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

1. … the members of this phylum have …

2. …, we find that there are …

3. They are widely distributed, but …

4. … the planes of symmetry radiate from …

5. This type of symmetry is in contrast to …

6. … two types of body organizations.

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

1. What does the word “coelenterate” mean?

2. Why are the coelenterates considered to be more highly developed than the sponges?

3. What do the coelenterates use their stinging cells for?

4. Do the coelenterates occur singly or in colonies?

5. Do they exhibit radial or bilateral symmetry?

6. How many types of body organization do the coelenterates have?

7. Are all coelenterates built on the same general plan?

4. Translate a passage into Russian in written form.

TEXT 8

Hydra - a carnivorous,

Multicellular, fresh-water animal

Habitat

The Hydra, which is the classical example of the coelenterate phylum, is a polyp type. Hydras are inhabitants of ponds and other bodies of fresh water. The water should not be stagnant. Increasing contamination by decaying debris leads to their death. Hydras usually cling to plants which discharge oxygen into water. On a bright summer day they can be seen as small, grayish tentacled cylinders, up to 2 cm long.

Structure

Hydras are usually found attached by their so-called foot to a leaf or a small twig. At the opposite end is the mouth, on a small cone. The mouth is surrounded with thread-like tentacles (six to twelve).

They move from place to place either by sliding along on the foot or by walking on tentacles.

The body is shaped like a sac. The mouth leads into a digestive cavity, the enteron. The cavity extends throughout the body and into the tentacles. In the cavity food is digested. The Hydra has no anus. Therefore, the undigested food is pushed out through the mouth. The body wall is composed of two layers of cells - ectoderm or the outer (protective) layer, and endoderm or the inner (digestive) layer. In between, there is a thin cell-less supporting layer, which performs the functions of a skeleton.

Capture of food

Often one can see how a water flea, swimming past a Hydra comes in contact with its tentacles and stops as if paralyzed. The tentacles stretch out and seize the flea which is pushed into the Hydra's mouth. The mouth of the Hydra can stretch considerably so that the animal can swallow not only water fleas but also young fish, considerably larger than the Hydra itself.

The ability to move freely and consume organic food are features specific to animals. Consequently, the Hydra is an animal and a carnivorous one.

Musculo-epithelial Cells

The outer layer of the Hydra's body mainly consists of musculo-epithelial cells. The end of the cell, which is directed to the outside of the animal, forms part of the body wall. The opposite end, adjoining the supporting layer, becomes elongated and forms two fibres that lie along the body and contract as a result of irritation. If the process simultaneously affects the fibres of all the musculo-epithelial cells, the body of the Hydra shrinks.

Nerve Cells

The nerve cells are found in the ectoderm and are star-shaped, with a big nucleus inside and long thin protoplasmic branches. Since the branches are connected with each other, the cells form a primitive network which can be regarded as a nervous system.

One of the characteristic features of the nerve cells is their quick response to stimuli and their ability instantaneously to convey impulses to other cells. If a passing water flea touches one of the Hydra's tentacles, the nerve cells of the tentacles react. The impulse spreads throughout the nervous system and is passed to the musculo-epithelial cells. The fibres of the musculo-epithelial cells contract, the tentacles lean towards the prey and seize it.

The contractile fibres of the musculo-epithelial cells do not contract of their own volition but under the influence of impulses originating in the nerve cells.

An organism's response to stimuli which occurs through the nervous system is called a reflex. The Hydra captures water fleas in response to the food reflex. The contraction of its body caused by a needle prick is called the defensive reflex.

Stinging Cells

The stinging cells are scattered in the ectoderm. The toxic fluid ejected from them numbs the prey and the latter dies. The tentacles contract, seize the prey and take it to the mouth.

The stinging cells are more numerous in some parts of the body surface. In the tentacles, they are found in groups. The stinging cells also have a defensive function but to larger animals and human beings, the threads are harmless.

Digestive Cells

The endoderm of the Hydra has two different types of cells, endodermal and gland cells. On the ends lining the body cavity (enteron) the endodermal cells develop flagella. The gland cells do not.

When food touches the walls of the enteron the gland cell secrete drops of digestive juice. The food is dissolved and drawn into the endodermal cells. The flagella of the endodermal cells quickly distribute the food over the entire cavity surface. As in the Amoeba, the cells can develop pseudopodia to embrace the food and draw it into the protoplasm. There, food vacuoles are formed, digestion takes place, and the dissolved food is passed to the other cells of the animal's body, and absorbed.

Therefore, the Hydra can digest its food in two ways:

1. (Firstly) In the enteron, with the help of the juice secreted by the gland cells (intracavitary digestion).

2. (Secondly) In the endodermal cells (intracellular digestion)

The endodermal cells also play a definite role in locomotion. The ends lying close to the supporting layer are elongated and form two fibres, which are arranged along the body. When they contract the body of the Hydra becomes thinner and longer. This means, the fibres perform the functions of ringed muscles.

Tissues

The study of the Hydra shows that the different cells in the body are not identical in their importance and functions. Some play a defensive role; others are used to digest food, etc. Groups of cells that are similar in structure and play similar roles in the life of the organism, are called tissues.

The cells which form the top layer of the Hydra are called cover tissue. The star-like cells immediately underneath are a compound of the nervous tissue. The cells lining the enteron belong to the digestive tissue. The Hydra is also known to have primitive muscular tissue.

Respiration and Excretion

Since the body wall of the Hydra consists of only two layers of cells, oxygen gets into each cell easily. The discharge of carbon dioxide is also simple. The Hydra has no specialized organs of respiration. Respiration takes place from the body surface.

Neither has it separate organs of excretion, but discharges harmful fluids into the water through all its cells.

Budding

In summer, when food is plentiful, parts of the Hydra's body begin to bulge outwards. Both layers are involved in the formation of these buds. The bulging continues until at the free end of the finger-like projection a mouth develops, around which grows a circle of tentacles. The buds grow into young Hydras.

For some time the enteron of the "daughter" is most separated from that of the "mother". Reproduction by budding is asexual.

Regeneration

If you cut a Hydra in two, each half will build up the part it lacks. The same process takes place if yon cut it into several pieces. New Hydras will grow from them. The restoration of lost parts is called regeneration. This is possible because the structure of the Hydra's body is very simple.

Sexual Reproduction

In autumn, when the water becomes cooler, food scarcer, and unfavourable conditions are approaching, the body of the Hydra begins to bulge in places. These projections, however, are different from the buds. They grow from only one layer, ectoderm. Some of them are large, with a big female germ cell, the ovum, inside; others give rise to a multitude of male germ cells, the so called sperm cells or spermatozoa.

Mature sperm cells are exceedingly mobile. They leave the body of the Hydra, swim in the water and penetrate the ovaries of other Hydras. Fertilization takes place and a new cell is formed out of two which fuse the ovum (egg) and the spermatozoid (sperm).

In autumn, the adult Hydras die, but their fertilized ova (eggs) are able to withstand the winter cold. Protected by a thick cyst they shelter at the bottom of the pond.

Division

Within the cyst, the fertilized ovum divides by fission into two, four, eight, sixteen, and more parts. They do not burst the cyst, as those of the Amoeba do, but stay linked together. The process of fission stops in winter to be resumed the following spring, when it leads to the development of a multicellular animal, a young Hydra.

Tasks

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

1. The Hydra is a polyp type.

2. Hydras are usually found in ponds and other bodies of fresh water.

3. The Hydra is a plant.

4. The Hydra doesn’t have nerve cells.

5. The endoderm of the Hydra has 5 different types of cells.

6. Mature sperm cells in Hydra are immobile.

7. The Hydra is also known to have very complex muscular tissue.

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

1. Hydras usually cling to plants … .

2. The body wall is composed of … .

3. The mouth of the hydra can … .

4. Since the branches are connected with … .

5. As in the Amoeba, the cells can … .

6. Groups of cells that are … .

7. The restoration of lost parts … .

8. Fertilization takes place and … .

9. The process of fission stops in … .

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

1. What is hydra?

2. What can you say about hydra’s body?

3. Is it a unicellular animal? If not, how many cells does it have?

4. How can hydra digest its food?

5. What are the characteristic features of hydra’s nerve cells?

6. How does it respire and excrete?

7. Does hydra reproduce in the same way as amoeba?

8. Is it possible for hydra to restore its lost parts? How does it occur?

4. Translate a passage into Russian in written form.