1916-17.] Experiments and Observations on Crustacea. 63 
forwards) are responsible for locomotion, which in the circumstances is 
highly insecure, the centre of gravity now lying beyond the point of 
support. The animal is forced to carry its head as high as possible, 
whereby its whole appearance assumes a comic aspect.” 
In popular descriptions of the moulting of Crustacea authors never fail 
to comment on the fact that the newly emerged animal is markedly larger 
than the cast-off shell. It is a curious fact that in Ligia, subsequent to 
posterior and prior to anterior moult, no increase in the girth of the 
posterior part of the body can be detected with the eye. In an Idotea 
under similar circumstances the disparity in thickness of the two portions 
is most striking, the anterior half of the animal then looking as if it had 
been tightly constricted with bands. Even when moult is complete Ligia 
is not obviously larger than before. 
Morphological. 
Under this heading two questions will be discussed. We have seen that 
in the process of exuviation as studied in Ligia, splitting is limited to the 
thoracic region ; further, that the splitting is of two kinds, transverse and 
longitudinal. The transverse splitting is obviously inter-segmental. The 
longitudinal splitting appears to involve a break in the actual exoskeletal 
somitic ring — a process quite unique, if true. 
Longitudinal Splitting . — As may be remembered, the longitudinal 
splitting involves the second to the seventh (free) thoracic segment, the 
first being unaffected. 
A typical walking limb of a malacostracan may for convenience be said 
to consist of seven separate segments. Many isopods show only six seg- 
ments in their walking legs. Hansen (1893) demonstrated that in such 
cases the proximal segment, or coxopodite, is fused with the corresponding 
body-ring. Among the Idotese he obtained illuminating cases of partial 
and of: complete fusion, on the basis of which he established his conclusion — 
for an improved series, with illustrations, consult Caiman (1909, p. 202). 
As a succinct epitome of the matter I quote Caiman’s account. “ The 
coxopodite has the form of a complete segment movably articulated with 
the body only in the Asellota. In all other Isopoda it is more or less com- 
pletely fused with the body and is often expanded into a coxal plate 
overhanging the attachment of the limb and replacing the pleural expansion 
of the somite to which it belongs. . . . When the suture line disappears, 
as in most Oniscoidea, it is impossible to distinguish the coxal plate from a 
true pleuron. In all Isopoda, however, with the single exception of the 
