Complex sentence: The cuticle forms the major part of the integument, that is to say the skin or outer skeleton, of the Arthropoda, including the insects, Crustacea, Arachnida, and Myriapoda.
Split into simpler sentences: The cuticle forms the major part of the integument of the Arthropoda. It includes most of the material of the exoskeleton of the insects, Crustacea, Arachnida, and Myriapoda.