Final answer:
Classifying mollusks solely based on shells presents challenges due to the absence of shells in some species, convergent shell forms, and the inability to discern non-shell features such as radula presence. Shell morphology also varies significantly within mollusk classes, necessitating a broader set of classification criteria.
Step-by-step explanation:
The student asked about the potential problems of using only shells to classify mollusks. There can be significant issues with this method, as the phylum Mollusca is highly diverse, comprising various species that exhibit vast differences in morphology, behavior, and anatomy. One of the primary issues with using shells for classification is that not all mollusks have shells; some have reduced or completely absent shells. Another problem is that shell forms can be convergent among unrelated species, leading to misclassification. Moreover, many other important characteristics are used in molluscan taxonomy, including radula presence, mantle structure, and details of the soft anatomy and physiology that are not apparent in the shell. For instance, some mollusks like bivalves do not possess a radula, which is a significant classification detail that cannot be discerned from shell features alone. Additionally, shell morphology can vary widely even within a single class of mollusks, such as Gastropoda, which includes both shell-bearing species like snails and shell-less species like slugs. Relying only on shells for classification would therefore omit critical aspects of molluscan diversity and lead to an incomplete understanding of the relationships between species.