Friends and Foes in the Environment 1: The Phylum Arthropoda Form Four focuses on the phylum Arthropoda, which is the largest group in the animal kingdom. The book is a supplementary book for Biology Four Four, on the topic, The Phylum Arthropoda. The phylum Arthropoda constitutes four major classes of animals, namely; Hexapoda (89%); Arachnids (6%); Crustaces (4%) and Myriapoda, the smallest group making only 1% of animals in the phylum..