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Proceedings of the Royal Society of Edinburgh. [Sess. 
the segments in extension of the body (cf. p.265); the intersegmental 
cuticle on the ventral aspect is much shorter. 
The axis of each hinge is not actually on the ventral surface, but cuts 
across just at the line of junction of the epimera with the tergites, i.e. 
some little distance above the floor of the permon cavity. Consequently 
in the movement of body flexion the two tergites of an adjoining pair are 
increasingly separated from each other, while the corresponding sternites 
are approximated ; the relative movement of the parts being like those of 
the gripping ends and handles respectively of a spring clothes-pin, the 
(more elongated) handles of the clothes-pin corresponding to the dorsal, 
the (shorter) gripping ends to the ventral parts of the somites — see fig. 9. 
Fig. 9. — Diagram to illustrate the sectional form of a thoracic 
somite of Glyptonotus and the nature of the articulation 
between the somites. 
On the right-hand side is shown the outline of a somite 
in end section ; on the left-hand side is represented the 
“rocker” articulation between the somites as it appears 
when viewed from the side. The bearing point in this 
rocker articulation lies at the level of the coxo-tergal 
junction. A line drawn across the somite at this level 
divides it into two approximately equal areas. 
It is obvious that by such an arrangement the internal volume of the 
perseon is not much altered by change from the flexed to the extended 
position. If we disarticulate a segment and, looking at the open end of the 
ring, imagine a line drawn across at the level of the axis of movement, we 
see that the area above this line is approximately equal to that below the 
line. Not only so, but the transverse diameter of the segment successively 
diminishes from below upwards, so that a large antero-posterior movement 
of any small slice taken across the dorsal part will cause a much less change 
of volume than an equal antero-posterior movement of a slice of similar 
depth taken across the ventral part. 
In many isopods, including Glyptonotus , the pleon is rigid (in this con- 
nection we may for the moment discount the cephalosome ; see, however, 
p. 286), and it seems probable on general grounds that this principle of 
constant internal volume might be applied to explain certain features in 
the conformation of Crustacea generally. In seeking to apply it, each 
animal or group of similar animals should, however, be considered on its 
own merits, for possible change of internal volume might turn out to be a 
