A coelomic cavity that has fused with the blastocoel to form a hemocoel (not lined with mesoderm), blood cavity where blood pools over the organs in an open circulatory system (hemolymph fluid) Soft cuticle covered in hydrophobic cells repels water ( velvet ) : ametabolic state at a teeny fraction of their life processes, basically dead; form dehydrated ball = tun. Nematodes most abundant, arthropods are the most specious (most named) Inverts with segmented bodies, jointed limbs, and an exoskeleton made of chitin. Marine, freshwater, terrestrial (fully adapted to live in totally dry along with amniotes) and the air. Human importance good (food) and bad (pest, disease) Development: triploblasts with protostome type of development. Tagmata: groups of segments that work together for a common function. Horseshoe crabs, spiders, ticks, scorpions, sea spiders. Appendages: chelicerae and pedipalps and four pairs of walking legs. No mandibles and no antennae (subphylum) myriapoda.