Which of the following statements is INCORRECT?

The correct answer is Option (2) - Mollusca is the largest animal phylum.

The incorrect statement is:  Mollusca is the largest animal phylum.

While Mollusca is a large phylum, it's not the largest. Here's a breakdown:

• Arthropoda is actually the largest animal phylum by far, with millions of described species including insects, crustaceans, arachnids, and myriapods.

• Mollusca is a diverse phylum with snails, clams, oysters, octopuses, as well as many other shelled and shell-less mollusks, but it has a smaller number of species compared to Arthropoda.
• Ctenophores are indeed commonly known as sea walnuts, which are fascinating gelatinous marine invertebrates.
• Chordates do have a post-anal tail at some stage in their development, though some groups like mammals may have a reduced or no visible tail in adults.

So, Arthropoda being the largest animal phylum is the statement that contradicts current scientific understanding.