Final answer:
The second largest animal phylum after Arthropoda is Mollusca, including invertebrates like snails and octopuses, whereas vertebrates belong to the phylum Chordata, which also contains some invertebrates.
Step-by-step explanation:
The second largest animal phylum, taking into account both invertebrates and vertebrates, is the Mollusca. While the phylum Arthropoda stands as the largest phylum overall, consisting primarily of invertebrates such as insects, arachnids, and crustaceans, Mollusca is recognized as the second largest. Mollusca includes diverse creatures such as snails, clams, squids, and octopuses. Unlike the members of phylum Chordata, which encompasses all vertebrates and some invertebrates, mollusks are exclusively invertebrates. Vertebrates, which belong to the phylum Chordata and include species like fish, amphibians, reptiles, birds, and mammals, represent a small proportion of animal diversity, with vertebrates accounting for about 5% of all animal species. The vast majority of animal species are invertebrates, which do not possess a backbone. The phylum Chordata is unique among animal phyla because it includes both invertebrate and vertebrate species.