Unit 8: Invertebrates                                                    1   2   3   4   5   6

4. Molluscs

a) Habitat

     Most part of Molluscs is aquatic. They live in the sea or in fresh water.

     Some of them are terrestrial.

b) Morphological characteristics

- They have bilateral symmetry.

-      They have a soft body divided into three parts:

           - Head.

                  - Visceral mass with organs.

                  - Muscular foot with different shapes

                     depending on the group.

- The body is covered by a thick membrane, the mantle.

   This produces a protective shell.

c) Vital functions:

     Nutrition:

The digestive system has a mouth, anus and glands. They can be herbivorous and carnivorous. 

Aquatic species breathe through gills, and terrestrial species breathe through lungs.

Molluscs have an open circulatory system. Blood exits from blood vessels to the tissues and then is collected by other blood vessels to return to the heart.

Interaction

They have a well developed brain and sense organs.

Reproduction:

Most are hermaphrodites and oviparous. Larva hatches and undergoes metamorphosis to become adult.

d) Classification:

- Gastropods (E. g. Snails, slugs and sea snails).

- Cephalopods (E. g. Octopus, squids and cuttlefish).

   They are aquatic.

   They have an internal shell, except octopus that doesn’t have it.

   Their head is well-developed with big eyes.

   Their foot is transformed into tentacles around the mouth, to catch prey.

   They are carnivores. The mouth has a special chewing structure

   called beak.

4.1. Complete the chart about the main groups of Molluscs and classify

        the molluscs that are represented.

Then, listen and indicate which sentece correspond to each group.

4.2. Listen and say which part of the Molluscs' body is described: