Sea spiders are chelicerates (the group of arthropods that includes arachnids and sea spiders) of the basal order Pantopoda, characterised by their extremely unusual body plan, having a sucking proboscis, a simple heart, a gut that branches into their extremities, and very, very elongate, thin limbs. They are believed to be basal chelicerates, only distantly related to true spiders, despite their name. Reproduction is external, and males typically care for their young.