Which of the following is the largest phylum in the animal kingdom in terms of number of known species?

Introduction / Context:
Animals are classified into various phyla based on their body plan, symmetry, and other structural characteristics. Some phyla contain only a small number of species, while others are extremely diverse. One phylum in particular dominates in terms of total number of described species, including insects, crustaceans, and many other familiar forms. This question asks you to identify the largest animal phylum by species count.

Given Data / Assumptions:

• The options include Annelida, Platyhelminthes, Chordata, Arthropoda, and Mollusca.
• Species count refers to the number of described species known to science.
• We assume standard textbook classifications of animal phyla.
• Only one phylum clearly has more species than all the others combined.

Concept / Approach:
Arthropoda is by far the largest phylum in the animal kingdom. It includes insects, spiders, scorpions, crustaceans, centipedes, and millipedes. Insects alone number in the millions of species (described plus estimated). In comparison, other phyla such as Chordata (which includes vertebrates like fish, amphibians, reptiles, birds, and mammals), Annelida (segmented worms), Platyhelminthes (flatworms), and Mollusca (snails, clams, octopus) have far fewer species. Therefore, the correct approach is to recall that most known animal species are arthropods, making Arthropoda the largest phylum.

Step-by-Step Solution:
Step 1: Recall that insects form the largest group of animals on Earth and belong to the phylum Arthropoda.Step 2: Note that Arthropoda also includes other groups like arachnids and crustaceans, further increasing its diversity.Step 3: Compare this with Chordata, which contains all vertebrates but still far fewer species than the combined arthropods.Step 4: Recognise that Annelida and Platyhelminthes contain many worms but not nearly as many species as Arthropoda.Step 5: Mollusca is very diverse but still ranks below Arthropoda in species numbers.Step 6: Therefore, choose Arthropoda as the correct answer.

Verification / Alternative check:
Biodiversity statistics in zoology and ecology literature consistently report that over half of all described animal species are insects, and when you add other arthropods, the percentage goes even higher. This makes Arthropoda the dominant phylum by species count. Textbooks also commonly state that Arthropoda is the largest phylum of the animal kingdom, confirming the answer.

Why Other Options Are Wrong:
Annelida (segmented worms) contain notable groups like earthworms and leeches but far fewer species than arthropods.Platyhelminthes (flatworms) include parasitic and free living species but are not numerically dominant.Chordata includes all vertebrates and some invertebrate groups, but its species count is much lower than Arthropoda.Mollusca is diverse (snails, clams, cephalopods) but still does not outnumber arthropods.

Common Pitfalls:
Students sometimes choose Chordata because it contains familiar large animals such as humans and other vertebrates, mistakenly equating visibility or size with diversity. Others may underestimate the sheer number of insect species. To avoid such mistakes, always recall that insects and other arthropods dominate global biodiversity, making Arthropoda the largest phylum by number of species.

Final Answer:
The correct answer is Arthropoda.