259 
1916-17.] Experiments and Observations on Crustacea. 
there is a marked difference in specific gravity between the fore and the hind 
part of the animal (a thing in itself unlikely), the position taken up by the 
limbs after death can hardly correspond with that adopted in walking. 
The dactylopodites of the first pair of perseopods can reach forwards to 
a considerable distance in front of the head ; those of the second pair can 
reach forward level with the front of the head ; and it is probable that at 
any phase of gait on a horizontal surface, at least one limb of the four is 
in contact with the ground at a point in front of the centre of gravity. 
While it is not possible to reconstruct the gait from an examination of 
the dead animal, one can formulate certain conditions with which the gait 
probably complies. General considerations may come first. In the forward 
progression of any reptant animal the distal extremity of each limb in 
contact with the ground may be considered as moving backward relatively 
to some point on the body of the animal ; there being no slip between the 
distal extremity and the walking surface, the path of this relative back- 
ward movement, where as in isopods there is no swaying of the body from 
side to side, is rectilinear and parallel to the antero-posterior axis of the 
animal. The two dactylopodites of a pair of limbs may thus be con- 
sidered as describing parallels of equal length equidistant from the axis 
of the body. 
If the pair of limbs is moved simultaneously, like pleopods or like 
swimming limbs, these parallels may be looked upon as forming opposite 
sides of a rectangle. If they move in alternate rhythm, the backward 
paths of the dactylopodites relatively to some fixed point in the mid line 
of the body also form opposite sides of a rectangle, with this difference, 
however, that the dactylopodite on one side is in a different phase of 
(periodic) movement from that on the other — see fig. 7. The mode of 
operation of any pair of limbs is to a first approximation determined if we 
can specify the length of stride in relation to the length of the animal, and 
the relation in phase of the movement on one side to that on the other. 
We now come to data pertaining specially to Glyptonotus. The first 
two pairs of perseopods have an antero-posterior reach exactly equal to the 
length of the body. The antero-posterior reach of the third pair is about 
three-quarters, that of the fourth pair one-half the length of the body. In 
the structural arrangements of the limbs there is nothing to indicate that 
any two successive limbs on one side ever intercross ; it may be taken for 
granted that the dactylopodite of a given limb is invariably in front of 
that of its successor. These considerations make it probable that, if all 
the limbs have the same period, the front three pairs are not employed in 
their full antero-posterior reach during ordinary progression, but with a 
