1. Bones and echinoderm skeletons have been tested for anelasticity.

2. Bone shows considerable, largely anelastic strains, continuing for at least 54 days, by which time the extra deflexion may be 130% of the original deflexion. The rate at which the anelastic deflexion appears is proportional to the applied stress.

3. Between 2 and 50° C. there is a 300% increase in the rate at which anelastic deflexions in bones appear. Drying seems not to affect irreversibly the anelastic properties of bone.

4. Echinoderm spines show no anelastic deflexion on prolonged loading.

5. The mechanical behaviour of the skeletal materials of the two phyla investigated are such as would be expected from their constitution.

6. The biological significance of these findings is discussed. It is concluded that anelasticity will, except perhaps in a few special cases, be of only marginal importance in vertebrates in life.

7. The adaptive significance of the difference between bone and the skeletal tissue of echinoderms is as yet unknown. It does not lie in their anelastic properties.

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