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1916-17.] Experiments and Observations on Crustacea. 
bi-alternate only ; occurring alternately with each joint that permits of 
movement in the principal plane is an articulation whose axis lies in 
the principal plane. In the isopod limb five articulations out of six 
permit of movement in the principal plane, forming a tri-alternate flexion- 
complex ; at the most proximal articulation there are three degrees of 
freedom of movement, while rotation about the long axis of the moving 
segment may also occur ; immediately distal to the elbow- or knee-bend 
occurs an articulation whose axis lies in the principal plane. 
The matter may be still more compactly embodied in a diagram — see 
fig. 1. Imagine the six movable segments of the limb in each case to be 
pulled outwards in a straight line from the body. If a and b represent 
simple hinges moving respectively about a vertical axis situated in the 
plane of the paper and about an axis normal to the plane of the paper, and 
if B represents a universal hinge capable among other things of the same 
Fig. 1 . — Diagram to show the flexion-complex of a brachyuran compared with 
that of an isopodan limb. 
a , simple hinge joint whose axis is vertical and in the plane of the paper ; b, 
simple hinge joint whose axis is normal to the plane of the paper ; B, universal 
hinge whose chief movement is like that of b. 
The upper row of letters represents the brachyuran, the lower row the isopodan 
flexion-complex. 
movement as at b, then the upper row of letters in the figure will re- 
present the full flexion-complex in the brachyuran, the lower row that in 
the isopodan limb. 
In both types of limb the linkage of segments confers on the dactylo- 
podite, within a restricted range, complete freedom of movement in space. 
In the isopod limb this end is attained in essentially the same way as in 
a reptant vertebrate (or “ tetrapodan ”) limb. The fact that there exists 
an alternate form of linkage, the brachyuran, which leads to the same 
result, might remind one that the problem of conferring, with the help of 
different varieties of hinges, upon the terminal link of a series complete 
freedom of movement in space, theoretically admits of an enormous 
number of solutions. This ideal problem however is, in the animal, subject 
to many conditions of restraint: the question of bending moment is in- 
volved ; the question of inertia ; the provision of a specially complex set 
of muscles at a universal joint; restriction of complete freedom of move- 
ment in correlation with the build of the body and with the position of 
the limb in the series of limbs; and so on. The point to insist upon — 
