Biological classification: insects, annelids, mollusks. . How to classify?

First of all, remember what is the class and what is the phylum, such as insects, mollusks, annelids, arthropods, coelenterates and crustaceans. Generally speaking, phylum summarizes the characteristics of this species group, such as chordates, but these are aimed at most organisms. In fact, as long as you have more contact with them, it is easy to systematically remember what to include. Look at Wikipedia's systematic classification method.