Final answer:
Crayfish belong to the phylum Arthropoda because they exhibit key features of this group, including body segmentation, jointed appendages, an exoskeleton made of chitin, and they undergo molting. They are part of the subphylum Crustacea.
Step-by-step explanation:
The crayfish is grouped in the phylum Arthropoda because it displays the defining characteristics of this group. Arthropods have a segmented body, jointed appendages, and an exoskeleton primarily made of chitin. Crayfish, specifically, belong to the subphylum Crustacea, which includes aquatic and some terrestrial species characterized by features such as two pairs of antennae, mandibles for mouthparts, and typically biramous appendages.
These traits are evident in crayfish, as seen in the presence of a carapace and a distinctive segmented body structure that align with those of other arthropods, such as insects, spiders, and trilobites. Moreover, as members of Ecdysozoa, crayfish also undergo the process of molting as they grow, which is common to all arthropods.