Unit 8: Invertebrates                                                    1   2   3   4   5   6

2. The simplest invertebrates

2.1. Poriferans

This phylum is also known as sponges.

They are the simplest invertebrates.

     a) Habitat

          They are aquatic animals, mainly marines, which live attached to rocks.

b) Morphological characteristics

      Their bodies are bag-shaped and are hollow.

   The inner space is called atrium.

   The body wall is full of pores.

   They have a bigger open called osculum.

   They don´t have organs or systems.

   There are little pieces of calcium carbonate or silica called spicules

   among the wall body cells that act as skeleton.

c) Vital functions:

They feed by filtration. Water enters by the pores and exits though the osculum. Nutritive particles of water are caught by the cells that covered the atrium (choanocytes). These cells have a flagellum

that is responsible for creating the water flow through the sponge.

They have asexual reproduction, through fragmentation or budding, and sexual reproduction through eggs.

2.2. Cnidarians

This phylum includes jellyfish, anemones, hydra and corals.

     a) Habitat

They are aquatic animals, mainly marines.

b) Morphological characteristics

They have radial symmetry.

They don´t have organs or systems.

Their body is soft with only one opening (mouth-anus) surrounded by tentacles.

The inside is hollow and it is called gastrovascular cavity.

They have two basic body forms:

   - Medusa

        Their bodies are umbrella-shaped. They are swimmers.

   - Polyp

        Their bodies are bag-shaped. They live attached to substrate

        (sessile).

Many Cnidarians species, such as corals, produce colonies

that are composed by many single organisms that live together.

c) Vital functions:

They are carnivorous. They use their tentacles to capture prey.

Tentacles have stinging-cells (cnidocytes). These cells contain a poisonous liquid and a filament that inject the poison when tentacles touch the prey.

Their life cycle is complex and includes individuals with sexual reproduction (medusa) and individuals with asexual reproduction (polyp).

2.1. The picture represents a............................

2.2. Relate the numbered parts of the picture with the words of the column and indicate which type of Cnidarians is every one of the represented animals.

a)    What type of reproduction do Cnidarians have?

b)    What type of organism corresponds to the sexual stage

     and to the asexual stage of Cnidarians life cycle?

2.3. Listen and indicate if these characteristics belong to Cnidarians

or Poriferans