Topic: Gastropods, bivalves and cephalopods

Author: Elżbieta Szedzianis

Target group

6th grade student of elementary school.

Core curriculum

7. molluscs. Student:

a) presents the living environment, morphological characteristics and lifestyle of snails, molluscs and cephalopods,

b) observation of representatives of molluscs (photos, films, diagrams, etc.) and presents the common features of this group of animals,

c) explains the importance of molluscs in nature and for humans;

Lesson objectives

Students describe the structure, mode of life and vital functions of bivalves and gastropods.

The criteria for success

• you will distinguish molluscs from other organisms as well as gastropods from bivalves;

• you will recognize some Polish gastropods on the basis of their shells;

• you will compare the adaptations of animals to a free and sedentary mode of life.

Key competences

• communication in the mother tongue;

• communication in foreign languages;

• mathematical competence and basic competences in science and technology; 

• digital competence;

• learning to learn;

• social and civic competences.

Methods/forms of work

Working with text, direct and alternative observation.

Individual activity and activity in pairs.

Teaching aids

• abstract;

• interactive whiteboard or traditional blackboard;

• tablets/computers;

• bivalves and gastropods’ shells.

Before classes

At the beginning of the school year, students are asked to collect shells of bivalves and gastropods. They bring collected specimens to the classes.

Lesson plan overview

Introduction

1. The teacher specifies the subject and the objective of the lesson as well as the criteria for success.

2. The teacher asks the students to fill in a table, listing the features of tapeworm, human roundworm and earthworm structure. The teacher points to the absence of skeleton and specialised sense organs in these animals.

Realization

1. The teacher asks the students to sort the shells according to the similarity and the environment inhabited by the animals to which the shells belonged. Students look at illustrations of gastropods and bivalves. They list the differences between the shells from both groups.

2. The students look at the illustration showing the structure of a gastropod. They indicate its adaptations to the terrestrial environment, analyse the importance of shells and sensory organs.

3. The students look at the illustration showing the structure of a bivalve. The teacher discusses its way of respiration and nutrition. The teacher asks students to indicate the adaptations of bivalves
   to a sedentary mode of life.

4. In pairs, the students design a table comparing the features of gastropods and bivalves. Then they submit their proposals to the whole class. Students discuss which ones to accept. After agreeing on individual table items, students draw up a table. The teacher watches over the correct wording describing the features of both mollusc classes.

5. Working in pairs, the students determine the generic names of gastropods and bivalves, shells of which they have collected. To do this, they use the illustrations in the abstract and the diagram presenting the key for identification of national species of bivalves.

Summary

1. Students list the known molluscs and describe their adaptations to the habitat
   which they occupy.

2. Students do interactive exercise no. 1.

3. The teacher asks the students to finish the following sentences:

• In today's lesson I have learnt...

• What I have learnt the best is...

• I have not sufficiently mastered information about ....

Homework for the interested students

Present information on „The significance of molluscs” in an attractive form.

The teacher explains what should be understood by „attractive form” and emphasizes that the mere presentation of a large amount of information does not guarantee a high mark.

The following terms and recordings will be used during this lesson

Terms

Texts and recordings

Molluscs

Molluscs are animals with a soft, unsegmented body, usually composed of 3 parts: head, trunk, called the visceral sac and foot. They usually have bilateral symmetry. On the head there is a mouth and sense organs, like tentacles and eyes. The foot is strongly muscled and is used for movement. The visceral sac contains the majority of internal organs. On its surface lies the skin fold, called the mantle, which in the majority of molluscs produces a hard, calcium salt‑saturated exoskeleton in the form of a shell. In cephalopods and slug, it is reduced to a smaller plate overgrown by the fold of the mantle or does not occur at all. On the inner surface of the shells, there are insertions of muscles used during movement.

The organs of gas exchange in molluscs living in water are gills located in the mantle cavity, the space between the mantle and the visceral sac. Terrestrial gastropods conduct gas exchange through the strongly supplied with blood surface of the mantle cavity which acts as a lung.

Gastropods are the most numerous group of molluscs. Their body consists of a clearly distinguished head, visceral sac and foot. It is usually covered with a spirally wound shell. On the head there are 1 or 2 pairs of tentacles with eyes mounted on them.

In the mouth of snails, on the tongue, an organ is placed, called a radula, consisting of several rows of hard, sharp teeth. The radula can be extended out and used to scrape food from the surface, and even to break off pieces of plants and drill holes in the shells of other molluscs.

Among the gastropods, there are algae eaters, herbivores, carnivores, saprophytes and parasites. Most terrestrial gastropods feed on plants. Slugs, e.g. arions, are particularly fond of fungi (both edible and poisonous). Most predatory gastropods occur among marine species. Some of them hunt for other gastropods and coelenterata, some feed on carrion. There are also poisonous species which hold the prey with radula teeth or a foot, and then paralyze it with venom.

The foot allows for movement of gastropods. Its underside secretes mucus, which makes it easier to move, reducing the friction of the foot against the ground. When a gastropod feels threatened, it pulls its head and foot into the shell. In autumn, before falling into a state of numbness, terrestrial gastropods hide in a shell, and they close its opening with a limestone lid. In this way, they protect themselves against predators and drying out. The gastropod's shell grows with it, is its exoskeleton and provides muscle support. At the same time, it has protective functions.

All bivalves live in the water and use gills to breathe. They are generally sedentary animals and do not have a distinguished head. Their shell consists of 2 parts, the so‑called valves connected to each other by a hinge usually formed from overlapping teeth or resilifers. Valves can be closed thanks to the contractions of the muscles in the visceral sac.

Bivalves are filter feeders, i.e. organisms feeding on organic matter molecules and microorganisms separated from the surrounding water. They suck in water, along with the food and oxygen through  incurrent syphon. In the gill cavity, the bivalve separates the organic debris and small organisms and directs them to the mouth. The gills absorb oxygen and release carbon dioxide, and the used water is removed through the excurrent syphon.

Cephalopods are predatory sea molluscs. They breathe using gills. They float using the force of water jet removed from the mantle cavity. They have a well‑developed head, a pair of large eyes, a mouth with a radula and massive jaws, which in some species (e.g. octopus) form a beak, resembling that of parrots.

The foot of the cephalopods is transformed into 8 or 10 muscular, mobile arms surrounding the mouth, provided with suckers and used to capture the prey. Cuttlefish and squids have 10 arms, 2 of which, often called tentacles, are clearly longer than others. The suckers on the squid’s tentacles have chitin hooks on the edges that facilitate capturing large preys. Some of the octopuses secrete a venom that paralyses the prey almost instantly.

Molluscs are very important in the aquatic environment. They are a source of food for many animals, mainly fish, birds, mammals, rarely amphibians and reptiles, as well as for invertebrates such as gastropods, crustaceans and starfish. Bivalves contribute to the purification of water by filtering organic matter particles out of it.

Molluscs have been used by humans as a source of food rich in wholesome, easily digestible protein and valuable micronutrients for a long time. They are particularly popular in countries that have direct access to the sea or the ocean. Shells of molluscs, especially pearls produced by bivalves, are valued for their decorative qualities and used for making decorations and jewellery.

Terrestrial gastropods, especially slugs, cause enormous damage to crop production. They also contribute to the rotting of accumulated fruits and vegetables. Predatory sea gastropods, such as Muricidae, are responsible for losses in the breeding of bivalves, mainly of oysters, mussels and pinctadas. Bivalves burrowing in a hard surface can destroy objects immersed in water, e.g. wooden port and hydrotechnical equipment as well as ship hulls.

• Molluscs live in fresh water, saltwater or on land.

• Molluscs are invertebrates that have a soft, unsegmented body, usually consisting of 3 parts: a head, a visceral sac and a foot.

• Molluscs, with the exception of land gastropods, breathe with gills that are located in the mantle cavity.

• Molluscs reproduce sexually.