73 
1916-17.] Experiments and Observations on Crustacea. 
hooks, of which the more distal is the longer. These hooks are bent in 
the plane of the propodo-flexure, the inclination of the bend corresponding 
with that of the flexure. 
The Flexion- Complex . — Let us now briefly pass in review the arrange- 
ment of one of these limbs, selecting, say, the limb of a land mammal for 
comparison. The basipodite evidently corresponds in function to an 
upper-arm or to a thigh, the ischio-flexure to an elbow or to a knee.* 
Beyond this point the correspondence is less exact. The post-ischial axis 
may indeed be likened to the forearm or to the leg of the mammal, and 
the dactylopodite to the (terminal phalanx with its) nail or claw. When 
we look, however, for a flexion system corresponding to that of the ankle, 
in its relation as regards sign of flexion to the knee, we have to select 
simply the propodo-flexure, leaving nothing to correspond to the phalangeal 
system of the mammal. In other words, the mammalian hind-limb 
exhibits an additional flexion system with new change of sign. In Ligia 
the “ flexion-complex ” is tri-alternate instead of quadri-alternate. j* 
While the general similarity to the arrangement of joints in a 
mammalian limb is obvious, it is, strange to say, less easy to trace an 
analogy between the limb of Ligia and the limb of a crab. The reason is 
that in the crab co-planar flexures do not occur in such regular sequence. 
Movement of the walking limb of a crab has been designed on quite a 
different plan from that observed in Ligia. 
Limb-Taxis and Clinging Function. — Under this heading I shall 
discuss the general disposition of the seven pairs of limbs in relation to 
the body of the animal, commenting on one purpose thereby served. 
As already emphasised, the flexion-complex is predominantly uniplanar, 
which means that the dactylopodite of each limb moves in approximately 
the same plane as its basipodite. The basipodite of the first walking limb 
moves in a vertical plane which is almost but not quite antero-posterior ; 
the dactylopodite in consequence hooks into the ground in such a way as 
to resist a backward pull on the part of the limb. The basipodites of the 
third and fourth limbs are arranged transversely, and their dactylopodites 
hook into the ground in such a way as to prevent lateral displacement of 
the animal. The basipodites of the seventh limb, again, move in a more 
* The use of the term “knee” to designate the carpo-propodal joint — Boas (1883), 
Hansen (1893) — is unhappy. On the other hand, Sars’ (1869) description of the basi-ischial 
joint as a “ geniculate bend ” is appropriate. 
t Properly speaking, the term “ flexion-complex ” should imply a complete specification 
of the movements of all the joints taken in order. With a series of six joints the possible 
complications are many. It is partly for simplicity, but also because this mode of 
description closely corresponds to the facts, that I describe the flexion-complex as uniplanar. 
