305 
1916-17.] Experiments and Observations on Crustacea. 
In Nephrops the pleonic somites are ring-shaped, the anterior ones 
being circular, the posterior elliptical ; in this case the axis of the inter- 
somitic joint goes right across the middle of the somite. In Astacus the 
pleonic somites are bow-shaped, the greatest transverse diameter being on 
the ventral aspect; here the axis of the intersomitic joint lies nearer 
the ventral than the dorsal aspect, the arrangement resembling that in 
Glyptonotus. The pleonic somites of Homarus, while not accurately 
circular, resemble those of Nephrops rather than those of Astacus, and the 
joint axis lies near the middle of the ring. Seeing that movement of these 
different types of pleon occurs in each case without change of internal 
volume, it is plain that the' position of the various hinges is intimately 
correlated with the form in cross-section of the somites, any variation in 
the one involving a corresponding variation in the other ; that is to say, 
the principle of constant internal volume, a physiological principle, can be 
invoked to account for a structural correlation observed in the pleon. 
I have to thank Dr H. C. Williamson and Mr J. Mackenzie for material, 
and Dr T. W. Fulton for granting facilities in the Marine Laboratory at 
Aberdeen. 
(. Issued separately July 9, 1917.) 
VOL. XXXVII. 
20 
