Which subphylum includes arachnids, horseshoe crabs, and sea spiders?

The subphylum that includes arachnids, horseshoe crabs, and sea spiders is Chelicerata.

Chelicerates are arthropods named for their chelicerae, paired mouthparts positioned in front of the mouth. In spiders, these structures have become fangs; in horseshoe crabs, they are small feeding appendages. Arachnids such as spiders, scorpions, mites, ticks, and harvestmen make up the best-known land-dwelling members.

Horseshoe crabs belong to the marine group Xiphosura, while sea spiders are commonly placed among the pycnogonids. Their classification has been debated, but the traditional and widely used grouping places all three in Chelicerata. Myriapoda includes centipedes and millipedes, while Crustacea includes animals such as crabs, lobsters, and shrimp; neither is the correct subphylum here.