71 
1916-17.] Experiments and Observations on Crustacea. 
altogether the result of the curved locus of origin of the basipodites, is 
connected with the function of the anterior limbs as pulling and of the 
posterior limbs as pushing agents. With lateral flexion of the thorax, 
say, to the right, the interspaces between the successive proximal ends 
of the right basipodites are obliterated, and they all lie in contact. 
Similarly, with ventriflexion of the thorax, the basipodites on both sides 
are brought into contact along their whole length. 
Each basipodite is capable of movement through a right angle at the 
coxo- basal joint. In the extended position the anterior four are per- 
pendicular to the surface of the ground, the posterior three sloping 
increasingly backwards. 
Ischiopodites and the I soldo- Flexure. — In the ordinary squatting 
position of the animal the extreme four distal segments of the limb 
(meropodite to dactylopodite) form an axis which runs parallel to the 
basipodite, but in the reverse direction, viz. medio-laterally. Between 
these two oppositely directed parts intervenes an ischiopodite, the length 
of which in successive limbs increases from before backwards. The basi- 
ischial joint, almost straight when extended, flexes to a right angle in the 
same plane as, but with opposite sign to, the coxo-basal flexure. A statement 
almost identical applies to the ischio-meral joint. As a consequence, the 
intervention of the ischiopodite marks an important turning-point in the 
line of the limb, at which the sharp medial flexion of the basipodite on 
the body is neutralised, as it were. For convenience in description we 
shall refer to this seat of double flexion at the basi-ischial and ischio- 
meral joints as the “ ischio-flexure ” (see fig. 2, A). 
In the death position of the limb and invariably after immersion in dis- 
tilled water (see the first communication of this series) the flexion at the 
ischio-flexure is right-angled only, for then the ischio-meral joint is ex- 
tended while the flexion at the more proximal joint remains unaltered. It 
would seem that flexion at the basi-ischial joint is, in Ligia as well as in 
many other isopods, more fundamental than flexion at the ischio-meral 
joint (see fig. 2, B). 
The Post-ischial Limb and the Propodo- Flexure. — From one point of 
view the post-ischial limb may be looked upon as a single axis or lever, 
which in the squatting position of the animal forms a very mild curve 
concave ventral wards. It can also be considered as furnishing the basis 
of a compound flexural unit like the ischio-flexure — and like it flexing 
in the same vertical plane as that in which the basipodite flexes. The 
meropodite and carpopodite lie in one straight line ; the joint between 
them permits of no bending in the vertical plane. Marked flexion, to 
