Body plans are extremely stable. Larvae which are free-living face assorted selective pressures and certain changes are quite common. For example, many gastropod lineages have gone from planktotrophic larvae to lecithotropy or direct development throughout geologic time. In bivalves, evolution of small adult size is strongly correlated with a change from planktotrophic larvae to brooding. Walter Garstang's poems on larval forms give several examples.
Development proceeds in characteristic time sequences and directions.
This is the top-down direction of development. Again, selection on the larvae must be taken into consideration. Also, some species are distinctive from very early stages of their growth. The shape of the developmental life span is curvilinear. A given kind of organism will have a lower size limit.