the ability to distinguish the leaves they use for food by their fragrance. This means that Earthworms have a sense of smell.

In their skin are organs which react to direct contact. They are called sensory nerve cells. The anterior end of the Earthworm is particularly sensitive: when the animal moves it is the first to come in contact with different objects.

The worm is also known to possess an organ of taste.

The eyes are absent but in the skin there are some light-sensitive cells. Worms do not see things but can distinguish between darkness and light. A bright ray of sunlight will immediately send a worm into hiding.

The sense of hearing is absent. Eyesight and hearing are not important for an animal that lives in the soil. That is why they are not developed in Earthworms. But the senses of touch, taste and smell are quite acute, helping them to find food easily, hide from immediate danger, and creep from dry to more favorable places.

Earthworms and Soil Formation

Earthworms do more than burrow into soil: they "eat" their passage by taking some of the earth into their body, passing it through the intestine and excreting it via the anus. In doing so, the animal transforms the organic matter which is present in the soil into a state more easily assimilated by plants.

Earthworms make the soil friable and mix different layers as they burrow through. They carry tiny particles into the subsoil and bring particles of subsoil to the surface. This deepens the fertile layer of the soil.

The famous English biologist Charles Darwin (1809-1882), who was one of the early students of earth-worms, estimated that every ten years about 1,5 inches of soil is added to the surface of the fields inhabited by earthworms. This is equal to about 180 tons per acre. Besides this, the burrows or channels dug by earthworms admit rainwater and air deep into the soil. Plants spread their roots along the channels burrowed by worms; these channels also facilitate the penetration of water and air into the soil - both of which plants need badly. By taking decayed leaves into their burrows worms increase the humus content of the soil.

The above shows that Earthworms are useful animals which contribute to soil formation and the creation of more favourable conditions for growing plants.

Tasks