The different bodyplans of the different phyla arose from a hypothetical, archaetypical Blastaea a hollow ball of cells. Once settled on the bottom, the Gastraea was also seen by Haekel to be a common ancestral type, although there are gastraea and gastraea. A little whirlpool in its wake maintained by a tasty store, A pocket sank to lodge it all and left a blastopore. Feature Protostome Deuterostome 1. It has been argued that small eggs and pelagic larvae are nowadays a feature only of rather large marine animals which produce large numbers of eggs.
Small animals produce few eggs and protect their brood.
This, of course, applies now, but not necessarily 600mya. when there were presumably less multicellular predators in the plankton. We should perhaps look a little more closely at the concept of a larval form. There is in fact no reason why any stage of a life cycle should not develop specialisations and, say feed in a different way or live in a different place. There is also no reason why changes in lifestyle should not be accompanied by changes in shape - metamorphoses. And there is really no difference in kind between a metamorphosis, which occurs suddenly and maturation which occurs rather more slowly, except the time scale.