Most respectable animals also have mesoderm. The derivation of this third germ layer is also a problem. In echinoderms it arises as outpushings of the primitive gut in the gastrula. There is nothing to suggest that 4d is ancestral to endomesoderm or vice versa. It looks as if these two types have evolved independently.
Coeloms also seem to have arisen separately in various groups.
The answer is we don't know, but the first two don't work very well. So we can set up at least two groups of animals, even if we don't follow Nielsen all the way. If these animals are all related to each other, as I suggest, and the differences arose by changes in the development of different kinds of animals then we have to start looking at how development might be changed in some detail. Early views were that environmental agents affected developing embryos directly - Lamarkism. The classic example of Lamarkism relates to the tails of mice.