Subphyla

Subphylum is a taxonomic rank used in zoological classification, positioned below phylum and above class. The rank is employed when a phylum contains distinct, well-supported lineages that warrant formal grouping beyond the broader phylum level. Subphyla are typically defined by shared anatomical features, developmental patterns, and increasingly by molecular data, though not all phyla use this rank consistently.

Within Chordata, several subphyla are commonly cited. The Vertebrata (or Vertebrates) includes animals with backbones, while Cephalochordata encompasses the lancelets, and Urochordata comprises the tunicates. In some classifications, Cephalochordata and Urochordata are treated as separate subphyla within Chordata, or as distinct phyla, reflecting ongoing debates about chordate relationships.

In Arthropoda, various classifications recognize subphyla such as Chelicerata, Crustacea, Hexapoda, and Myriapoda, though the exact arrangement and rank can vary among taxonomic systems. Hexapoda, for example, groups the six-legged organisms including insects and their close relatives, while Crustacea contains crabs, lobsters, and related forms, and Myriapoda includes centipedes and millipedes. The use of subphyla in Arthropoda illustrates how large, diverse phyla are subdivided to reflect major evolutionary lineages.

The concept of subphylum overlaps with cladistic approaches, where ranks can be replaced by clades representing monophyletic groups. As such, the usage of subphyla is more prevalent in traditional Linnaean taxonomy and may vary with new phylogenetic data and differing taxonomic philosophies.