Bauplän (pl. Baupläne): What is a body plan? This is a difficult question to define, and it is usually answered with examples: the insect body plan, the jellyfish body plan, or whale body plan. Loosely defined, a body plan is primarily morphological, involving the shared structural homologies of upper taxa. For example, the vertebrate bauplän might be described as comprising a "cephalised, sensate, bilaterally symmetrical, motile, ceolomate gnathostome having a segmented endoskeleton, a dorsal hollow nerve chord, and a ventral gut" (Ostrom 1992, p. 119).
Body plans, or baupläne, affect development at its most basic level, thus developmental constraints strongly influence the range of body plans possible. Even in simple animals, axes of symmetry are so fundamental that significant internal co-adaptation is required for viable body plan mutations to occur.
This raises the question of why a number of different body plans arose at the cusp of the Cambrian Period – the so-called Cambrian Explosion. Some paleobiologists believe the answer lies in the Ediacaran fossil remains dating back before the Cambrian, in the Vendian Period (565-45 million years ago).
Ostrom, John H. (1992): Chapter 5 – A History of Vertebrate Successes. In, Schopf, J. William (ed.) Major Events in the History of Life. Jones and Bartlett.