According to modern cladism, the taxonomy of the Gastropoda should be written in terms of strictly monophyletic groups. That means only one lineage of gastropods in each group. It will be difficult to do this, and still have a practical taxonomy for working biologists. Classifying animals in practice means using morphology (what they look like). But there are differences between the older groupings got from morphology, and those based on genome sequences. Convergent evolution, which has occurred often in Gastropods, may account for this.