Topic: Cnidarians

Author: Elżbieta Szedzianis

Target group

Students of the 6th grade of an elementary school.

New core curriculum

7 . Diversity and unity of the animal world:

2) cnidarians – student:

A) presents the living environment, morphological characteristics and life style of parachutists,

B) observes representatives of cnidarians (photos, films, schemes, etc.) and presents the common features of this group of animals,

C) explains the importance of paratroids in nature.

The general aim of education

The students describe the structure of cnidarians, their life activities and meaning.

Criteria of success

• You will identify cnidarians in a photo and in a film, you will distinguish a medusa from a polyp;

• giving three examples, you will justify why cnidarians are animals, not plants;

• you will show cnidarians leading a free lifestyle and a settled lifestyle;

• you will give two examples of how cnidarians play an important role in the aquatic environment.

Key competences

• communication in foreign languages;

• communication in the mother tongue;

• mathematical competence and basic competences in science and technology; 

• learning to learn.

Methods / techniques

Replacement observation, and work with text.

Individual work, work in pairs.

Teaching aids

• abstract;

• interactive or traditional board;

• tablets/computers.

Lesson plan overview (Process)

Introduction

1. The teacher asks the students to listen to a story about a group of organisms. When listening, the students are to draw how they imagine these organisms. The teacher says (this information can be repeated once): „The organisms we are going to learn about today do not look like animals. Maybe they look a bit like plants. They have no head, no front or back of the body, no right or left side. They do not have legs or fins, although they live in the aquatic environment. They have no heart or intestine. Their bodies are blindly ended sacks, so the mouths also function as anuses.” When the drawings are ready, the students exchange them and make comments.

2. The teacher displays video recording 1: „Hydra” and a film from gallery 1: “Medusas”. The teacher explains that these cnidarians can be found in Poland: medusas in the Baltic Sea, and hydras in ponds and lakes. However, most of the cnidarians are found in warm seas.

3. The teacher gives the students the criteria of success.

Realization

1. The teacher divides the class into three groups. It informs that the task of each of them will be to develop the assigned issue on the basis of available sources of information (e‑textbook, internet, etc.)

• group I – Present life activities anemone;    

• group II – Are parzydełkowce plants or animals? Justify the answer;    

• group III – Which of the parzydełkowce lead free, and which settled the lifestyle?     Justify the answer.

2. The teacher can give pupils advice on finding videos of sea animals online, e.g. Cnidarians: Life on the Move | the Shape of Life | The Story of the Life, and used their fragments in their presentations.

3. After the set time, the groups present their studies. Other students can complement their colleagues' statements and ask questions. The teacher watches over the correctness of the information provided

4. The teacher displays Illustration 4: „Structure of a polyp and a medusa”, repeats the text on cnidarians used in the introduction of the lesson, and when discussing it the teacher points to the characteristics of the structure of a polyp and a medusa.

5. The students do interactive exercise No. 1 in pairs. The teacher informs that if they have a problem with its solution, they can use the content in the abstract.

6. The teacher asks the students to give a title to the completed text after solving the task.

Summary

1. The teacher asks the students to perform interactive tasks No. 2, 3 and 4 by themselves.

2. The teacher asks the students about what they learnt on the lesson.

Homework

Make a medusa in a bottle. The instructions to make a medusa in a bottle are presented in the film entitled „How to make a medusa model?”.

The following terms and recordings will be used during this lesson

Terms

Texts and recordings

Cnidarians

Animals are heterotrophic, unable to produce food on their own. They take the energy required for functioning from food. They feed on plants and other organisms they actively hunt for, or on carrion and organic debris found in water or soil. Most animals travel in search of food or a partner for breeding, while other animals lead a sedentary lifestyle.

Cnidarians are characterized by a little complicated structure. Their body consists of 2 layers of cells that create tissues with a structure similar to the epithelial and nervous tissues of other animals. They do not have respiratory, cardiovascular and excretory systems. Their nervous and muscular systems are poorly developed. The interior of the body is a large gastrovascular cavity, in which digestion and absorption of food takes place. Undigested remains are removed from the gastrovascular cavity through the mouth.

Adult cnidarians are animals with a radial symmetry. They can be found in two forms – a sedentary polyp and jellyfish(medusa) floating in water. However, for most of their life cycle they take only one form.

The body of the polyp looks like a bag attached to the ground. The mouth is located in the top part of the body, it is directed upwards and surrounded by a crown of tentacles, which help them to move and capture food, and to protect against predators. The body of the jellyfish is in the form of a bell. The mouth hole in this form is located in the lower part of the body and is also surrounded by tentacles.

Cnidarians live in all climatic zones, most species in salt water. They are very sensitive to temperature changes and salinity of water. They usually occur in very clean waters, which is why they belong to the so‑called indicator organisms (bioindicators).

Cnidarians, like jellyfish and some polyps, can live alone. However, most polyps form clusters called colonies, which are formed when individuals after division do not move away from each other. Units that form a colony communicate with each other, and sometimes specialize in performing functions that condition the existence of the colony (e.g., some are focused on nutrition and others on reproduction). Polyps most often lead sedentary lives, less often they move on the bottom of water reservoirs, striding or tumbling. When feeding, they stretch the entire length and comb water with their tentacles. If alarmed, they shrink to a small size.

Jellyfish actively swim in water, using the jet propulsion. It is evoked by contractions of their bells that cause rapid ejection of water sucked earlier into the gastrovascular cavity. Jellyfish are classified as planktonic organisms because they do not have structures that would allow them to resist the current of water or pursue a victim.

Hydra vulgaris is only 1‑3 mm long. It attaches to the ground and directs food into its mouth using its tentacles. Threatened, it moves the body.

Most cnidarians feed on organic matter suspended in water and small planktonic organisms. Only few species, especially jellyfish and slow‑floating polyps in colonies, like the Portuguese man‑of‑war, can catch larger animals, such as fish.

The name cnidarians comes from cnidocytes used to defend, capture and incapacitate the victim. These cells appear on the surface of tentacles. They are a powerful weapon – some species of cnidarians can kill even a human.

The activation of cnidocytes occurs when the victim, passing by an animal, touches its tentacles.

Cnidarians reproduce sexually and asexually. Adult polyps usually reproduce asexually by budding or less frequently by crosswise division. In many species, such as hydra, the descendant is separated from the body of the parent and begins an independent life. In other species, the descendant becomes attached to the parent's body, resulting in the formation of a colony that grows by budding of another polyps.

Transverse division is the way certain species reproduce, and during this division jellyfish are formed from polyps. Initially, tiny narrowings appear on the body of the polyp, which become clearer and eventually allow the plate‑like structures to be formed – young jellyfish.

Cnidarians are dioecious organisms and they fertilize externally. During reproduction, males release sperm into water and females egg cells. Reproductive cells (gametes) combine with each other during the fertilization process, resulting in a zygote being formed. A ciliated larva is hatched, which after some time settles at the bottom of a reservoir and transforms into a polyp.

Among cnidarians, corals have a special significance in nature. They are the only ones that produce hard, limestone external or internal skeletons. After a polyp dies, the skeletons do not decompose but accumulate to form a coral reef, which is the basis for the growth of subsequent generations of corals. Over time, the reef grows so much that it can rise above the water surface and form an island. Hundreds of islands in the Pacific Ocean were formed this way. The growth rate of the reef is extremely slow and amounts to 1 m per 1000 years only.

Coral reefs are the habitat of many organisms. Among the colonies of cnidarians there are numerous species of fabulously colourful fish, crustaceans, snails, starfish and sea urchins. They are also the place of occurrence of many algae and protozoa. Cnidarians are also a source of food for many marine animals. Skeletons of corals, especially the red coral, are used to make jewellery and other decorations. Some species of cnidarians are very dangerous for humans, they can cause burns and even death.

• Cnidarians are water animals living in both salty and sweet waters.

• Cnidarians are tissue animals with a radiant body symmetry; they are found in the forms of a polyp and a medusa.

• Polyps lead a sedentary or semi‑sedentary way of life, and medusas float in water.

• Cnidarians have a gastrovascular cavity, in which digestion of food and absorption of nutrients obtained there from takes place. It is behind a hole that performs the function of a mouth and an anus.

• Cnidarians reproduce asexually by budding or body division, or sexually.