In the last paper of the present series it was pointed out that the articulations between the thoracic somites of Glyptonotus are so designed as to minimise change of internal volume during flexion and extension of the body. The pleon of the long-tailed Decapoda forms a system comparable to the series of thoracic somites of Glyptonotus; and as the macrurous decapods execute very rapid strokes of the pleon, one might expect that change of volume during movement is slight, for otherwise there would be waste of energy owing to inertia. It was decided to examine the matter experimentally.