Classification of living organisms in kingdoms

Question

What feature justifies the placing of prawns and grasshoppers in the same phylum

Answer 1

Kingdoms represent phylogenetic relationship. As such there is no need for specific phenotypic features to group lineages together but only the recognition of their phylogenetic relationship. This being said, lineages from a certain kingdoms do share some phenotypic features.

As both the prawns and the grasshoppers belong to the kingdoms Animalia (aka. Metazoa), you can just look at the wikipedia article for animal and repeat the listed features. In short, they are both eukaryotic, multicellular, have motile cells and are heterotrophs.