304 Proceedings of the Royal Society of Edinburgh. [Sess. 
XVI. — Experiments and Observations on Crustacea : Part V. A 
Functional Interpretation of certain Structural Features 
in the Pleon of Macrurous Decapods. By John Tait, 
M.D., D.Sc. (From the Marine Laboratory, Aberdeen, and the 
Department of Physiology, Edinburgh University.) 
(MS. received March 31, 1917. Read May 7, 1917.) 
In the last paper of the present series * it was pointed out that the articula- 
tions 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. 
The animals used for experiment were Homarus, Astacus, and Neplirops , 
and the first observations were made on formalin-fixed specimens. After 
disarticulation of the pleon from the thorax the internal contents were 
scooped out, the cavity was filled with water, and the pleon subjected to 
passive movement. The internal volume was found to diminish with 
flexion, the water flowing over the edge of the first somite. The experi- 
ments, however, could hardly be taken to imitate the natural conditions, 
for, owing to hardening, the ventral soft cuticle was seen to fold unequally 
and irregularly. 
When the same experiment is carried out on a recently exuviated 
pleon, or even on an exuviated pleon, which has lain for some time in 
preservative, passive flexion causes no change of internal volume. In the 
exuviated pleon the ventral soft cuticle is seen to fold exactly in the 
middle, the internal face of one half being accurately applied to that of 
the other half. To make the experiment still more delicate, the first 
pleonic somite was sealed with a waxed cork, while a narrow glass tube 
passing through the cork served to narrow down the opening at the upper 
end and thus to act as a gauge. In this way it was proved that change 
from full extension to full flexion occurs without any change of volume 
whatsoever. 
* Ante, p. 268. 
