#5 is a highly modified arachnid (spiders, scorpions, ticks). The front part of the cephalothorax holding the chelicerae has elongated, forming a distinct "head." It has no eyes, and its front pair of legs have grown feelers (the short "legs" at the very front are called pedipalps, mouthparts that all arachnids have). The tail has a defensive set of blades that it swings back and forth to deter predators.
#7 is a centipede-like creature whose rear legs have developed webbing, allowing it to use them as a swimming tail. It swims very fast and is equally at home in water or on land.
#6 is a centipede-like creature with an armored head and a reduced number of legs. It lives in saltwater. It has webbed feet for faster scurrying on the seafloor and numerous grasping "fingers" for carrying nesting material or food and for tool use (they are unusually intelligent and social). The frontmost right "hand" carries a set of bristles that it uses as a "key" of sorts to open its symbiotic-bivalve home. Each is unique.
I have no idea what #4 is. Help me.