In animal classification, which of the following is considered the second largest animal phylum after Arthropoda in terms of described species?

Introduction / Context:
The animal kingdom is divided into several phyla based on body plan, symmetry, body cavity, segmentation, and many other characteristics. Phyla differ not only in structure but also in how many species they contain. Knowing which phyla are largest in terms of species richness is a common objective question in basic zoology and general knowledge exams. Here, you are asked to identify the second largest animal phylum after Arthropoda.

Given Data / Assumptions:

• Arthropoda is widely recognised as the largest animal phylum by number of species.
• Options include Mollusca, Chordata, Coelomates, Annelida, and Arthropoda.
• We consider the approximate number of described species as reported in standard zoology texts.
• Coelomates is not a single phylum but a broad category for animals with a true coelom.

Concept / Approach:
Arthropoda includes insects, crustaceans, arachnids, and many other groups and is by far the largest phylum. The second largest in terms of species numbers is Mollusca, which includes snails, slugs, clams, octopus, and squid. Chordata, although important and including vertebrates like fishes, amphibians, reptiles, birds, and mammals, has fewer described species than Mollusca. Annelida is a smaller group of segmented worms. Coelomates is not a single phylum and therefore does not qualify in this question.

Step-by-Step Solution:
Step 1: Recall that Arthropoda is the largest animal phylum and is already mentioned in the question. Step 2: Identify which remaining phylum has the next highest species diversity. Step 3: Mollusca includes many marine, freshwater, and terrestrial soft bodied animals with shells or internal skeletons. Step 4: Chordata includes all vertebrates but has fewer total species than Mollusca. Step 5: Annelida is a smaller phylum mainly of segmented worms. Step 6: Coelomates is not a phylum but a body cavity category, so it cannot be counted as one phylum. Step 7: Therefore, the second largest animal phylum is Mollusca.

Verification / Alternative check:
Zoology reference books and competitive exam guides often state that Arthropoda is the largest phylum and Mollusca is the second largest. Lists of phyla ordered by number of species consistently place Mollusca near the top. Examples like gastropods, bivalves, and cephalopods illustrate the vast diversity within molluscs. In contrast, the total number of known chordate species is lower than that of molluscs, confirming that Mollusca holds the second position in species richness.

Why Other Options Are Wrong:
Chordata is an important phylum but not as species rich as Mollusca, so option B is incorrect. Coelomates is a descriptive term for animals with a true coelom and includes several phyla, so it is not a single phylum. Annelida is a smaller phylum of segmented worms and clearly does not exceed Mollusca in species number. Arthropoda is the largest phylum overall, but the question asks for the second largest.

Common Pitfalls:
Students sometimes pick Chordata because it contains familiar animals like humans, fishes, and birds, but familiarity does not equal species count. Another pitfall is misunderstanding the term coelomates and treating it as a phylum. Always remember that phylum names are specific like Mollusca, Arthropoda, Chordata, and Annelida, whereas terms such as coelomates, protostomes, or deuterostomes describe broader groups or developmental patterns.

Final Answer:
The second largest animal phylum after Arthropoda is Mollusca.