An internal skeleton is used to help an organism hold its body up and keep from falling, while a stem stores nutrients, and produces new living tissue, and transports fluids.
<span>They have very stable outer electron shells</span>
C) the sun warms the ground.
I would say that the fishes exhibited the greatest diversity (though the brachiopods also had considerable diversity) and mostly were of the ostracoderms (with a platey or shell-like skin and no jawbone) which exhibited many varieties and also the placoderm which had gills, a jawbone and fins so was developing characteristics of modern fish.
