Which of the following pairs is INCORRECT regarding the grade of organisation and its example?

To determine which of the given pairs is incorrect, let us analyze the levels (grades) of structural organization in animals and see if each organism is correctly matched with its level of organization.

1. Protoplasmic grade of organisation - Paramecium:
This grade is found in unicellular organisms (protozoans) where all life activities are confined within a single cell. Paramecium is a unicellular eukaryotic protozoan, so it exhibits a protoplasmic grade of organization. This pair is correct.

2. Cellular grade of organisation - Sycon:
In this grade, the body consists of a loose division of labor among cells that are not organized into tissues. Sponges, belonging to the phylum Porifera, show this level of organization. Sycon is a marine sponge, and thus exhibits a cellular grade of organization. This pair is correct.

3. Cell-tissue grade of organisation - Jellyfish:
Here, similar cells are organized into tissues to perform a common function. Cnidarians (coelenterates) display this grade. Jellyfish (such as Aurelia) belongs to the phylum Cnidaria and exhibits tissue-level organization. This pair is correct.

4. Tissue-organ grade of organisation - Euplectella:
In this level, tissues are grouped together to form organs. This grade is typically seen in Platyhelminthes (flatworms) and higher phyla. However, Euplectella (Venus' flower basket) is a glass sponge belonging to the phylum Porifera. As a sponge, it exhibits only a cellular grade of organisation, not a tissue-organ grade. Therefore, this pair is incorrect.

Thus, the incorrect pair is "Tissue-organ grade organisation - Euplectella".