The criterion of permanence may be used to divide skeletons or parts thereof into permanent and transient structures; the former can be subdivided according to whether they are immutable or modifiable. 2. Permanent immutable exoskeletons which grow by accretion can adapt to the changing need of the animal by the development of time-line structures of various kinds; without these, the skeleton is of the same shape throughout postlarval life and cannot be adapted to changing conditions. 3. A permanent immutable accretionary exoskeleton is subject to two restrictions: (a) newly incorporated time-line structures must harmonize with previously formed portions of the skeleton so that the latter can function optimally as an integrated unit; and (b) time-line structures can be generated only at the growing margin (generating curve) and not over the surface of the skeleton as a whole. 4. Permanent immutable endoskeletons, transient exoskeletons, and modifiable skeletons are different solutions to the problems imposed by these restrictions, and are propagated as a generating surface rather than a curve; each is well suited to and embodies the strategy necessary for a particular mode of life.
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Geerat J. Vermeij (1970) studied this question.